GSEB Class 10 Science Solutions Chapter 11 Human Eye and Colourful World

Question 8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer: Ciliary muscles in the human eye can contract the eye lens only to a specific limit. Because of this limitation, a person with normal vision can see nearby objects clearly only if they are placed at or beyond 25 cm. If an object is placed closer than 25 cm to the eye, the ciliary muscles cannot contract further to adjust the focal length, and therefore, the eye cannot see the object clearly.
In simple words: Your eye muscles can only tighten so much to change focus. If something is closer than 25 cm, the muscles can't tighten enough to focus it, so it looks blurry.

Exam Tip: The minimum distance of distinct vision (25 cm) is the closest point at which the ciliary muscles can maximally contract to focus an image sharply on the retina.

Question 9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer: The image distance in the eye remains constant. When the distance of an object from the eye increases, the ciliary muscles change the focal length of the eye lens. This adjustment helps the eye to focus the object's image onto the retina, ensuring the image is always formed at the same distance (on the retina).
In simple words: No matter how far or close an object is, the image in your eye always forms at the same spot on the retina. Your eye's muscles just change the lens's focus.

Exam Tip: Emphasize that the retina acts as a fixed screen; therefore, the image distance is constant. The ciliary muscles adjust the focal length of the lens to maintain this constant image distance.

Question 10. Why do stars twinkle?
Answer: Stars appear to twinkle because of the atmospheric refraction of light from the stars and the constantly changing density of the air around the Earth. As starlight passes through Earth's atmosphere, it bends multiple times due to varying atmospheric conditions, causing the apparent position and brightness of the star to fluctuate rapidly, which we perceive as twinkling.
In simple words: Stars twinkle because their light bends and changes direction many times as it passes through Earth's moving and uneven air, making them seem to shimmer.

Exam Tip: The key factors for twinkling are atmospheric refraction, the continuously changing refractive index of air, and the star's point-like appearance.

Question 11. Explain why the planets do not twinkle.
Answer: Planets do not twinkle because they are much closer to Earth and are considerably larger in size compared to stars. They appear as extended sources of light rather than point sources. Although light from planets also undergoes atmospheric refraction, the overall effect averages out over their larger apparent disc, resulting in a steady, non-twinkling appearance.
In simple words: Planets don't twinkle because they are close and look bigger than stars, so their light doesn't get messed up as much by our atmosphere.

Exam Tip: Contrast planets (extended sources, no twinkling) with stars (point sources, twinkling) when explaining this phenomenon.

Question 12. Why does the sun appear reddish early in the morning?
Answer: When the sun rises early in the morning (or sets in the evening), its light travels through a thicker layer of air and a longer distance within the atmosphere surrounding the Earth. This causes the sunlight to scatter the most. Blue light scatters the most, while red light scatters the least. Consequently, most of the blue light is scattered away, allowing the red light to reach our eyes, making the sun appear reddish.
In simple words: In the morning, sunlight travels far through the air. Blue light gets scattered a lot, so only the red light reaches our eyes, making the sun look red.

Exam Tip: This phenomenon is due to Rayleigh scattering, where shorter wavelengths (blue) are scattered more intensely than longer wavelengths (red) by atmospheric particles.

Question 13. Why does the sky appear dark instead of blue to an astronaut?
Answer: In space, there are no particles, air, gases, or water droplets present to scatter light. So, when astronauts look at the sky from space, there is no light entering their eyes from atmospheric scattering. Therefore, the sky appears dark to them.
In simple words: Astronauts see a dark sky because there's no air or particles in space to scatter sunlight, unlike on Earth.

Exam Tip: The presence of an atmosphere and its scattering properties are essential for seeing a blue sky. Without it, light travels directly, and the background appears black.

Gujarat Board Class 10 Science Human Eye and Colourful World Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1. Name the transparent membrane through which light enters first in the eye.
Answer: Cornea.
In simple words: Light first enters the eye through the cornea.

Exam Tip: The cornea is the eye's outermost lens, providing most of the eye's focusing power.

Question 2. What is the approximate diameter of the human eye?
Answer: 2.3 cm
In simple words: The human eye is about 2.3 cm across.

Exam Tip: Knowing the approximate dimensions of eye parts can help in understanding related concepts like focal length and image formation.

Question 3. Name the light-sensitive part of the eye where the image of an object is formed.
Answer: Retina
In simple words: The retina is the light-sensing part where images form in the eye.

Exam Tip: The retina contains specialized photoreceptor cells (rods and cones) essential for vision.

Question 4. What is the function of iris?
Answer: The iris controls the size of the pupil.
In simple words: The iris adjusts how big or small the pupil is.

Exam Tip: The iris's action regulates the amount of light entering the eye, similar to the aperture of a camera.

Question 5. What are light-sensitive cells?
Answer: Rods and cones.
In simple words: Rods and cones are the cells in your eye that sense light.

Exam Tip: Remember that rods detect dim light and shades of gray, while cones detect bright light and color vision.

Question 6. What type of image is formed on the retina?
Answer: Real, inverted image.
In simple words: A real and upside-down image is formed on the retina.

Exam Tip: Although the image formed is inverted, our brain processes it to give us an upright perception.

Question 7. Why is an inverted image formed on the retina of the human eye?
Answer: The inverted image is formed because the eye lens is convex in shape. This convex lens converges the light rays that enter through it, creating a real and inverted image on the retina.
In simple words: The eye's convex lens bends light to form a real, upside-down image on the retina.

Exam Tip: Convex lenses always form real and inverted images when the object is beyond the focal point, which is generally the case for objects we observe.

Question 8. What type of signals are generated and sent to the brain by light-sensitive cells of the retina?
Answer: Electrical signals.
In simple words: The light-sensitive cells in the retina create electrical signals that are sent to the brain.

Exam Tip: These electrical signals are transmitted via the optic nerve to the brain for interpretation.

Question 9. Which part of the human eye controls the amount of light entering the eye?
Answer: Pupil.
In simple words: The pupil controls how much light gets into your eye.

Exam Tip: The pupil's size changes in response to light intensity, a reflex controlled by the iris.

Question 10. What is the function of the crystalline lens of the human eye?
Answer: The crystalline lens provides the correct focal length needed to focus objects at varying distances onto the retina.
In simple words: The eye's lens changes its shape to properly focus objects onto the retina.

Exam Tip: The crystalline lens is flexible, allowing for accommodation, unlike a fixed camera lens.

Question 11. What holds the crystalline lens in the human eye?
Answer: Ciliary muscles.
In simple words: Ciliary muscles hold the eye's lens in place.

Exam Tip: These muscles are crucial for changing the curvature of the lens and thus its focal length.

Question 12. Which part of the human eye helps in changing the thickness of lens?
Answer: Ciliary muscles.
In simple words: The ciliary muscles change the thickness of the eye's lens.

Exam Tip: By changing the thickness (and thus curvature) of the lens, the ciliary muscles enable the eye to focus on objects at different distances.

Question 13. Name the disease in which the crystalline lens of the human eye becomes opaque.
Answer: Cataract.
In simple words: Cataract is the disease where the eye's lens turns cloudy.

Exam Tip: Cataracts impair vision by preventing light from passing clearly through the lens to the retina, often requiring surgical removal.

Question 14. Which is the range of vision of the normal eye?
Answer: 25 cm to infinity.
In simple words: A normal eye can see clearly from 25 cm away to as far as you can imagine.

Exam Tip: This range defines the limits of accommodation for a healthy human eye.

Question 15. Define the least distance of distinct vision.
Answer: The minimum distance at which objects can be seen clearly without any strain on the eye is called the least distance of distinct vision. It corresponds to the near point of the eye and is typically 25 cm for a young adult with normal vision.
In simple words: The closest distance at which you can see an object clearly without straining your eyes is called the least distance of distinct vision.

Exam Tip: This value is crucial for understanding hypermetropia, where the near point shifts beyond 25 cm.

Question 16. What is the persistence of vision?
Answer: Persistence of vision refers to the property of the eye where an image formed on the retina remains there for about 1/16th of a second after the actual object is removed. This phenomenon allows us to perceive a series of still images as continuous motion.
In simple words: Persistence of vision means your eye holds onto an image for a short moment, even after it's gone. This helps us see movies as moving pictures.

Exam Tip: This property is the basis for motion pictures and animation, where a rapid succession of still images creates the illusion of movement.

Question 17. In which type of eye defect far point of the eye gets reduced?
Answer: Myopia.
In simple words: In myopia, the far point of the eye becomes closer than it should be.

Exam Tip: A reduced far point means a person can only see objects clearly up to a certain closer distance, not far away.

Question 18. In which type of eye defect near the point of the eye becomes more than 25 cm?
Answer: Hypermetropia.
In simple words: Hypermetropia is when your eye's near point moves further away than 25 cm.

Exam Tip: An increased near point indicates difficulty seeing nearby objects clearly, a characteristic of hypermetropia.

Question 19. What is presbyopia?
Answer: Presbyopia is an eye defect where a person cannot clearly see both nearby and far-off objects. This condition is generally caused by the natural aging process, which leads to weakening of the ciliary muscles and reduced flexibility of the eye lens.
In simple words: Presbyopia is an age-related eye problem where people struggle to see both close and distant things clearly.

Exam Tip: Presbyopia differs from myopia and hypermetropia as it affects both near and far vision due to aging, often requiring bifocal lenses.

Question 20. What type of lens should be used to correct the presbyopia?
Answer: A bi-focal lens, which is a combination of concave and convex lenses, should be used to correct presbyopia.
In simple words: To fix presbyopia, a bi-focal lens is needed.

Exam Tip: Bifocal lenses have different powers for different parts, typically with the upper part for distant vision (concave) and the lower part for near vision (convex).

Question 21. What is the dispersion of light?
Answer: Dispersion of light is the process where white light splits into its various component colors (like the seven colors of a rainbow) when it passes through a transparent medium, such as a prism.
In simple words: Dispersion of light is when white light separates into its different colors, like a rainbow, as it passes through something.

Exam Tip: Remember that dispersion occurs because each color of light has a different wavelength and thus a different refractive index in a given medium, causing them to bend at different angles.

Question 22. Define angle of prism.
Answer: The angle of prism is the angle formed between the two lateral faces of the prism where light enters and exits. This angle is crucial as it determines the extent of deviation and dispersion of light passing through the prism.
In simple words: The angle of prism is the angle between the two flat sides of the prism where light goes in and out.

Exam Tip: The angle of the prism is also known as the refracting angle of the prism, and it affects the angle of deviation.

Question 23. What is spectrum?
Answer: A spectrum is the band of seven distinct colors (Violet, Indigo, Blue, Green, Yellow, Orange, Red) obtained when white light undergoes dispersion, typically by passing through a prism.
In simple words: A spectrum is the colorful band you get when white light splits into its different colors, like a rainbow.

Exam Tip: Recall the acronym VIBGYOR to remember the sequence of colors in the visible spectrum.

Question 24. In visible spectrum which colour has the longest wavelength.
Answer: Red.
In simple words: In the rainbow of colors, red light has the longest wavelength.

Exam Tip: Longer wavelengths scatter less, which is why red light is used in warning signals and is prominent in sunsets.

Question 25. Give one main difference between the lens of the human eye and the lens of a camera.
Answer: The lens of the human eye has a flexible aperture, meaning its focal length can be changed by the ciliary muscles. In contrast, the focal length of a camera lens is fixed and cannot be changed without physically replacing the lens.
In simple words: The human eye's lens can change its focus, but a camera lens has a fixed focus.

Exam Tip: This flexibility of the human eye lens is what enables accommodation and allows us to focus on objects at various distances.

Question 26. What is the Tyndall effect?
Answer: The Tyndall effect is the phenomenon of scattering of light by small particles suspended in a medium. It is visible when a beam of light passes through a colloidal solution or through the atmosphere containing dust and smoke particles, making the path of light visible.
In simple words: The Tyndall effect is when light scatters off tiny particles, making the path of the light beam visible, like dust in a sunbeam.

Exam Tip: Examples of the Tyndall effect include the visible beam of headlights in fog, the blue color of the sky, and the red appearance of sunsets.

Question 27. Which phenomenon is responsible for making the path of light visible?
Answer: Tyndall effect.
In simple words: The Tyndall effect makes light paths visible.

Exam Tip: The Tyndall effect helps differentiate between true solutions and colloidal solutions or suspensions.

Question 28. State one function of the iris in the human eye.
Answer: Iris controls the size of the pupil.
In simple words: The iris adjusts the pupil's size.

Exam Tip: By controlling pupil size, the iris regulates the amount of light that enters the eye, protecting the retina from excessive light and allowing more light in dim conditions.

Question 29. State one role of ciliary muscles in the human eye.
Answer: Ciliary muscles help the eye lens to focus the image of an object on the retina by increasing or decreasing the curvature of the eye lens. This action allows the eye to accommodate for objects at various distances.
In simple words: Ciliary muscles change the shape of the eye lens to focus images clearly on the retina.

Exam Tip: This function of ciliary muscles is vital for the eye's power of accommodation, ensuring clear vision at different distances.

Question 30. State one function of the crystalline lens in the human eye.
Answer: The crystalline (eye) lens forms a real and inverted image of the object on the retina.
In simple words: The eye's lens makes a real, upside-down image on the retina.

Exam Tip: Besides forming the image, the crystalline lens's ability to change shape is crucial for focusing at different depths.

Short Answer Type Questions

Question 1. How does eye control the amount of light entering it?
Answer: The amount of light entering the eye is controlled by the pupil. Furthermore, the size of the pupil is regulated by the iris. In bright light, the iris contracts, making the pupil smaller to reduce light entry, while in dim light, it relaxes, making the pupil larger to allow more light in.
In simple words: The pupil controls light entry. The iris makes the pupil bigger or smaller to let in more or less light, depending on how bright it is.

Exam Tip: This pupil reflex is an automatic response that protects the retina from damage and optimizes vision in varying light conditions.

Question 2. The image formed on the retina is inverted but we see the object erect. Why?
Answer: The image formed on the retina is indeed inverted. This image is created by the light-sensitive cells, called rods and cones, of the retina, which then generate electrical signals. These signals travel to the brain via the optic nerve. It is the brain that interprets this inverted image and, through complex processing, helps us perceive objects as upright and erect.
In simple words: The image on your retina is upside down, but your brain flips it to make you see things the right way up.

Exam Tip: This highlights the brain's role in interpreting sensory input, not just passively receiving it. The brain's processing corrects the inverted retinal image.

Question 3. Why do birds fly back to their nest in the evening?
Answer: Birds often fly back to their nests in the evening because many bird species lack specific light-sensitive cells called rods, or have very few of them. Rods are crucial for seeing in low or dim light. Due to the shortage of these cells, many birds cannot see objects clearly in less bright or dim light conditions.
In simple words: Many birds go home in the evening because they don't have enough cells (rods) to see well in dim light.

Exam Tip: This answer connects the structure of the eye (presence/absence of rods) to an animal's behavior (nocturnal/diurnal activity).

Question 4. Why do you take time to see objects when you enter a dimly lighted room from outside in the sun?
Answer: When you move from bright sunlight to a dimly lit room, your eyes need time to adjust. In bright light, the pupil contracts to a small size to limit light entry. When you enter a dim room, it takes some time for the iris to adjust the pupil's size by dilating it to allow more light in, and for the light-sensitive cells (especially rods) to become fully activated for low-light vision. This short delay is why objects are not immediately clear.
In simple words: When you go from bright sun to a dark room, your eye's pupil needs time to get bigger to let in more light, and your eye cells need time to become active for dim vision.

Exam Tip: This describes visual adaptation, a process involving both pupil adjustment (iris action) and retinal sensitivity (rod activation).

Question 5. Why are two eyes more helpful for us to see as compared to one?
Answer: Two eyes offer several advantages over one. While a single eye provides a view of approximately 150° horizontally, two eyes greatly extend this to about 180°, widening the field of view. Additionally, two eyes help us see objects more clearly in dim light or darkness. Most importantly, having two eyes provides stereoscopic vision, which enables us to perceive depth and assess distances more accurately.
In simple words: Two eyes are better because they give you a wider view, help you see in dim light, and let you judge distances better (depth perception).

Exam Tip: Emphasize the three main benefits: wider field of view, improved low-light vision, and crucially, stereoscopic vision for depth perception.

Question 6. When white light enters the prism, which colour of light deviates/bends the least and which colour bends the most?
Answer: When white light enters a prism, the light that bends the least is red, and the light that bends the most is violet. This happens because red light has the longest wavelength and travels fastest through the prism, refracting less, while violet light has the shortest wavelength and travels slowest, refracting the most.
In simple words: Red light bends the least, and violet light bends the most when white light goes through a prism.

Exam Tip: Remember the order of dispersion (VIBGYOR) and that deviation is inversely proportional to wavelength in a prism.

Question 7. Explain the phenomenon which causes twinkling of stars.
Answer: The phenomenon that causes stars to twinkle is atmospheric refraction. Stars are like point sources of light, very far from us. As their light travels through Earth's atmosphere, it constantly bends due to changing densities of air (which cause changes in the refractive index). This continuous bending makes the amount of light entering our eyes vary each moment, which causes the twinkling effect.
In simple words: Stars twinkle because their light keeps bending and shifting as it passes through the Earth's moving atmosphere, making them seem to flicker.

Exam Tip: Key terms to include are "atmospheric refraction," "changing density/refractive index of air," and "point source of light."

Question 8. Why does a ray of light split into different colours on passing through a glass prism?
Answer: When light rays enter a glass prism, the angle at which they bend causes the light to split into its seven component colors. This occurs because the speed of each color component of light is different inside the glass. Due to these varying speeds, each component refracts at a slightly different angle, resulting in their separation.
In simple words: Light splits into colors when passing through a prism because each color travels at a different speed through the glass, making them bend differently.

Exam Tip: The core concept here is that the refractive index of glass is different for different wavelengths (colors) of light.

Question 9. In the dispersion of white light through a prism, which colour deviates most and which deviates least? Why do they deviate differently?
Answer: In the dispersion of white light through a prism, violet light deviates the most, and red light deviates the least. This difference in deviation happens due to the variation in wavelength and speed of each color of light. Colors with shorter wavelengths (like violet) slow down more and bend more sharply when entering and exiting the prism, while colors with longer wavelengths (like red) slow down less and bend less, resulting in less deviation.
In simple words: Violet light bends the most, and red light bends the least when passing through a prism. This is because each color has a different wavelength and speed, causing them to bend at different angles.

Exam Tip: Link the degree of deviation directly to the wavelength and speed of light within the prism: shorter wavelength = slower speed = greater deviation.

Question 10. What is the direction of rainbow formation? What is the position of red colour in the rainbow?
Answer: A rainbow always forms in the direction opposite to the sun. The red color's position in the rainbow is at the top.
In simple words: Rainbows appear opposite the sun, and red is always the highest color.

Exam Tip: Remember that red light has the longest wavelength and is refracted the least, which places it at the top of the rainbow arc.

Question 11. What is internal reflection?
Answer: When light rays move from a denser medium to a rarer medium (for example, from water to air), a ray of light, instead of passing through, reflects back into the original medium. This phenomenon is called internal reflection of light.

Exam Tip: For internal reflection, always remember the two conditions: light must travel from a denser to a rarer medium, and the angle of incidence must be greater than the critical angle.

Question 12. A short-sighted person cannot see clearly beyond 5 m. Calculate the power of the lens required to correct his vision to normal?
Answer:A short-sighted person has a far point of 5 m.
This means the corrective lens should form a virtual image of a distant object (at infinity) at 5 m.
Object distance \( u = -\infty \)
Image distance \( v = -5 \) m
Using the lens formula: \( \frac { 1 }{ v } - \frac { 1 }{ u } = \frac { 1 }{ f } \)
\( \frac { 1 }{ -5 } - \frac { 1 }{ -\infty } = \frac { 1 }{ f } \)
\( \frac { 1 }{ -5 } - 0 = \frac { 1 }{ f } \)
\( \frac { 1 }{ f } = \frac { -1 }{ 5 } \)
\( f = -5 \) m
Power of the lens \( P = \frac { 1 }{ f } \)
\( P = \frac { 1 }{ -5 } = -0.2 \) Dioptre
Therefore, the required power of the lens is \( -0.2 \) Dioptre.
In simple words: To help a short-sighted person see far away, we need a lens that makes things at infinity look like they are 5 meters away. This lens will have a power of -0.2 Dioptre.

Exam Tip: For myopia correction, the image formed by the corrective lens should be at the person's far point, and always use a concave lens, resulting in negative power.

Question 14. What is night blindness and colour blindness?
Answer: Night blindness occurs when a person lacks rod cells in the retina, making them unable to see objects clearly in dim light. Colour blindness is a defect where a person lacks cone cells in the retina, leading to an inability to distinguish between different colors.
In simple words: Night blindness is when you can't see well in low light because of missing rod cells, while color blindness means you can't tell colors apart due to missing cone cells.

Exam Tip: Remember that rod cells are responsible for vision in low light (black and white), and cone cells are responsible for color vision in bright light.

Question 15. What is myopia? How can it be corrected?
Answer: Myopia, also known as short-sightedness, is an eye defect where a person can clearly see nearby objects but struggles to see far-off objects distinctly. This condition can happen if the eyeball increases in size or if the focal length of the eye lens decreases. It can be corrected by using a concave lens with an appropriate focal length.
In simple words: Myopia means you see near things clearly but distant things are blurry. It's fixed with special concave glasses.

Exam Tip: Always remember that myopia requires a concave (diverging) lens to shift the image onto the retina.

Question 16. What is hypermetropia? How can it be corrected?
Answer: Hypermetropia, or long-sightedness, is an eye defect where a person can see far-off objects clearly but cannot see nearby objects well. This occurs because the image forms behind the retina. The causes include the eye lens having too long a focal length or the eyeball being too small. It can be corrected by using a convex lens of appropriate power.
In simple words: Hypermetropia means seeing distant things clearly but nearby things blur. It's fixed with convex glasses because the image forms too far back.

Exam Tip: For hypermetropia, a convex (converging) lens is needed to converge light rays onto the retina.

Question 18. Give the difference between myopia and hypermetropia.
Answer:Myopia:

• Short-sightedness - A person can see nearby objects clearly but cannot see far-off objects distinctly.
• The image is formed in front of the retina.
• The size of the eyeball increases.
• The focal length of the eye lens decreases.
• Corrected by using a concave lens.

• Long-sightedness - A person can see far-off objects clearly but cannot see nearby objects distinctly.
• The image is formed beyond the retina.
• The size of the eyeball decreases.
• The focal length of the eye lens increases.
• Corrected by using a convex lens.

Exam Tip: When distinguishing eye defects, focus on where the image forms and the type of corrective lens required, as these are key differentiating factors.

Question 19. Distinguish between presbyopia and hypermetropia.
Answer:Hypermetropia:

• Only far-sightedness; people see distant objects clearly.
• The eyeball becomes shorter or the focal length of the eye lens increases.
• Corrected by using a convex lens.

• Both far and short-sightedness are experienced; people cannot see nearby as well as far-off objects clearly.
• Ciliary muscles become weak and lose their ability to adjust the focal length of the eye lens. This is usually due to aging.
• Corrected by using a bifocal lens, which contains both concave and convex lenses.

Exam Tip: Note that presbyopia is an age-related condition affecting accommodation, whereas hypermetropia can occur at any age due to eyeball shape or lens focal length.

Question 20. The near point of a hypermetropic eye is 80 cm. What is the nature and power of the lens required to enable him to read a book placed at 25 cm from the eyes?
Answer:For a hypermetropic eye, the near point is 80 cm. The person wants to read a book at 25 cm. Object distance \( u = -25 \) cm The image of the object at 25 cm needs to be formed at the near point of the defective eye, which is 80 cm. Image distance \( v = -80 \) cm Using the lens formula: \( \frac { 1 }{ v } - \frac { 1 }{ u } = \frac { 1 }{ f } \)
\( \frac { 1 }{ -80 } - \frac { 1 }{ -25 } = \frac { 1 }{ f } \)
\( \frac { 1 }{ f } = \frac { -1 }{ 80 } + \frac { 1 }{ 25 } \)
To find a common denominator (400):
\( \frac { 1 }{ f } = \frac { -5 }{ 400 } + \frac { 16 }{ 400 } \)
\( \frac { 1 }{ f } = \frac { 11 }{ 400 } \)
\( f = \frac { 400 }{ 11 } \) cm \( \approx 36.36 \) cm
Converting focal length to meters: \( f = 0.3636 \) m
Power of the lens \( P = \frac { 1 }{ f } \)
\( P = \frac { 1 }{ 0.3636 } \approx +2.75 \) Dioptre
The nature of the lens required is convex (since the power is positive).
In simple words: To read a book at 25 cm with a hypermetropic near point of 80 cm, a convex lens with a power of approximately +2.75 Dioptres is needed.

Exam Tip: For hypermetropia, always expect a positive power and a convex lens, as the lens needs to converge light rays to bring the near point closer.

Question 21. What is meant by the dispersion of white light? Draw a ray diagram to show the dispersion of white light by a glass prism. Give a reason why do we get different colours of light?
Answer: Dispersion of light is the phenomenon where white light splits into its seven constituent colors when passing through a transparent medium like a glass prism.

Diagram: Dispersion of light

Exam Tip: Remember the acronym VIBGYOR to recall the order of colors in a spectrum and their relative deviation (Violet deviates most, Red deviates least).

Question 22. A student can see objects clearly only when the objects are lying at distances between 60 cm and 320 cm from the eye.
(a) What kind of eye defect he is suffering from?
(b) What kind of lens will be required to increase his range from 25 cm to infinity? Explain briefly.
Answer:
(a) The student is suffering from both myopia and hypermetropia.
(b) To correct this, a bifocal lens with appropriate focal length and power will be required. The upper part of the bifocal lens will be a concave lens to correct distant vision (myopia), allowing the student to see objects at infinity. The lower part will be a convex lens to correct near vision (hypermetropia), enabling the student to see objects clearly at 25 cm.
In simple words: (a) The student has both blurry near and distant vision. (b) They need special glasses with two different lens strengths, called bifocal lenses, to see clearly at all distances.

Exam Tip: When a person has both near and far vision defects, it's typically presbyopia, which is corrected using bifocal lenses. This suggests age-related weakening of ciliary muscles.

Question 23. When we see any object through the hot air over the fire, it appears to be wavy moving slightly. Explain.
Answer: Objects viewed through hot air over a fire appear wavy because the air medium through which light passes changes constantly. Light travels from denser to rarer air and then back to denser air, causing refraction. Moreover, the refractive index of the hot air keeps changing, which makes the object appear wavy and slightly moving.
In simple words: Hot air above a fire moves light around because its temperature and density change quickly. This constant change makes objects seen through it look blurry and wobbly.

Exam Tip: This phenomenon is a direct consequence of atmospheric refraction, where varying densities of air cause light to bend unpredictably, similar to stars twinkling.

Question 24. Study the diagram given below and answer the questions that it follows:
(a) Name the defect and give the reason.
(b) Give 2 causes for this defect.
(c) Give the correction - draw a diagram for the same.
Answer:(a) The defect shown in the diagram is myopia, also known as short-sightedness. The reason is that the image of a distant object is formed in front of the retina, instead of directly on it.
(b) Two common causes for this defect are:
(i) A decrease in the focal length of the eye lens.
(ii) An increase in the size of the eyeball.
(c) Correction: The defect can be corrected by using a concave lens (diverging lens) which shifts the image to form at the retina. The virtual image is formed at F using the concave lens.

Exam Tip: For diagrams of eye defects, clearly label the retina, the lens, and the point where the image forms, and indicate the path of light rays accurately.

Question 25. In the given diagram label A, B, C and D and give the function of B and D.
Answer:A = Cornea
B = Ciliary muscles
C = Retina
D = Optic nerve

Exam Tip: For diagrams of the human eye, ensure clear labels for all major parts and a concise description of their primary functions.

Question 26. Draw a labelled diagram of rainbow formation. Also, explain the phenomenon of rainbow formation.
Answer:

Diagram: Rainbow formation

Exam Tip: The key processes for rainbow formation are dispersion, internal reflection, and refraction. Ensure your diagram clearly shows these steps within a single water droplet.

Question 27. What do understand by the term pupil reflex? Explain with the diagram.
Answer: Pupil reflex refers to the changes in the size of the pupil that control the amount of light entering the eye. In bright light, the pupil's size is reduced to prevent too much light from damaging the retina. This smaller pupil size allows only a limited amount of light to enter. Conversely, in dim light, the pupil's size increases to allow as much light as possible to enter the eye, improving vision.
The retina detects the brightness of light entering the eye. An impulse passes to the brain along sensory neurons, and the message then travels back to the iris muscles via motor neurons, triggering a response that changes the pupil's size due to the contraction of radial or circular muscles.

Exam Tip: When explaining pupil reflex, emphasize the role of both circular and radial muscles and how their contraction/relaxation controls pupil size in response to light intensity.

Question 28. What is the difference in the circular muscles and the ciliary muscles in the human eye?
Answer: The circular muscles control the size of the iris, which in turn regulates the amount of light entering the eye by adjusting the pupil's opening. In contrast, the ciliary muscles control the size and shape of the eye lens, enabling it to change its focal length for focusing on objects at various distances.
In simple words: Circular muscles manage how big the iris is, controlling light entry, while ciliary muscles adjust the lens to help the eye focus on things far and near.

Exam Tip: Remember that circular muscles affect the pupil (light entry), and ciliary muscles affect the lens (focusing power).

Question 29. A person needs a lens of power 5.5 D for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 D. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
Answer:(i) For distant vision: Power of lens \( P_1 = -5.5 \) D Focal length \( f_1 = \frac { 1 }{ P_1 } = \frac { 1 }{ -5.5 } \) m \( \approx -0.18 \) m (or \( -18 \) cm) (ii) For near vision: Power of lens \( P_2 = +1.5 \) D Focal length \( f_2 = \frac { 1 }{ P_2 } = \frac { 1 }{ 1.5 } \) m \( \approx 0.67 \) m (or \( 67 \) cm)
In simple words: For distant vision, the lens focal length is about -0.18 meters, and for near vision, it's about 0.67 meters.

Exam Tip: A negative focal length indicates a concave (diverging) lens for myopia, while a positive focal length indicates a convex (converging) lens for hypermetropia.

Question 30. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Answer:To correct myopia, the person needs a concave lens (diverging lens) of a suitable focal length.
For a myopic eye, the far point is 80 cm. The corrective lens should form a virtual image of a distant object (at infinity) at the person's far point (80 cm).
Object distance \( u = -\infty \)
Image distance \( v = -80 \) cm
Using the lens formula: \( \frac { 1 }{ v } - \frac { 1 }{ u } = \frac { 1 }{ f } \)
\( \frac { 1 }{ -80 } - \frac { 1 }{ -\infty } = \frac { 1 }{ f } \)
\( \frac { 1 }{ -80 } - 0 = \frac { 1 }{ f } \)
\( f = -80 \) cm
Converting focal length to meters: \( f = -0.80 \) m
Power of the lens \( P = \frac { 1 }{ f } \)
\( P = \frac { 1 }{ -0.80 } = -1.25 \) Dioptre
The nature of the lens required is concave (since the power is negative).
In simple words: To correct a myopic eye with a far point of 80 cm, a concave lens with a power of -1.25 Dioptre is needed.

Exam Tip: For myopia, the far point of the defective eye becomes the virtual image distance (v), and the object distance (u) for a perfectly corrected eye is infinity.

Question 31. Describe and explain the action of eye changes seen while changing the focus from a distant object to a near object.
Answer: When the eye changes focus from a distant object to a near object, the ciliary muscles contract. This contraction relaxes the suspensory ligaments, allowing the eye lens to become thicker and more convex. This change increases the lens's converging power and decreases its focal length, enabling the eye to form a clear image of the nearby object on the retina. Conversely, when focusing on a distant object, the ciliary muscles relax, increasing the focal length of the lens.
In simple words: To see nearby things clearly after looking far away, the eye's ciliary muscles tighten, making the lens thicker and rounder so it can focus the image correctly on the retina.

Exam Tip: The ability of the eye lens to change its focal length is called accommodation, and it is primarily controlled by the ciliary muscles and suspensory ligaments.

Question 32. State the difference between rods and cones cells in the retina of the human eye, Draw a diagram of the human eye.
Answer:The differences between rods and cones cells are:

Exam Tip: When drawing the human eye, ensure proportional representation of parts and clear labeling. For the table, focus on key differences like light sensitivity and color perception.

Long Answer Type Questions

Exam Tip: For explaining eye parts, describe each component's structure and its specific role in the process of vision. A well-labeled diagram is crucial.

Question 2. Describe with the help of a diagram, how the refraction of light takes place through a glass prism.
Answer: When a ray of light (PE) enters a glass prism, it undergoes refraction. As it moves from a rarer medium (air) to a denser medium (glass), the incident ray PE bends towards the normal and forms an angle of refraction \( \angle r \). The angle of refraction is smaller than the angle of incidence \( \angle i \). This refracted ray (EF) then travels within the prism. When it exits the prism, moving from a denser medium (glass) back to a rarer medium (air), it bends away from the normal, forming the emergent ray (FS). This emergent ray is at an angle to the direction of the original incident ray. This final angle of deviation is denoted as \( \angle D \).

Exam Tip: For prism diagrams, clearly show the incident ray, refracted ray, emergent ray, and the angles of incidence, refraction, and emergence. Mark the deviation angle correctly.

Question 3. Name three refractive defects of vision with the help of a diagram. Explain the reasons for and correction of these defects.
Answer: The three refractive defects of vision are:
1. Myopia (Short-sightedness):
Reasons: This defect occurs when the eyeball is too long or the eye lens has too much converging power (focal length is too short). As a result, the image of distant objects is formed in front of the retina.
Correction: Myopia is corrected by using a concave lens (diverging lens) of appropriate power. This lens diverges the incoming light rays before they enter the eye, ensuring that the image focuses precisely on the retina.

Exam Tip: When explaining each defect, clearly state the cause, the location of image formation, and the type of corrective lens. The diagrams should visually support these explanations.

Questions On High Order Thinking Skills (Hots)

• If the particles are very small (smaller than the wavelength of visible light), they predominantly scatter blue light, which has a shorter wavelength. This is why the sky appears blue.
• If the particles are larger (comparable to or larger than the wavelength of visible light), they scatter light of longer wavelengths more effectively, such as red light. If the particles are large enough, they scatter all wavelengths of light almost equally, making the light appear white (e.g., clouds).

Exam Tip: This question relates to Rayleigh scattering. Remember that smaller particles scatter shorter wavelengths (blue) more, while larger particles scatter all wavelengths more uniformly, leading to white appearance.

Question 2. Give one use of the following properties of light:
(i) Scattering of light
(ii) Persistence of vision
(iii) Power of accommodation
(iv) Refraction of light
(v) Reflection of light
Answer:

• (i) Due to the scattering of light, we can see the different colors of the sky and observe rainbow formation.
• (ii) Persistence of vision is used in cinematography (making movies).
• (iii) The eye's power of accommodation allows it to focus on both nearby and far-off objects.
• (iv) Through lenses, eye defects can be corrected, and we can see underwater.
• (v) Our eyes can see an object only due to the reflection of light by the object into our eyes.

Exam Tip: For each property, provide a concise, real-world application or example to illustrate its use or effect.

Question 3. To correct myopia why we use the concave lens and to correct hypermetropia, why do we use convex lens? Why can't we do vice-versa?
Answer:Myopia is a defect where the image forms in front of the retina. We use a diverging (concave) lens to correct it because it spreads out the light rays before they enter the eye, allowing them to focus directly on the retina.
In hypermetropia, the image forms behind the retina. We use a converging (convex) lens because it brings the light rays together, ensuring they meet on the retina.
We cannot use the lenses vice-versa because using a convex lens for myopia would converge the light rays even more, moving the image further in front of the retina and worsening the vision. Similarly, using a concave lens for hypermetropia would diverge the light rays further, pushing the image even further behind the retina and making near vision even blurrier.
In simple words: Concave lenses fix myopia by spreading light to move the focus back onto the retina. Convex lenses fix hypermetropia by bringing light together to move the focus forward onto the retina. Using the wrong lens would make vision problems even worse because they do the opposite of what's needed.

Exam Tip: Clearly explain how the refractive properties (converging/diverging) of each lens type directly counteract the specific image formation problem of each eye defect.

Question 4. In presbyopia, we use a bi-focal lens with an upper portion concave lens and lower portion convex lens. Why is the arrangement so?
Answer: In presbyopia, a person experiences difficulty seeing both distant and nearby objects clearly due to age-related weakening of ciliary muscles and reduced lens flexibility. The arrangement of a bifocal lens addresses both issues:
The upper portion consists of a concave lens. This part is used for distant vision, as when we look at far-off objects, our eyes typically gaze straight ahead or slightly upwards. The concave lens diverges parallel rays from infinity, allowing them to focus correctly on the retina.
The lower portion consists of a convex lens. This part facilitates seeing near objects. When reading or performing close-up tasks, our eyes naturally look downwards. The convex lens converges divergent rays from nearby objects, ensuring they form a clear image on the retina.
In simple words: Bifocal lenses have two parts for presbyopia. The top part is concave for seeing distant things, as we look up for those. The bottom part is convex for seeing nearby things, as we look down for those.

Exam Tip: Explain that the lens types in bifocals are positioned to align with natural eye movements for distant and near viewing, effectively combining corrections for both myopia and hypermetropia symptoms.

Question 5. Why does light split into spectrum when it passes through prism only and does not split when it passes through glass slab?
Answer: Light splits into a spectrum when passing through a prism but not through a glass slab primarily due to the difference in the geometry of their refracting surfaces and how light interacts with them.
A rectangular glass slab has parallel refracting surfaces. When white light enters the slab, it undergoes dispersion and splits into its constituent colors. However, when these dispersed colors emerge from the second parallel surface, they recombine or undergo an equal and opposite deviation. This effectively negates the dispersion, and the emergent light remains parallel to the incident light, only slightly displaced laterally.
In contrast, a prism has non-parallel refracting surfaces. When white light enters the prism, dispersion occurs, and the colors separate. Because the surfaces are angled, each color deviates at a different angle as it exits the prism. The deviation is not canceled out; instead, it is compounded, leading to a noticeable splitting of white light into its constituent spectrum.
In simple words: A prism splits light because its sides are angled, so colors bend differently and stay separate. A glass slab doesn't split light into a clear spectrum because its sides are parallel, causing the colors to bend equally as they enter and leave, effectively canceling out the separation.

Exam Tip: Highlight the crucial difference: parallel surfaces in a slab lead to cancellation of dispersion, while angled surfaces in a prism lead to the persistence and enhancement of dispersion.