CBSE Class 10 Science The Human Eye and the Colourful World Assignment Set 01

Question. To get line spectrum, the substances are excited in their
(a) solid state
(b) molecular state
(c) gaseous state
(d) atomic state
Answer : D

Question. Sometimes blurred and less sharply defined images are formed. This defect is called
(a) Chromatic aberration
(b) Spherical aberration
(c) Blurred lens
(d) None of the above
Answer : B

Question. A prism ABC (with BC as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in figure. In which of the following cases, after dispersion, the sixth colour from the top corresponds to the colour of the sun?

Question. A glass slab is placed over a page on which the word VIBGYOR is printed with each letter in corresponding colour. Then, which of the following is correct?
(a) The images of all the letters will be in the same place as that on paper
(b) Letter V is raised more
(c) Letter R is raised more
(d) None of the above
Answer : B

Question. The following one is not a primary colour
(a) Yellow
(b) Red
(c) Green
(d) Blue
Answer : A

Question. A person cannot see objects clearly which are nearer than 75 cms from his eyes, the disease he suffering from is
(a) Astigmatism
(b) Myopia
(c) Hypermetropia
(d) Presbyopia
Answer : C

Answer : D

Question. A person with a myopic eye cannot see object beyond -1.2m distinctly. The power of the corrective lens used to restore proper vision is
(a) -0.83 D
(b) -0.92 D
(c) +0.21D
(d) +0.91D
Answer : A

Question. Fraunhofer lines in the sun’s spectrum are present because
(a) Vapours of certain elements present in the atmosphere absorb certain colours
(b) The temperature of the sun is very high 
(c) The sun does emit certain light
(d) Certain elements present in the sun interfere
Answer : D

Question. When a mirror is rotated an angle the reflected ray moves through double that angle, the instrument based on the above principle is
(a) Periscope
(b) Odometer
(c) Refractometer
(d) Sextant
Answer : D

Question. The splitting of white light into several colours on passing through a glass prism is due to
(a) refraction
(b) reflection
(c) interference
(d) diffraction
Answer : A

Question. At sun rise or at sun set the sun appears to be reddish while at mid-day it looks white. This is because
(a) Scattering due to dust particles and air molecules causes this phenomenon
(b) The sun is cooler at sun rise or at sunset
(c) Refraction causes this phenomenon
(d) Diffraction sends red rays to the earth at these times.
Answer : A

Question. The frequency of light whose wavelength is 5000 Ac is
(a) 15#1013 cycles per second
(b) 5000 cycles per second
(c) 6#1014 cycles per second
(d) 15#1016 cycles per second
Answer : C

Question : Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?
(a) Dispersion of light
(b) Scattering of light
(c) Total internal reflection of light
(d) Reflection of light from the earth.
Answer : B

Question : While performing the experiment to trace the path of a ray of light passing through a glass prism, four students marked the incident ray and the emergent ray in their diagrams in the manner shown below.

The correct path of the rays has been shown by:
(a) I
(b) II
(c) III
(d) IV
Answer : C

Matching Questions

Question. 

Answer : A-s, B-r, p, C-q, D-q

Answer : A-q, B-p, C-r, D-s

Answer : (a) A-s, r, B-r, p, C-q, D-q

Answer : (a) A-t, B-p, C- q, s, D-r

Very Short Answer

Question :  What is the relation between intensity of scattered light and its amplitude? 

Answer :  The intensity of scattered light is directly proportional to the square of the amplitude 

a. of scattered light, i.e 

 

means greater the amplitude greater will be intensity of refraction and vice versa.

Question : Which phenomenon is responsible for making the path of light visible?
Answer :  Tyndall effect is responsible for making the path of light visible. 

Question : What will the color of the sky be for an astronaut staying in the international space station orbiting the earth?
Answer :  For an astronaut staying in the international space station orbiting the Earth the color of the sky will be black because the light reaching it does not scatter.

  

Short Answer 

 

Question : Why are we not able to see the object clearly for some time when we enter bright light to a room with dim light ? 

Answer :  The pupil of the eye acts as a variable aperture whose size can be varied with the help of the iris. When the light is very bright, the iris contracts the pupil to allow less light to enter the eye. However in dim light the iris expands the pupil to allow more light to enter the eye. When we enter from a bright light area to a dim light area, the pupil is  not able to expand quickly to allow more light into the eye. We are therefore not able to see the objects in dim light room momentarily. 

Question : When white light passes through a glass prism; seven colors namely red, orange, yellow, green, blue, indigo and violet are seen on the white screen. All these colors have different angles of deviation. Explain why?
Answer :  Speed of color in a medium depends upon its wavelength. All the colors have different wavelength. The red color has the longest wavelength and violet color has the last wavelength. Therefore, red color has the highest speed in the glass prism and the violet color has the lowest speed in the glass prism. Hence, all colors of white light are refracted by different amounts while passing through the glass prism. Therefore, all the colors have different angles of deviations. 

Question : (a) What is hypermetropia?
(b) What are the two causes of the defect of vision?
(c) How can this defect of the eye be corrected? Illustrate your answer by drawing ray diagrams to show the formation of image by
(i) a hypermetropic eye
(ii) a hypermetropic eye corrected with a suitable lens.
Answer : (a) Hypermetropia: It is also called long-sightedness. The person with this eye defect can see far off objects but cannot see nearby objects clearly. The image is formed beyond retina.
(b) Two causes
(i) Focal length of the lens increases
(ii) Eye ball becomes smaller
(c) Correction: It can be corrected by using a convex lens. It is a converging lens which shifts the image of the object on the retina.

Question : What is hypermetropia? State the two causes of hypermetropia. With the help of ray diagram show:
(i) the eye defect hypermetropia.
(ii) correction of hypermetropia by using a lens
Ans. Hypermetropia: Defect of a vision in which a person can see distant objects clearly but cannot see nearby objects clearly.
Causes: (a) Focal length of the eye lens is too large.
(b) Eye ball has become too small.

Long Answer

Question : A narrow beam PQ of white light is passing through a glass prism ABC as shown in the diagram.
(i) Write the name and cause of the phenomenon observed.
(ii) Where else in nature is this phenomenon observed?
(iii) Based on this observation, state the conclusion which can be drawn about the constituents of white light.

HOTS Questions and Answers

Question 1. What is the least distance of distinct vision of a normal human eye?
Answer: The least distance of distinct vision for a normal human eye is 25 cm.
In simple words: It is the closest distance at which an object can be seen clearly without any strain on the eye.

Exam Tip: Remember the standard value 25 cm as it is frequently asked in numerical problems involving defects of vision.

Question 2. Name the muscle responsible for bringing change in the focal length of the eye lens?
Answer: Ciliary muscles are responsible for changing the focal length of the eye lens by altering its curvature.
In simple words: Ciliary muscles act like a focal adjuster by squeezing or relaxing the eye lens.

Exam Tip: Power of accommodation is directly linked to the contraction and relaxation of ciliary muscles.

Question 3. Name one defect of vision which cannot be corrected by any type of spectacle lenses?
Answer: Cataract is the defect of vision which cannot be corrected by spectacle lenses; it requires surgical removal of the opaque eye lens and replacement with an artificial lens.
In simple words: Cataract happens when the eye lens becomes cloudy or milky, which spectacles cannot fix.

Exam Tip: Cataract is caused due to the formation of a membrane over the eye lens in old age.

Question 4. State one effect produced by the scattering of light by the atmosphere?
Answer: The Tyndall effect or the blue colour of the clear sky is an effect produced by the scattering of light by atmospheric particles.
In simple words: Scattering makes light visible as it bounces off tiny dust and gas particles in the air.

Exam Tip: Mentioning the Tyndall effect or red sunrise/sunset gives full credit.

Question 5. What is the nature of image formed on the retina of the eye?
Answer: The image formed on the retina of the eye is real and inverted.
In simple words: The eye lens creates a real picture that is upside down on the back screen of the eye.

Question 6. What type of lens is used for correcting hypermetropia?
Answer: A convex lens (converging lens) is used for correcting hypermetropia (farsightedness).
In simple words: Convex lenses help converge light rays earlier so they focus correctly on the retina.

Exam Tip: Hypermetropia = Farsightedness = Corrected by Convex lens.

Question 7. Who was the first person to obtain the spectrum of sunlight?
Answer: Sir Isaac Newton was the first person to use a glass prism to obtain the spectrum of sunlight.
In simple words: Sir Isaac Newton showed that white light splits into seven colours using a prism.

Exam Tip: Memorize the scientist name Sir Isaac Newton for dispersion experiments.

Question 8. What is the function of optic nerve in human eye?
Answer: The optic nerve carries the electrical signals generated by the light-sensitive cells on the retina to the brain for processing.
In simple words: It acts as a data cable connecting the eye's screen (retina) to the computer (brain).

Exam Tip: Always mention 'electrical signals' and 'brain' to score full marks.

Question 9. What is range of vision?
Answer: The range of vision of a normal human eye is between its near point and far point, i.e., from 25 cm to infinity.
In simple words: It is the span of distances from closest to farthest where objects can be seen clearly.

Exam Tip: Lower limit is 25 cm and upper limit is infinity.

Question 10. Why do different colours deviate through different angles on passing through a prism?
Answer: This is because different colours travel through glass with different speeds, and glass has a different refractive index for different colours.
In simple words: Each colour bends by a different amount because their speeds inside the glass prism vary.

Exam Tip: Violet bends the most because of shortest wavelength and highest refractive index.

Question 11. As light rays pass from air into glass prism, are they refracted towards or away from the normal?
Answer: When light rays pass from air (rarer medium) into glass prism (denser medium), they are refracted towards the normal.
In simple words: Light bends towards the central normal line when entering a denser material.

Exam Tip: Rarer to denser medium means bending towards the normal.

Question 12. Which color has largest wavelength?
Answer: Red colour has the largest wavelength.
In simple words: Red light has the longest waves in the visible spectrum.

Exam Tip: Red wavelength is roughly 1.8 times larger than violet wavelength.

Question 13. Which defect of vision can be rectified using a concave lens?
Answer: Myopia (nearsightedness) can be rectified using a concave lens.
Answer: Myopia.
In simple words: Concave lenses diverge light rays to correct nearsighted vision.

Exam Tip: Myopia = Nearsightedness = Corrected by Concave lens.

Question 14. What phenomenon causes twinkling of star on a clear night?
Answer: Atmospheric refraction causes the twinkling of stars on a clear night.
In simple words: Starlight bends continuously as it passes through changing layers of air, making the star appear to twinkle.

Exam Tip: Mention atmospheric refraction of starlight.

Question 15. What is meant by scattering of light?
Answer: Scattering of light is the phenomenon of change in the direction of light on striking a scatterer (like atoms, molecules, or dust particles in the atmosphere).
In simple words: Light gets bounced and spread in different directions when it hits tiny particles in the air.

Question 16. Why do the sky appear black instead of blue to an astronaut?
Answer: The upper atmosphere does not have particles or dust etc. as a result there is no scattering of light, and hence the sky appears dark or black in space to an astronaut.
In simple words: Without an atmosphere to scatter sunlight, space has no blue colour and looks black.

Exam Tip: Mention absence of atmosphere and absence of scattering.

Question 17. What is the basic cause of atmospheric refraction?
Answer: The basic cause of refraction is variation in optical density of different layers of earth's atmosphere. The sun passes through earth's atmosphere.
In simple words: Different air layers have different temperatures and densities, which bend light rays.

Exam Tip: Temperature differences lead to density differences, causing varying refractive indices.

Question 18. Why does clear sky look blue?
Answer: Because blue colour having smallest wavelength is scattered most, amount of scattering is directly proportional to \(\frac{1}{\lambda^4}\).
In simple words: Shorter wavelengths (blue) scatter much more than longer wavelengths when hitting air molecules.

Exam Tip: Quote Rayleigh scattering law \(I \propto \frac{1}{\lambda^4}\) for extra marks.

Question 19. Can visible light be scattered by atoms/molecules in earth's atmosphere?
Answer: Yes, as size of molecules/atoms is much less than wavelength of light.
In simple words: Yes, particles smaller than the wavelength of light cause effective scattering.

Exam Tip: Mention particle size compared to wavelength of light.

Question 20. What is a spectrum?
Answer: The band of seven colours obtained by dispersion of white light is called spectrum.
In simple words: It is the multi-coloured band formed when white light splits.

Exam Tip: Remember the acronym VIBGYOR.

Question 21. Name the defect of vision in person
a. Whose near point is more than 25 cm away?
b. Whose far point is less than infinity.
Answer:
a. Hypermetropia
b. Myopia
In simple words: Farsightedness shifts the near point away, while nearsightedness brings the far point closer.

Exam Tip: Hypermetropia near point > 25 cm; Myopia far point < infinity.

More Questions for Practice

Question 1. A person can see only objects beyond 1 m. From his eyes. Name the defect of the eye.
Answer: Myopia (nearsightedness).
In simple words: Since the person cannot see distant objects clearly beyond 1m, or rather can only see beyond 1m wait - if a person can see *only* objects beyond 1m, this is hypermetropia or presbyopia where near vision is defective. Let's provide standard textbook solution: Hypermetropia (or near vision defect).

Exam Tip: Check near and far point limits carefully before answering.

Question 2. Out of light of blue and red colours which one is scattered most?
Answer: Blue colour is scattered most because it has a shorter wavelength compared to red colour.
In simple words: Blue light scatters much more easily than red light.

Exam Tip: Scattering is inversely proportional to the fourth power of wavelength.

Question 3. What is the function of crystalline lens in the human eye?
Answer: The crystalline lens focuses a real and inverted image of objects at various distances onto the retina by adjusting its focal length.
In simple words: It focuses light properly onto the back of the eye so we can see clearly.

Question 4. Which phenomenon is responsible for increasing the apparent length of the day by 4 minute?
Answer: Atmospheric refraction is responsible for advanced sunrise and delayed sunset, increasing the day length by 4 minutes.
In simple words: Bending of sunlight through the atmosphere makes the sun visible 2 minutes before actual sunrise and 2 minutes after sunset.

Question 5. What is the far point of a person suffering from Myopia?
Answer: The far point of a person suffering from myopia is closer than infinity (at a finite distance).
In simple words: Myopic persons cannot see distant objects clearly, so their far point is less than infinity.

Exam Tip: Normal far point is infinity; myopic far point is finite.

Question 6. What name is given to front transparent part of human eye?
Answer: The cornea.
In simple words: Cornea is the clear outer window at the front of the eye.

Exam Tip: Most of the refraction of light entering the eye occurs at the cornea.

Question 7. Where do we see: (1) Concave and (2) Convex lens in bifocal lenses?
Answer: (1) The concave lens portion is placed at the lower part of the bifocal lens (for reading/near vision).
(2) The convex lens portion is placed at the upper part of the bifocal lens (for distant vision).
In simple words: Lower part helps in reading close-up, upper part helps in viewing far objects.

Exam Tip: Bifocals correct presbyopia where both near and far vision are impaired.

Question 8. What is the nature of image formed by our eye?
Answer: Real and inverted.
In simple words: The retina receives a real, upside-down picture.

Exam Tip: Standard property of convex eye lenses.

Question 9. Name the liquid which is present between eye lens and cornea.
Answer: Aqueous humour.
In simple words: A watery fluid that nourishes the front part of the eye.

Exam Tip: Do not confuse with vitreous humour which is behind the eye lens.

Question 10. Where does most of the refraction of light in an eye occurs?
Answer: At the outer surface of the cornea.
In simple words: Most bending of incoming light happens at the cornea surface.

Exam Tip: Cornea provides the major refractive power of the eye.

Question 11. Which kind of lens is an eye lens?
Answer: A convex lens (made of jelly-like proteinaceous material).
In simple words: It is a natural converging lens inside our eye.

Exam Tip: Natural eye lens is a convex lens.

Question 12. What is the cause of dispersion of light?
Answer: The different speeds of different coloured light rays in a refracting medium (like glass prism), causing them to bend by different angles.
In simple words: Different colours travel at different speeds through the prism, separating them out.

Exam Tip: Dispersion is splitting of white light into its constituent seven colours.

Question 13. Under very dim light, we are able to see the objects but can not distinguish between colours why?
Answer: In dim light, rod cells (which are sensitive to light intensity) function, whereas cone cells (which are sensitive to colours) require bright light to get activated.
In simple words: Rod cells work in the dark for vision without colour, while cone cells need bright light to show colours.

Question 14. What is the range of vision for normal human eye?
Answer: From 25 cm to infinity.
In simple words: Normal eyes can see clearly from 25 cm up to infinity.

Exam Tip: Standard physiological data for human vision.

Question 15. How is the amount of light entering the eye controlled?
Answer: By the iris, which adjusts the size of the pupil.
In simple words: The iris expands or contracts the pupil to control how much light goes inside.

Question 16. What is the colour of danger signal? Why?
Answer: Red, because red colour has the maximum wavelength and is scattered the least by fog or smoke, making it visible from a long distance.
In simple words: Red light travels the farthest without getting scattered much.

Exam Tip: Long wavelength means minimum scattering.

Question 17. What is rainbow? How is rainbow formed?
Answer: A rainbow is a natural spectrum appearing in the sky after a rain shower. It is formed due to dispersion, refraction, and internal reflection of sunlight by tiny water droplets suspended in the atmosphere.
In simple words: Tiny raindrops act like tiny prisms that reflect and split sunlight into a colourful rainbow arc.

Exam Tip: Mention the three key processes: refraction, internal reflection, and dispersion.

Question 18. State two causes of myopic vision.
Answer: 1. Excessive curvature of the eye lens.
2. Elongation of the eyeball.
In simple words: The eyeball becomes too long or the lens becomes too curved, causing light to focus in front of the retina.

Exam Tip: Remember elongation of eyeball and excessive lens curvature.

Question 19. How an uncorrected myopic eye sees far off objects.
Answer: An uncorrected myopic eye sees far off objects blurred because the image is formed in front of the retina rather than on it.
In simple words: Distant objects appear blurry and unclear.

Exam Tip: Image focuses in front of retina in myopia.

Question 20. What is presbyopia? Name the type of lens which can be used to correct presbyopia.
Answer: Presbyopia is the defect of vision due to ageing where the power of accommodation of the eye decreases, resulting in loss of near vision. It is corrected using bifocal lenses (containing both concave and convex lenses).
In simple words: Age-related farsightedness where old people find it hard to read nearby objects clearly.

Exam Tip: Caused by weakening of ciliary muscles and diminishing flexibility of eye lens.

Question 21. Explain why planet do not twinkle at night?
Answer: Planets are much closer to earth and are considered as extended sources of light (collection of multiple point sources). The total variation in light entering the eye from all point-sized sources averages out to zero, hence they do not twinkle.
In simple words: Planets are bigger and closer than stars, so their light fluctuations cancel out and they don't twinkle.

Exam Tip: Contrast stars (point sources) with planets (extended sources).

Question 22. Explain about the colour of the sun at sunrise and sunset.
Answer: At sunrise and sunset, sunlight travels a much longer distance through the atmosphere. Most of the blue light and shorter wavelengths are scattered away by particles, leaving only the red and longer wavelength light to reach our eyes, making the sun appear red.
In simple words: Long distance travel through air scatters away blue light, leaving only red light behind at sunrise and sunset.

Exam Tip: Mention long distance through atmosphere and scattering of blue light.

Question 23. Define the term (1) Near point (2) Far point
Answer: (1) Near point: The minimum distance at which objects can be seen most distinctly without strain (25 cm for normal eye).
(2) Far point: The maximum distance up to which the normal eye can see objects clearly (infinity for normal eye).
In simple words: Near point is closest clear view distance; far point is farthest clear view distance.

Exam Tip: Standard definitions: 25 cm and infinity.

Question 24. Why is a normal eye not able to see clearly the object closer than 25 cm.
Answer: Because ciliary muscles cannot contract beyond a certain limit to increase the curvature and focal length of the eye lens sufficiently to focus images of objects closer than 25 cm on the retina.
In simple words: The eye lens cannot adjust its focal length anymore when an object is brought closer than 25 cm.

Question 25. Draw a ray diagram to show the refraction of light through a glass prism on the diagram mark.
(a) Incident ray (b) Emergent ray and (c) Angle of deviation
Answer:
Construction / Explanation:
1. Draw a triangular outline representing a glass prism ABC.
2. Draw an incident ray hitting the face AB, a refracted ray inside the prism, and an emergent ray leaving face AC.
3. Extend the incident ray forward and the emergent ray backward to meet at an angle, which represents the angle of deviation (\(D\)).

Exam Tip: Label the arrows on incident and emergent rays clearly in exams.

Question 26. How is the dispersed white light recomposed?
Answer: By placing an identical second glass prism in an inverted position with respect to the first prism, the dispersed spectrum of colours recombines to form white light again.
In simple words: Putting an upside-down second prism in front of the first one brings all the split colours back together into white light.

Exam Tip: Newton's re-combination experiment setup.

Question 27. The near point of a hypermetropic eye is at 75 cm from the eye. What is the power of the lens required to enable him to read clearly a book held at 25 cm from the eye.
Answer:
Given data:
Object distance, \(u = -25\text{ cm}\) (normal near point)
Image distance, \(v = -75\text{ cm}\) (defective near point)
Using lens formula:
\(\frac{1}{f} = \frac{1}{v} - \frac{1}{u}\)
\(\frac{1}{f} = \frac{1}{-75} - \frac{1}{-25}\)
\(\frac{1}{f} = -\frac{1}{75} + \frac{1}{25}\)
\(\frac{1}{f} = \frac{-1 + 3}{75} = \frac{2}{75}\text{ cm}^{-1}\)
Focal length, \(f = \frac{75}{2}\text{ cm} = +37.5\text{ cm} = +0.375\text{ m}\)
Power of lens, \(P = \frac{1}{f\text{ (in meters)}}\)
\(P = \frac{1}{0.375} = +\frac{8}{3}\text{ D} \approx +2.67\text{ D}\)
In simple words: A convex lens of power \(+2.67\) D is required to correct this hypermetropic eye.

Exam Tip: Always convert focal length to meters before calculating power in Dioptres.