CBSE Class 10 Science The Human Eye and the Colourful World Assignment Set 07

This is Assignment Set 03 for CBSE Class 10 Science Chapter 10, The Human Eye and the Colourful World, with 22 chapter-wise practice questions arranged by marks, drawn from board-style question banks. It covers the parts and working of the eye, accommodation, eye defects, prism and dispersion, the scattering of light and the blue sky, and every question has a step-wise answer.

Very Short Answer Questions (1 Mark)

Question 1. [1 mark] What is the function of the retina in the human eye?
Answer:
The retina acts as a screen on which the eye lens forms the image. Its light-sensitive cells convert the image into electrical signals sent to the brain.
In simple words: The retina is the screen of the eye.

Teacher's Note:
1. Mention the signals to the brain.
2. The image on the retina is real and inverted.

Question 2. [1 mark] Which colour of light has the maximum velocity in glass?
Answer:
Red light, because its refractive index in glass is the smallest.
In simple words: Red travels fastest in glass.

Teacher's Note:
1. Speed in glass = c/n.
2. Red bends least.

Question 3. [1 mark] Which colour of light has the minimum velocity in glass?
Answer:
Violet light, because its refractive index in glass is the largest.
In simple words: Violet travels slowest in glass.

Teacher's Note:
1. It is the opposite of red.
2. Violet bends most.

Question 4. [1 mark] What does VIBGYOR refer to?
Answer:
VIBGYOR stands for the seven colours of the spectrum of white light: Violet, Indigo, Blue, Green, Yellow, Orange and Red.
In simple words: It is a way to remember the colours of the spectrum.

Teacher's Note:
1. Violet is at one end, red at the other.
2. Learn the order carefully.

Question 5. [1 mark] What is the angle of dispersion?
Answer:
The angle between the red light and the violet light after dispersion by a prism is called the angle of dispersion.
In simple words: It is the angular spread of the spectrum.

Teacher's Note:
1. It is the angle between the extreme colours.
2. Not the same as the angle of deviation.

Question 6. [1 mark] Name the optical phenomenon due to which stars twinkle.
Answer:
Atmospheric refraction of light.
In simple words: Moving air bends starlight, making stars twinkle.

Teacher's Note:
1. The star is a point source.
2. Planets do not twinkle.

Question 7. [1 mark] When a light ray passes obliquely through the atmosphere in an upward direction, how does its path generally change?
Answer:
The ray passes from a denser to a rarer layer, so it bends away from the normal.
In simple words: Going up through thinner air, the ray bends away from the normal.

Teacher's Note:
1. Going down it bends towards the normal.
2. The reverse of starlight coming down.

Question 8. [1 mark] How would the sky appear in the absence of the Earth's atmosphere?
Answer:
The sky would appear dark (black), because there would be no air particles to scatter sunlight.
In simple words: No atmosphere, no scattering, so a black sky.

Teacher's Note:
1. The blue colour is due to scattering.
2. An astronaut sees a dark sky.

Question 9. [1 mark] Which liquid fills the space behind the cornea?
Answer:
Aqueous humour.
In simple words: A clear liquid sits between the cornea and the lens.

Teacher's Note:
1. Do not confuse with vitreous humour.
2. It nourishes the front of the eye.

Question 10. [1 mark] What is the eye lens made of?
Answer:
The eye lens is made of a transparent, fibrous, jelly-like material.
In simple words: The eye lens is soft and jelly-like, not glass.

Teacher's Note:
1. It is a convex lens.
2. Its curvature is changed by the ciliary muscles.

Short Answer Questions (2 and 3 Marks)

Question 11. [3 marks] State in brief the functions of the following parts of the human eye: (a) Iris (b) Cornea (c) Ciliary muscles (d) Pupil.
Answer:
(a) Iris: controls the size of the pupil.
(b) Cornea: the transparent front layer that protects the eye and does most of the refraction of light entering it.
(c) Ciliary muscles: adjust the curvature, and so the focal length, of the eye lens.
(d) Pupil: the opening that regulates and controls the amount of light entering the eye.
In simple words: Iris sets the pupil, cornea bends light, ciliary muscles focus, pupil lets light in.

Teacher's Note:
1. Write one function in one line for each part.
2. Do not mix up the iris and the pupil.

Question 12. [2 marks] Why does it take some time to see objects in a dim-lit room when we enter the room from bright sunlight outside?
Answer:
In bright light the pupil is small to limit the amount of light entering the eye.
When we enter a dim-lit room, the pupil takes some time to expand and allow more light to enter the eye. Only then can we see things clearly.
In simple words: The pupil needs time to open wider in dim light.

Teacher's Note:
1. Iris controls the pupil.
2. State both the small and the widened pupil.

Question 13. [2 marks] (a) What is meant by the least distance of distinct vision? (b) How does the thickness of the eye lens change when we shift from looking at a distant tree to reading a book?
Answer:
(a) The minimum distance of an object from the eye at which it can be seen clearly without strain is the least distance of distinct vision. It is about 25 cm for a healthy person.
(b) While reading a book, the ciliary muscles contract and the eye lens becomes thicker than when looking at a distant tree.
In simple words: Reading needs a thicker lens than looking at faraway objects.

Teacher's Note:
1. The least distance is also called the near point.
2. Thicker lens means smaller focal length.

Question 14. [3 marks] What do you mean by dispersion of light? What is its cause?
Answer:
When white light passes through a glass prism, it splits into its seven constituent colours (VIBGYOR). This is called dispersion.
Cause: light of different colours travels at the same speed in vacuum or air, but at different speeds in a medium like glass. Violet travels slowest and red fastest, so the refractive index of glass is greatest for violet and least for red.
As a result violet is deviated the most and red the least, and the colours separate.
In simple words: Glass slows each colour differently, so each bends differently.

Teacher's Note:
1. State the speed in vacuum and in glass.
2. Give the order of deviation.

Question 15. [2 marks] What is the scattering of light? Explain with the help of an example.
Answer:
The change in the direction of propagation of light caused by the large number of particles present in the atmosphere (or any medium) is called scattering of light.
Example: the path of a beam of light becomes visible through a colloidal solution because of scattering of light.
In simple words: Particles send the light in different directions.

Teacher's Note:
1. Give one clear example.
2. The Tyndall effect is the same idea.

Question 16. [2 marks] Why does the clear sky appear blue?
Answer:
The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light.
They scatter light of shorter wavelengths (the blue end) much more than light of longer wavelengths (the red end). The scattered blue light reaches our eyes from all directions, so the sky appears blue.
In simple words: Air scatters blue light the most, so the sky looks blue.

Teacher's Note:
1. Size of particles smaller than the wavelength.
2. Shorter wavelength means more scattering.

Question 17. [2 marks] How can a change in the size of the eyeball be one of the reasons for (a) a myopic eye, (b) a hypermetropic eye?
Answer:
(a) Myopic eye: the eyeball is longer than normal, so the image formed by the eye lens lies in front of the retina.
(b) Hypermetropic eye: the eyeball is shorter than normal, so the image lies behind the retina.
In simple words: A long eyeball causes myopia and a short eyeball causes hypermetropia.

Teacher's Note:
1. Think about where the image falls.
2. Both defects can have other causes too.

Question 18. [2 marks] What type of spectacles should be worn by a person having the defects of myopia as well as hypermetropia? How does it help?
Answer:
Such a person should wear bifocal lenses.
The upper part is a concave lens to correct myopia (used for distant vision) and the lower part is a convex lens to correct hypermetropia (used for reading).
In simple words: A bifocal lens has a concave top and a convex bottom.

Teacher's Note:
1. Concave for distant, convex for near.
2. Presbyopia often needs them.

Question 19. [3 marks] Differentiate between a glass slab and a glass prism. What happens when a narrow beam of (a) monochromatic light and (b) white light passes through (i) a glass slab and (ii) a glass prism?
Answer:
Glass slab: it has parallel refracting faces; the emergent ray is parallel to the incident ray, only shifted sideways, and it does not split white light.
Glass prism: its refracting faces are inclined at an angle; the emergent ray is deviated towards the base, and it splits white light into a spectrum.
(a) Monochromatic light: through a slab it emerges parallel to the incident ray; through a prism it is deviated, but no colours are formed.
(b) White light: through a slab it emerges as white light; through a prism it splits into seven colours.
In simple words: A slab only shifts the beam; a prism turns it and spreads the colours.

Teacher's Note:
1. Compare the faces, the emergent ray and the colours.
2. State all four cases.

Question 20. [2 marks] Explain how the power of accommodation of the human eye is achieved.
Answer:
The eye lens is a crystalline lens of variable focal length. The ciliary muscles change its curvature.
For a distant object, the ciliary muscles relax, the lens becomes thinner and its focal length increases. For a nearby object, the ciliary muscles contract, the lens becomes thicker and its focal length decreases. In both cases the image is formed on the retina.
In simple words: The ciliary muscles change the lens shape to keep the image on the retina.

Teacher's Note:
1. Mention both near and distant objects.
2. Focal length changes; image distance does not.

Long Answer and Case Based Questions (4 and 5 Marks)

Question 21. [5 marks] Explain the common defects of the eye, their causes and methods of correction.
Answer:
(a) Myopia (near-sightedness): distant objects cannot be seen clearly. Causes: the eyeball becomes longer (the retina is too far from the lens) or the focal length of the eye lens becomes too small. Correction: a concave lens of suitable power.
(b) Hypermetropia (far-sightedness): nearby objects cannot be seen clearly. Causes: the eyeball becomes shorter or the focal length of the eye lens becomes too large. Correction: a convex lens of suitable power.
(c) Presbyopia: the power of accommodation decreases with age due to weakening of the ciliary muscles and loss of flexibility of the lens. Correction: bifocal lenses, with a concave upper part for distant vision and a convex lower part for reading.
In simple words: Myopia: concave lens; hypermetropia: convex lens; presbyopia: bifocal lens.

Teacher's Note:
1. Write defect, cause and correction for each.
2. Use headings to keep the answer neat.

Question 22. [4 marks] Read the passage and answer the questions.
Newton found that when a beam of white light passes through a prism it is spread out into a band of all the colours of the rainbow, from red to violet. The band of colours is called a spectrum and the separation of the colours by the prism is known as dispersion. He concluded that white light is a mixture of light of various colours. All colours of light travel at the same speed in a vacuum. When they enter a transparent substance like glass, they all slow down but by different amounts, so different colours are refracted through different angles.
(i) Where in nature can you find evidence that white sunlight may be made of different colours? (ii) When a monochromatic light passes through a prism, will it show dispersion?
Answer:
(i) In the formation of a rainbow in the sky.
(ii) No. A monochromatic light will not show dispersion, as there is only one colour; it will show only deviation.
In simple words: A rainbow is natural evidence of colours in sunlight; a single colour cannot disperse.

Teacher's Note:
1. Use the passage for the reasons.
2. Dispersion needs more than one colour.