CBSE Class 10 Science The Human Eye and the Colourful World Assignment Set 08

Assignment Set 04 for CBSE Class 10 Science Chapter 10, The Human Eye and the Colourful World, brings 21 more marks-wise practice questions with solutions from board-style question banks. It tests the parts of the eye and their functions, near and far points, myopia, hypermetropia, presbyopia and cataract, the prism and the colours of light, atmospheric refraction and scattering, and each solution is worked out clearly.

Very Short Answer Questions (1 Mark)

Question 1. [1 mark] A person is advised to wear spectacles with concave lenses. What type of defect of vision is he suffering from?
Answer:
Myopia (short-sightedness).
In simple words: Concave lenses are prescribed for short-sightedness.

Teacher's Note:
1. Concave lens: myopia.
2. Convex lens: hypermetropia.

Question 2. [1 mark] A person suffering from an eye defect uses lenses of power +1 D. Name the defect he is suffering from and the nature of the lens used.
Answer:
The power is positive, so the lens is convex. The defect is hypermetropia (long-sightedness).
In simple words: Positive power means a convex lens for long-sightedness.

Teacher's Note:
1. The sign of the power gives the type of lens.
2. A convex lens also corrects presbyopia.

Question 3. [1 mark] How is the sense of vision carried from the eye to the brain?
Answer:
Through the optic nerve, which carries the electrical signals from the retina to the brain.
In simple words: The optic nerve connects the eye to the brain.

Teacher's Note:
1. The retina produces the signals.
2. The brain interprets them.

Question 4. [1 mark] Name the phenomenon on which the working of a prism depends.
Answer:
Refraction of light.
In simple words: A prism bends light by refraction.

Teacher's Note:
1. The light bends at both faces.
2. Dispersion is a result of refraction.

Question 5. [1 mark] In which direction does a ray of light bend while emerging out of a prism?
Answer:
The ray bends towards the base of the prism.
In simple words: A prism always turns the ray towards its base.

Teacher's Note:
1. The apex is the thin end.
2. The angle of deviation measures the turn.

Question 6. [1 mark] Name the factor on which the colour of light depends.
Answer:
The colour of light depends on its wavelength (equivalently its frequency).
In simple words: Each colour has its own wavelength.

Teacher's Note:
1. Red has the longest wavelength.
2. Violet has the shortest.

Question 7. [1 mark] Give an example in nature which shows that sunlight may be made up of different colours.
Answer:
The formation of a rainbow.
In simple words: A rainbow splits sunlight into colours.

Teacher's Note:
1. Water drops act as tiny prisms.
2. Dispersion is the key idea.

Question 8. [1 mark] State one function of the iris in the human eye.
Answer:
The iris is a dark muscular diaphragm that controls the size of the pupil (and thus the amount of light entering the eye).
In simple words: The iris sets the size of the pupil.

Teacher's Note:
1. Do not call the iris the opening.
2. Pupil is the opening.

Question 9. [1 mark] State one function of the crystalline lens in the human eye.
Answer:
The crystalline lens provides the finer adjustment of the focal length required to focus objects at different distances on the retina.
In simple words: The lens fine-tunes the focus on the retina.

Teacher's Note:
1. The cornea does most of the bending.
2. Lens changes the focal length.

Short Answer Questions (2 and 3 Marks)

Question 10. [3 marks] (i) What is meant by the power of accommodation of the eye? (ii) What do you mean by the far point of the human eye? (iii) What do you mean by the near point of the human eye?
Answer:
(i) The ability of the eye to focus on distant objects as well as on nearby objects on the retina by changing the focal length of its lens is the power of accommodation.
(ii) The most distant point at which an object can be seen clearly by the eye is the far point (infinity for a normal eye).
(iii) The closest point at which an object can be seen clearly by the eye is the near point (about 25 cm for a normal eye).
In simple words: The eye focuses by changing its lens; near and far points are the limits.

Teacher's Note:
1. Give the normal values of both points.
2. Accommodation is done by the ciliary muscles.

Question 11. [3 marks] (i) What is hypermetropia? (ii) How can hypermetropia be corrected? (iii) How is myopia corrected?
Answer:
(i) Hypermetropia (long-sightedness) is the defect in which a person can see objects at long distances clearly but cannot see nearby objects distinctly.
(ii) It is corrected by spectacles with a convex lens of suitable power or focal length.
(iii) Myopia is corrected by spectacles with a concave lens of suitable power or focal length.
In simple words: Convex lens for hypermetropia and concave lens for myopia.

Teacher's Note:
1. Define first, then correct.
2. The lens power depends on the near or far point.

Teacher's Note:
1. Write at least three points.
2. The image is in front of the retina in myopia and behind it in hypermetropia.

Teacher's Note:
1. Keep the same three points for each defect.
2. Cataract is treated by replacing the lens.

Teacher's Note:
1. Give an example of each.
2. Rainbow and blue sky are the standard examples.

Question 15. [3 marks] Write the structure of the eye lens and state the role of the ciliary muscles in the human eye.
Answer:
The eye lens is a convex lens, thick in the middle and thin at the edges, made of a transparent jelly-like material. It converges the incident light and forms the image on the retina.
Role of the ciliary muscles: they hold the lens and change its shape (curvature) so that its focal length changes, helping the eye focus on near and distant objects.
In simple words: The eye lens is a convex lens, whose shape the ciliary muscles change.

Teacher's Note:
1. Mention thick in the middle, thin at the edges.
2. Relaxed muscles: thin lens, long focal length.

Question 16. [3 marks] What is atmospheric refraction? List two phenomena which can be explained on the basis of atmospheric refraction.
Answer:
The atmosphere has layers of different densities and refractive indices. When light passes through these layers it is refracted; this is atmospheric refraction.
Two phenomena: (1) twinkling of stars; (2) advance sunrise and delayed sunset (the Sun is seen about 2 minutes before it actually rises and after it actually sets).
In simple words: The bending of light in the layers of air is atmospheric refraction.

Teacher's Note:
1. Other examples: the Sun looks flattened near the horizon.
2. Do not confuse with scattering.

Question 17. [2 marks] The danger signal is red. Why?
Answer:
The wavelength of red light is the longest, so it is scattered the least by the air molecules of the atmosphere (and by fog and smoke).
It can therefore travel a long distance without becoming weak and can be seen from far away.
In simple words: Red is scattered least, so it is visible from afar.

Teacher's Note:
1. Link wavelength to scattering.
2. Use fog or smoke as the example.

Question 18. [3 marks] What is scattering of light? Explain how the colour of the scattered light depends on the size of the scattering particles.
Answer:
Scattering of light is the phenomenon in which part of the incident light is sent in different directions by the particles of the medium.
Dependence on size: (1) When the particles (dust, water droplets) are large, they scatter all colours and the scattered light appears white. (2) When the particles are extremely small (like air molecules), they scatter mainly the blue light of sunlight, so the scattered light appears blue.
In simple words: Large particles give white scattered light; tiny particles give blue.

Teacher's Note:
1. Cloud is white, sky is blue.
2. Mention the size compared to the wavelength.

Question 19. [3 marks] Due to gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens, a certain defect of vision arises. Write the name of this defect. Name the type of lens required by such persons to improve the vision. Explain the structure and function of such a lens.
Answer:
The defect is presbyopia.
The lens required is a bifocal lens.
A bifocal lens consists of both a concave and a convex lens. The upper part is concave, to correct myopia (distant vision), and the lower part is convex, to correct hypermetropia (near vision or reading).
In simple words: Bifocal lenses have a concave upper part for far and a convex lower part for near.

Teacher's Note:
1. Name the defect, the lens and the structure.
2. Presbyopia often comes with myopia.

Question 20. [3 marks] Parthiv, with a normal near point (25 cm), reads a book with small print using a magnifying glass, a thin convex lens of focal length 5 cm. What are the closest and farthest distances at which he can read the book viewing through the magnifying glass?
Answer:
f = +5 cm.
Closest distance: the image is at the near point, v = −25 cm. 1/u = 1/v − 1/f = −1/25 − 1/5 = −6/25, so u = −25/6 = −4.17 cm. The book is at 4.17 cm from the lens.
Farthest distance: the image is at infinity (relaxed eye), v = ∞, so 1/u = −1/f and u = −5 cm. The book is at 5 cm from the lens.
He can read the book when it is between 4.17 cm and 5 cm from the lens.
In simple words: The book must be placed between about 4.2 cm and 5 cm from the lens.

Teacher's Note:
1. The image of a magnifier is virtual and on the same side as the object.
2. Use the sign convention carefully.

Long Answer Question (5 Marks)

Question 21. [5 marks] With the help of the ciliary muscles the human eye can change its curvature and thus alter the focal length of its lens. State the changes that occur in the curvature and focal length of the eye lens while viewing (i) a distant object, (ii) a nearby object. Explain why a normal eye is not able to see distinctly the objects placed closer than 25 cm without putting any strain on the eye.
Answer:
(i) Distant object: the ciliary muscles relax, the curvature of the lens decreases and the lens becomes thin, so the focal length increases. The distant object is focused on the retina.
(ii) Nearby object: the ciliary muscles contract, the curvature increases and the lens becomes thick, so the focal length decreases. The nearby object is focused on the retina.
Closer than 25 cm: the focal length of the eye lens cannot be reduced below a certain minimum. For an object closer than 25 cm the image cannot be formed on the retina, and the ciliary muscles are strained.
In simple words: Thin lens for far, thick lens for near, and the limit of thickening is the 25 cm near point.

Teacher's Note:
1. Give curvature and focal length for both cases.
2. Explain the 25 cm limit in terms of the minimum focal length.