CBSE Class 10 Science HOTs Human Eye and Colourful World Set 05

A & R Questions

In the following questions a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.

Question. Assertion : Rainbow is an example of the dispersion of sunlight by the water droplets.
Reason : Light of shorter wavelength is scattered much more than light of larger wavelength.
Answer: (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.

Question. Assertion : A beam of white light gives a spectrum on passing through a hollow prism.
Reason : Speed of light outside the prism is different as the speed of light inside the prism.
Answer: (d) Assertion is wrong statement but reason is correct statement.

Question. Assertion : Myopia is due to the increased converging power of the eye lens.
Reason : Myopia can be corrected by using spectacles made from concave lenses.
Answer: (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.

Multiple Choice Questions

Question. What is the nature of image formed by eye lens?
(a) Real, upright and enlarged
(b) Real, upright and diminished
(c) Real, inverted and diminished
(d) Virtual, inverted and diminished
Answer: (c) Real, inverted and diminished

Question. The intensity of light entering our eye is controlled by
(a) pupil
(b) iris
(c) ciliary muscles
(d) cornea.
Answer: (a) pupil

Question. A far sighted man who has lost his spectacles, speculated the radius of a plate by looking through a small hole in a sheet of paper. The reason is
(a) in doing so, equivalent focal length remains the same as with the spectacles
(b) in doing so, the focal length of the eye lens is effectively decreased
(c) the hole produces an image of the plate at a longer distance
(d) none of the options.
Answer: (b) in doing so, the focal length of the eye lens is effectively decreased

Question. When a ray of light emerges out of a glass prism
(a) \( i + \delta = A + e \)
(b) \( i + A = \delta + e \)
(c) \( i + e = A + \delta \)
(d) none of the options.
Answer: (c) \( i + e = A + \delta \)

Question. Which of the following is not the cause of atmospheric refraction?
(a) Apparent image of sun is formed closer to the earth.
(b) Dawn or dusk are formed.
(c) Sun can be seen 2 minutes before actual sunrise and 2 minutes after actual sunset
(d) Clouds look white.
Answer: (d) Clouds look white.

VSA Type Questions

Question. When is the thickness of the eye lens minimum?
Answer: The thickness of the eye lens is minimum when the ciliary muscles are fully relaxed, and the eye is focused on a distant object (at infinity).

Question. What type of cells are lacking in a person who is colour blind?
Answer: A person who is colour blind lacks cone-shaped light-sensitive cells in the retina.

Question. What is the order of wavelength of visible light?
Answer: The wavelength of visible light is in the order of \( 10^{-7} \) m (ranging approximately from 400 nm to 700 nm).

Question. What is the angle of deviation of a prism ?
Answer: The angle between the incident ray and the emergent ray is known as the angle of deviation of a prism.

SA I Type Questions

Question. Why do chickens can see only in bright light?
Answer: The retina of chickens contains mostly cone-shaped cells and very few rod-shaped cells. Since rod cells are responsible for vision in dim light, chickens can see only in bright light.

Question. (a) Which type of retinal cells respond to brightness of light? (b) Why does it take some time to see objects in a cinema hall, when we just enter the hall?
Answer: (a) Rod-shaped cells respond to the intensity or brightness of light. (b) When entering a dark cinema hall from bright sunlight, it takes time for the pupil to expand and for the rod cells to become active to allow more light into the eye for vision in dim light.

Question. A person with a myopic eye cannot see objects beyond 1.6 m distinctly. What should be the nature of the corrective lens used to restore proper vision?
Answer: A concave (diverging) lens of suitable power should be used. Using the lens formula with \( u = \infty \) and \( v = -1.6 \) m:
\( 1/f = 1/v - 1/u = 1/(-1.6) - 1/(\infty) = -1/1.6 \)

\( \implies \) \( f = -1.6 \) m.
Power \( P = 1/f = 1/(-1.6) = -0.625 \) D.

Question. Give reason : We cannot see an object clearly if it is placed very close to the eyes.
Answer: Ciliary muscles can decrease the focal length of the eye lens only up to a certain limit. If an object is placed closer than 25 cm (the least distance of distinct vision), the lens cannot become convex enough to focus the rays on the retina.

Question. The extent of deviation of a ray of light on passing through a prism depends on the colour.
Answer: Different colours of light have different speeds in the glass of the prism. Violet light travels the slowest and deviates the most, while red light travels the fastest and deviates the least.

SA II Type Questions

Question. A person needs a lens of power – 4.5 D for correction of her vision.
(a) What kind of defect in vision is she suffering from ?
(b) What is the focal length of the corrective lens?
(c) What is the nature of the corrective lens?
Answer: (a) The person is suffering from Myopia (short-sightedness).
(b) \( f = 1/P = 1/(-4.5) \text{ m} = -0.222 \text{ m} = -22.2 \text{ cm} \).
(c) The corrective lens is a concave lens.

Question. Why does the sun appear oval at sunrise and sunset, and circular at noon?
Answer: At sunrise and sunset, the sun is near the horizon. Rays from the top and bottom of the sun traverse different thicknesses of the atmosphere, leading to different amounts of refraction, which flattens the disc. At noon, the sun is overhead and the rays pass through the atmosphere normally, resulting in negligible refraction and a circular appearance.

Question. What is meant by range of vision? What is its value for a person with normal vision?
Answer: The distance between the near point and the far point of an eye is called the range of vision. For a normal eye, it is from 25 cm to infinity.

LA Type Questions

Question. Explain the structure and functioning of the human eye. How are we able to see nearby as well as distant objects?
Answer: The human eye works like a camera. Light enters through the cornea, passes through the pupil (size controlled by iris), and is focused by the crystalline lens onto the retina. The retina converts light into electrical signals sent to the brain via the optic nerve. We see nearby and distant objects through 'accommodation': ciliary muscles change the lens curvature—making it thin for distant objects and thick for nearby ones—to focus images precisely on the retina.

Question. List three common refractive defects of vision. Suggest the way of correcting these defects.
Answer: 1. Myopia: Corrected by a concave lens.
2. Hypermetropia: Corrected by a convex lens.
3. Presbyopia: Corrected by bifocal lenses.

Question. Demonstrate an activity with a well labelled diagram to prove that white light is made up of seven colours.
Answer: Newton's prism experiment: Allow a narrow beam of sunlight to pass through a glass prism. The light splits into a band of seven colours (VIBGYOR) on a screen. Placing a second identical inverted prism in the path recombines the colours into white light, proving white light consists of seven colours.

Question. State the relation between colour of scattered light and size of the scattering particle.
Answer: Very fine particles scatter mainly blue light (shorter wavelengths). Large particles scatter light of longer wavelengths. If the particles are large enough, the scattered light may even appear white.

Question. An astronaut in space finds sky to be dark. Explain reason for this observation.
Answer: In outer space, there is no atmosphere. Without air molecules or dust particles, there is no scattering of sunlight. Therefore, the sky appears dark to astronauts.