CBSE Class 10 Science The Human Eye and the Colourful World VBQs Set 03

Question. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to
(a) presbyopia.
(b) accommodation.
(c) near-sightedness.
(d) far-sightedness.
Answer: (b) accommodation.

Question. The human eye forms the image of an object at its
(a) cornea.
(b) iris.
(c) pupil.
(d) retina.
Answer: (d) retina.

Question. The least distance of distinct vision for a young adult with normal vision is about
(a) \( 25\text{ m} \).
(b) \( 2.5\text{ cm} \).
(c) \( 25\text{ cm} \).
(d) \( 2.5\text{ m} \).
Answer: (c) \( 25\text{ cm} \).

Question. The change in focal length of an eye lens is caused by the action of the
(a) pupil.
(b) retina.
(c) ciliary muscles.
(d) iris.
Answer: (c) ciliary muscles.

Question. A person needs a lens of power \( -5.5\text{ dioptres} \) for correcting his distant vision. For correcting his near vision he needs a lens of power \( +1.5\text{ dioptre} \). What is the focal length of the lens required for correcting (i) distant vision (ii) near vision?
Answer: (i) Focal length of the lens for distant vision
\( = \frac{1}{\text{Power}} = \frac{1}{-5.5} = \frac{-100}{55}\text{ cm} \approx -18\text{ cm} \).
(ii) Focal length of the lens for near vision
\( = \frac{100}{1.5}\text{ cm} = 66.66\text{ cm} \).

Question. The far point of a myopic person is \( 80\text{ cm} \) in front of the eye. What is the nature and power of the lens required to correct the problem?
Answer: The far point of a normal eye is infinity. Since the far point of the defective eye is given as \( 80\text{ cm} \), the eye is short-sighted. To correct it, the lens should be such that an object at infinity must form its image at the far point of defective eye.
\( \therefore u = -\infty, v = -80\text{ cm} \)
Using lens formula,
\( \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \)
\( \frac{1}{f} = \frac{1}{-80} - \frac{1}{-\infty} \)
\( \frac{1}{f} = -\frac{1}{80} \)
\( \therefore \) Focal length of lens is \( -80\text{ cm} \).
The correction is done by using a concave lens of focal length \( 80\text{ cm} \).
Power of the lens \( = \frac{100}{f(\text{in cm})} = -\frac{100}{80} = -1.25\text{ D} \).

Question. Why is a normal eye not able to see clearly the objects placed closer than \( 25\text{ cm} \)?
Answer: The focal length of the eye lens cannot be reduced below a certain limit.

Question. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Or
If we increase the distance of an object from the eye, how will the distance of image formed in the eye change?
Answer: In eye, the image is always formed on the retina. The image distance is the distance between the eye lens and the retina. When we increase the distance of the object from the eye, the focal length of the eye lens increases due to the action of ciliary muscles so that the image of object is formed on the retina and therefore, the image distance remains the same.

Question. Why do stars twinkle?
Answer: The stars twinkle at night, because the intensity of starlight reaching our eyes increases and decreases continuously due to atmospheric refraction. When the starlight reaching our eyes increases, the stars look bright and when the starlight reaching our eyes decreases, they appear dim. This gives us the twinkling effect.

Question. Explain why the planets do not twinkle.
Answer: The planets are much nearer to the earth as compared to the stars so they can be treated as a collection of large number of point-sized source of light. Due to varying condition of atmosphere, the darkest part of the twinkling effect from one point source may be overlapped by the focussed light from the point source of other region of planet, so the total amount of light entering into the eye remains constant. Thus, the planets look steady and do not appear twinkle.

Question. Why does the sky appear dark instead of blue to an astronaut?
Answer: At such huge heights due to absence of atmosphere, no scattering of the light takes place. Therefore, sky appears dark.

Question. A person cannot see distinctly objects kept beyond \( 2\text{ m} \). This defect can be corrected by using a lens of power
(a) \( +0.5\text{ D} \)
(b) \( -0.5\text{ D} \)
(c) \( +0.2\text{ D} \)
(d) \( -0.2\text{ D} \)
Answer: (b) \( -0.5\text{ D} \)
Person cannot see distant objects clearly. So he is suffering from myopia. The defect is corrected by using concave lens of power
\( P = \frac{1}{f} = \frac{1}{-2\text{ m}} = -0.5\text{ D} \).

Question. Twinkling of stars is due to atmospheric
(a) dispersion of light by water droplets
(b) refraction of light by different layers of varying refractive indices
(c) scattering of light by dust particles
(d) internal reflection of light by clouds.
Answer: (b) refraction of light by different layers of varying refractive indices

Question. The clear sky appears blue because
(a) blue light gets absorbed in the atmosphere.
(b) ultraviolet radiations are absorbed in the atmosphere.
(c) violet and blue lights get scattered more than lights of all other colours by the atmosphere.
(d) light of all other colours is scattered more than the violet and blue colour lights by the atmosphere.
Answer: (c) violet and blue lights get scattered more than lights of all other colours by the atmosphere.

Question. The danger signals installed at the top of tall buildings are red in colour. These can be easily seen from a distance because among all other colours, the red light
(a) is scattered the most by smoke or fog
(b) is scattered the least by smoke or fog
(c) is absorbed the most by smoke or fog
(d) moves fastest in air
Answer: (b) is scattered the least by smoke or fog

Question. The bluish colour of water in deep sea is due to
(a) the presence of algae and other plants found in water
(b) reflection of sky in water
(c) scattering of light
(d) absorption of light by the sea
Answer: (c) scattering of light

Question. When light rays enter the eye, most of the refraction occurs at the
(a) crystalline lens
(b) outer surface of the cornea
(c) iris
(d) pupil
Answer: (b) outer surface of the cornea

Question. The focal length of the eye lens increases when eye muscles
(a) are relaxed and lens becomes thinner
(b) contract and lens becomes thicker
(c) are relaxed and lens becomes thicker
(d) contract and lens becomes thinner
Answer: (a) are relaxed and lens becomes thinner

Question. A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his text book. Which of the following statements is correct?
(a) The near point of his eyes has receded away
(b) The near point of his eyes has come closer to him
(c) The far point of his eyes has come closer to him
(d) The far point of his eyes has receded away
Answer: (a) The near point of his eyes has receded away

Question. Which of the following phenomena of light are involved in the formation of a rainbow?
(a) Reflection, refraction and dispersion
(b) Refraction, dispersion and total internal reflection
(c) Refraction, dispersion and internal reflection
(d) Dispersion, scattering and total internal reflection
Answer: (c) Refraction, dispersion and internal reflection

Question. Which of the following statements is correct regarding the propagation of light of different colours of white light in air?
(a) Red light moves fastest.
(b) Blue light moves faster than green light.
(c) All the colours of the white light move with the same speed.
(d) Yellow light moves with the mean speed as that of the red and the violet light.
Answer: (c) All the colours of the white light move with the same speed.

Question. Which of the following statement is correct?
(a) A person with myopia can see distant objects clearly.
(b) A person with hypermetropia can see nearby objects clearly.
(c) A person with myopia can see nearby objects clearly.
(d) A person with hypermetropia cannot see distant objects clearly.
Answer: (c) A person with myopia can see nearby objects clearly.

Question. How are we able to see the nearby as well as distant objects clearly?
Answer: Accommodation: The ability of the ciliary muscles to adjust the curvature and thereby the focal length of eyelens to get a clear view of the objects is called accommodation. There is always a limit up to which ciliary muscles can increase or decrease the focal length of eye lens. This change enables us to see nearer and far-off objects clearly.

Question. A person needs a lens of power \( – 4.5 \text{ D} \) for correction of his vision.
(a) What kind of defect in vision is he suffering from?
(b) What is the focal length of the corrective lens?
(c) What is the nature of the corrective lens?
Answer: (a) The defect is myopia (short-sightedness).
(b) Focal length = \( \frac{1}{\text{Power}} = -\frac{1}{4.5} = -\frac{100}{4.5} \text{ cm} = -22.2 \text{ cm} \)
(c) The lens is a concave lens.

Question. Is the position of a star as seen by us its true position? Justify your answer.
Answer: No. Light from the star passing through various layers of the atmosphere bends towards the normal due to the changing refractive indices of different layers of the atmosphere and appears as if it is coming from a higher level than their true position. As a result, the star appears slightly higher than its actual position.

Question. Red colour is used as danger sign because
(a) red colour is scattered least by smoke
(b) red colour is scattered most by smoke
(c) red colour moves fast in smoke
(d) red colour is absorbed by smoke
Answer: (a) red colour is scattered least by smoke

Question. Which of the following is controls the amount of light entering the eye?
(a) Pupil
(b) Iris
(c) Eye lens
(d) Cornea
Answer: (b) Iris

Question. A star appears slightly higher (above) than its actual position in the sky. It is due to
(a) atmospheric reflection
(b) atmospheric refraction
(c) atmospheric scattering
(d) none of the options
Answer: (b) atmospheric refraction

Question. A normal eye is not able to see objects closer the 25 cm because
(a) distance between eye lens and retina is 25 cm.
(b) eye lens has a fixed focal length of 25 cm.
(c) beyond a certain limit eye cannot decrease its focal length
(d) both (a) and (b)
Answer: (c) beyond a certain limit eye cannot decrease its focal length

Question. Assertion: When a white light beam passes through the glass prism, a spectrum is obtained.
Reason: The different colours have the same speed while passing through the glass prism.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (c) Assertion is true but the Reason is false.

Question. Assertion: A person suffering from hypermetropia cannot see nearby objects distinctly.
Reason: A converging lens is used for the correction of hypermetropic eye as it can form real as well as virtual images of the objects placed in front of it.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. It is observed that the power of an eye to see nearby objects as well as far off objects diminishes with age.
(i) Give reason for the above statement.
(ii) Name the defect that is likely to arise in the eyes in such a condition.
(iii) Draw a labelled diagram to show the type of corrective lens used for restoring the vision of such an eye.
Answer: (i) The power of accommodation of the eye usually decreases with aging. This is due to the gradual weakening of the ciliary muscles and diminishing flexibility of the crystalline lens. (ii) This defect is called Presbyopia. (iii) Bifocal lenses, containing both concave and convex lenses, are often used to correct this condition.

Question. A star at sometimes appears bright and at sometimes fainter. What is this effect called? State the reason for this effect.
Answer: This effect is called the twinkling of stars. Reason: It is due to the atmospheric refraction of starlight. Since the physical conditions of the earth's atmosphere are not stationary, the path of rays of light coming from the star goes on varying slightly, and the apparent position of the star fluctuates. Also, the amount of starlight entering the eye fluctuates, making the star sometimes appear bright and sometimes faint.

Question. When a ray of light incident on a solid glass prism, it will split and the band of colour obtained on the screen is VIBGYOR. During when light ray passes through the prism, it suffers refraction twice and hence the ray deviates through a certain angle from its original path. This angle is called angle of deviation. Now answer the following questions:
(a) Name the phenomenons that are observed when a white light falls on a prism.
(b) For which colour of spectrum, the angle of deviation is minimum?
(c) What is the cause of obtaining VIBGYOR on the screen? Explain it. List an example of that cause in nature.
Or
(c) How did Newton, using two identical glass prisms, showed that white light is made of seven colours?
Answer: (a) Refraction and Dispersion. (b) Red colour. (c) Cause: Dispersion. White light is made of seven colours. In glass, different colours travel with different speeds, so they bend by different amounts. An example in nature is a rainbow. (Or) Newton used an inverted second prism behind the first one. The first prism split the light into seven colours, and the second inverted prism recombined them to produce white light again, proving that white light is composed of seven colours.