CBSE Class 10 Science HOTs Human Eye and Colourful World Set 03

Question. A person can see clearly objects only when the object is lies between 40 cm and 350 cm. In order to increase the distances of distinct vision to infinity, find the power of corrective lens and type of lens used by the person to see the objects clearly.
Answer: Here, \( v = -350 \text{ cm} = -3.5 \text{ m} \).
\( u = -\infty \), \( f = ? \)
Using lens formula,
\( \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \)
\( = \frac{1}{-3.5} - \frac{1}{-\infty} = \frac{1}{-3.5} \)

\( \implies f = -3.5 \text{ m} \)
Negative sign shows that corrective lens is concave lens and its power is
\( P = \frac{1}{f(\text{m})} = \frac{1}{-3.5} = -0.28 \text{ D} \)

Question. The far point of myopic person is 150 cm in front of the eye. Calculate the focal length and power of the lens required to enable him to see distant objects clearly. 
Answer: For myopic person, \( u = \infty \), \( v = -150 \text{ cm} \), \( f = ? \)
From lens formula, \( \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \)
\( = \frac{1}{-150} - \frac{1}{\infty} = \frac{1}{-150} \)

\( \implies f = -150 \text{ cm} = -1.5 \text{ m} \)
Power of corrective lens, \( P = \frac{1}{f(\text{m})} = \frac{1}{-1.5} = -\frac{2}{3} \)

\( \implies P = -0.66 \text{ D} \)
So, concave lens of power, 0.66 D enable him to see distant objects clearly.

Long Answer Type Questions 

Question. What is presbyopia? State its cause. How is it corrected?
Answer: (i) Presbyopia: Presbyopia is a condition that occurs as a part of normal ageing. Due to loss of power of accommodation of the eye, with age, objects at a normal near working distance will appear blurry. The near point gradually recedes away. This defect of eye is called Presbyopia. Sometimes, a person may suffer from both myopia and hypermetropia.
(ii) Presbyopia is caused due to: • weakening of ciliary muscles, and • eye lens becomes less flexible and elastic, i.e. reducing ability of eye lens to change its curvature with the help of ciliary muscles.
(iii) Bifocal lens will be required to see clearly nearby as well as the distant object. For myopic defect, upper part of bifocal lens consists of a concave lens used for distant vision and to correct hypermetropia, lower part of bifocal lens consists of a convex lens. It facilitates near vision.

PRACTICE QUESTIONS

Question. The defect of vision in which eyeball is elongated is
(a) myopia
(b) hypermetropia
(c) presbyopia
(d) cataract
Answer: (a) myopia

Question. Far-sightedness is caused by large focal length of the eye lens. It is corrected by using
(a) divergent lens
(b) converging lens
(c) bifocal lens
(d) plano-concave lens
Answer: (b) converging lens

Question. When a person is myopic, he/she can clearly see
(a) distant object clearly
(b) nearby objects clearly
(c) both near and distant objects
(d) between infinity and 25 cm
Answer: (b) nearby objects clearly

Question. Name the defect of vision caused by
(i) Excessive curvature of cornea
(ii) Power of accommodation of eye decreases.
Answer: (i) Myopia (ii) Presbyopia.

Question. When do we consider a student sitting in the class to be (a) myopic (b) hypermetropic?
Answer: (a) When the student cannot see the writing on the blackboard from the back bench. (b) When the student cannot read his textbook clearly when placed at a normal distance.

Question. Why is concave lens used to correct myopia? Why not convex lens?
Answer: A concave lens is used because it diverges parallel rays from a distant object so they appear to come from the far point of the myopic eye, allowing the eye lens to focus them on the retina. A convex lens would further increase converging power, making the rays focus even further in front of the retina.

Question. The size of image of an object by a mirror having a focal length of 20 cm is observed to be reduced to 1/3rd of its size. At what distance the object has been placed from the mirror? What is the nature of the image and the mirror?
Answer: If the image is real (as in concave mirror), \( m = -1/3 \). Therefore, \( -v/u = -1/3 \).
Using mirror formula, \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \), we get
\( \frac{1}{-20} = \frac{1}{u/3} + \frac{1}{u} = \frac{3}{u} + \frac{1}{u} = \frac{4}{u} \) \( \implies u = -80 \text{ cm} \).

If the image is virtual (as in convex mirror), \( m = 1/3 \). Therefore, \( -v/u = 1/3 \).
Using mirror formula, \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \), we get
\( \frac{1}{20} = \frac{1}{-u/3} + \frac{1}{u} = \frac{1}{u}(1 - 3) = \frac{-2}{u} \) \( \implies u = -40 \text{ cm} \).

TOPIC : Refraction and Dispersion of White Light through a Prism

Question. A student traces the path of ray of light through a triangular glass prism for different values of angle of incidence. On analysing the ray diagrams, which one of the following conclusions is he likely to draw? 
(a) The emergent ray is parallel to incident ray.
(b) The emergent ray bends at an angle to the direction of incident ray.
(c) The emergent ray and refracted ray are at right angles to each other.
(d) The emergent ray is perpendicular to the incident ray.
Answer: (b) The emergent ray bends at an angle to the direction of incident ray.

Question. A prism is used to
(a) change the path of light by reflection and refraction both
(b) rotate the image
(c) disperse the light into its components
(d) All of the options
Answer: (c) disperse the light into its components

Question. When white light enters a prism, it gets split into its constituent colours. This is due to
(a) different refractive index for different wavelength of each colour
(b) each colours has same velocity in the prism.
(c) prism material have high density.
(d) scattering of light.
Answer: (a) different refractive index for different wavelength of each colour

Question. If a beam of red light and a beam of violet light are incident at the same angle on the inclined surface of a prism from air medium and produce angles of refraction ‘r’ and ‘v’ respectively, which of the following is correct? 
(a) r = v
(b) r > v
(c) r = \( \frac{1}{v} \)
(d) r < v
Answer: (d) r < v

Question. The phenomenon of dispersion prove that
(a) white light passes through a transparent medium.
(b) sunlight is made up of seven colours.
(c) light travels in a straight line path.
(d) different colours travel with the same speed while passing through a glass prism.
Answer: (b) sunlight is made up of seven colours.

Question. When a narrow beam of white light is passed through two identical prisms, one placed inverted with respect to the other, the emerging beam will be
(a) white beam
(b) blue light
(c) red light
(d) all seven colours
Answer: (a) white beam

Question. When a ray of light passes through a glass prism it suffers two refractions. During these refractions the ray bends: 
(a) Away from the base in both cases
(b) Towards the base in both cases
(c) Towards the base in first case and away from the base in second case
(d) Away from the base in first case and towards the base in second case
Answer: (b) Towards the base in both cases

Question. In the formation of rainbow, the role of water droplet presents in water fountains is to act as a 
(a) glass slab
(b) convex lens
(c) concave lens
(d) prism
Answer: (d) prism

Question. When sunlight enters the water droplet suspended in the atmosphere after rainfall, it will get
(a) refracted only
(b) refracted internally
(c) refracted and dispersed
(d) first refracted and then dispersed while coming out of the water droplet.
Answer: (c) refracted and dispersed

Very Short Answer Type Questions

Question. No rainbow could be observed from the surface of the moon by the astronauts. What could be the possible reason?
Answer: The Moon does not have any blanket of air or atmosphere and consequently, in the absence of water droplets no dispersion of sunrays is possible. Therefore, rainbow could not be observed from the surface of the Moon by the astronauts.

Question. What is dispersion of light? Name component of white light that deviate (i) the least (ii) the most while passing through a glass prism.
Answer: Dispersion: Splitting of white light into its component colours is called dispersion. The component of white light that deviate
(i) the most – violet
(ii) the least – red
while passing through the glass prism.

PRACTICE QUESTIONS

Question. The property of light used by the prism to form a spectrum is
(a) refraction of light
(b) reflection of light
(c) dispersion of light
(d) diffused reflection of light.
Answer: (c) dispersion of light

Atmospheric Refraction and Scattering of Light

Question. The air layer of atmosphere whose temperature is less then the hot layer behave as optically
(a) denser medium
(b) rarer medium
(c) inactive medium
(d) either denser or rarer medium
Answer: (a) denser medium

Question. Refraction of light by the earth’s atmosphere due to variation in air density is called
(a) atmospheric reflection
(b) atmospheric dispersion
(c) atmospheric scattering
(d) atmospheric refraction
Answer: (d) atmospheric refraction

Question. The deflection of light by minute particles and molecules of the atmosphere in all direction is called ___________ of light.
(a) dispersion
(b) scattering
(c) interference
(d) tyndell effect
Answer: (b) scattering

Question. One cannot see through the fog, because
(a) refractive index of the fog is very high
(b) light suffers total reflection at droplets
(c) fog absorbs light
(d) light is scattered by the droplets
Answer: (d) light is scattered by the droplets

Question. To an astronaut the sky on the moon appear dark because 
(a) there is no light on the moon.
(b) there is no atmosphere on the surface of the moon.
(c) moon is non-luminous object.
(d) the surface of the moon absorbs all the sunlights.
Answer: (b) there is no atmosphere on the surface of the moon.

Question. Rohan lit an incense stick in his room and after an hour observed that when a beam of sunlight entered his room through a small gap in the window, he was able to see the path of the beam. Which of the following is most likely TRUE about the air present in the room? 
(a) It is a pure substance.
(b) It is a compound.
(c) It is a solution.
(d) It is a colloid.
Answer: (d) It is a colloid.

Question. Which one of the following is the correct reason for twinkling of stars? 
(a) Atmospheric reflection of starlight
(b) Atmospheric refraction of starlight
(c) Scattering of starlight
(d) Dispersion of starlight
Answer: (b) Atmospheric refraction of starlight

Question. Blue colour of clear sky is due to
(a) refraction of light
(b) absorption of light
(c) reflection of light
(d) scattering of light
Answer: (d) scattering of light

Very Short Answer Type Questions

Question. Space is mostly vacuum, devoid of any medium. (a) What colour does the sun appear to the astronauts on International Space Station? (b) Give reason for your answer to (a). 
Answer: (a) White (b) Since there is no medium to disperse or scatter the light coming from the Sun, it appears white.

Question. What is meant by scattering of light? State its two effects that are produced by the atmosphere.
Answer: Scattering of Light: The phenomenon in which light rays are directed in different direction when it interacts with the large number of molecules, such as smoke, tiny water droplets, suspended particles of dust and molecules of air present in the earth’s atmosphere, is called scattering of light. The effects are (i) Tyndall effect (ii) Blue colour of sky

Question. The colour of clear sky from the earth appears blue but from the space it appears black. Why?
Answer: When sunlight passes through the earth’s atmosphere, it is scattered in all directions by the gaseous and other fine particles present in the atmosphere. The blue colour has a shorter wavelength than the red. So, according to Rayleigh scattering law, the blue colour of sunlight is scattered more strongly by the large number of fine particles having size smaller than the wavelength of visible light in the earth’s atmosphere. The scattered blue light enters our eyes, hence the sky appears blue. From space sky appears black because at such huge heights due to absence of atmosphere no scattering of light takes place.

Short Answer Type Questions

Question. Explain giving reason why the sky appears blue to an observer from the surface of the earth. What should the appearance of the sky be during the day for an astronaut staying in the international space station orbiting the Earth? State reason to justify your answer. Or Explain giving reason why the sky appears blue to an observer from the surface of the earth? What will the colour of the sky be for an astronaut staying in the international space station orbiting the earth? Justify your answer giving reason. 
Answer: When sunlight passes through the earth’s atmosphere, it is scattered in all directions by the gaseous and other fine particles present in the atmosphere. The blue colour has a shorter wavelength than the red. So, according to Rayleigh scattering law, the blue colour of sunlight is scattered more strongly by the large number of fine particles having size smaller than the wavelength of visible light in the earth’s atmosphere. The scattered blue light enters our eyes, hence the sky appears blue. For an astronaut, staying in the international space station orbiting the earth, the colour of the sky will be black, i.e. sky will appear dark. Reason: Sunlight does not scatter in the absence of atmosphere.

Question. Why is Tyndall effect shown by colloidal particles? State four instances of observing the Tyndall effect. 
Answer: Tyndall effect: Scattering of beam of visible light by the colloidal particles present in the colloidal solution is called Tyndall effect. It occurs when the dimension of the dispersed particle that are causing the scattering are larger than the wavelength of incident light i.e \( 10^{-7} \) m. Accordingly, the colour of the scattered light depends on the size of the scattering particles in colloidal solution. Four instances of observing the Tyndall effect are (a) Headlight of a car shining through fog. (b) When a fine beam of sunlight enters a smoke filled room through a small hole. (c) When sunlight passes through a canopy of a dense forest. (d) Blue colour of sky.

Question. Which of the following statements is not true for scattering of light?
(a) Colour of the scattered light depends on the size of particles of the atmosphere.
(b) Red light is least scattered in the atmosphere.
(c) Scattering of light takes place as various colours of white light travel with different speed in air.
(d) The fine particles in the atmospheric air scatter the blue light more strongly than red. So the scattered blue light enters our eyes.
Answer: (c) Scattering of light takes place as various colours of white light travel with different speed in air.

Question. Tyndall effect is colloidal solution is due to
(a) reflection of light
(b) absorption of light
(c) scattering of light
(d) refraction of light
Answer: (c) scattering of light

Question. At noon, the sun appears white as
(a) light is least scattered.
(b) all the colours of the white light are scattered away.
(c) blue colour is scattered the most.
(d) red colour is scattered the most.
Answer: (a) light is least scattered.

Question. The apparent flattening of the sun’s disc at sunrise and sunset is due to:
(a) Dispersion of light
(b) Scattering of light
(c) Atmospheric refraction of light
(d) Tyndall effect
Answer: (c) Atmospheric refraction of light