CBSE Class 10 Science Human Eye and Colourful World VBQs

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

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

Question. Even in absolutely clear water, a diver cannot see very clearly because
(a) rays of lights get diffused
(b) velocity of light is reduced in water
(c) ray of light passing through the water makes it turbid.
(d) the focal length of the eye lens in water gets changed and the image is no longer focussed sharply on the retina.
Answer : D

Question. Which amongst the given radiation is preferred for taking photographs in fog?
(a) Ordinary visible light
(b) Infrared
(c) Microwave
(d) X-rays
Answer : B

Question. On entering a glass prism, sun rays are
(a) Deviated but not dispersed
(b) Deviated and dispersed
(c) Dispersed but not deviated
(d) Neither deviated nor dispersed.
Answer : B

Question. When ciliary muscales are relaxed, focal length of eye lens is
(a) maximum
(b) minimum
(c) Neither maximum nor minimum
(d) Cannot say
Answer : A

Question. A person uses a lens of power +3 D to normalise vision. Near point of hyper-me-tropic eye is
(a) 1.66 m
(b) 0.66 m
(c) 0.33 m
(d) 1m
Answer : D

Question. A near sighted person wears eye glass of power 5.5 D for distant vision. His doctor prescribes a correction of +1D in near vision part of his bi-focals, which is measured relative to the main part of the lens. Then, the focal length of his near vision part of the lens is
(a) -18.18 cm
(b) -20 cm
(c) -22.22 cm
(d) +20.22 cm
Answer : C

Question. A thin prism P₁ with angle 4c and made from glass of refractive index 1.54 is combined with another prism P₂ made from glass of refractive index 1.92 to produce dispersion without deviation. Then, the angle of the
prism P₂ is
(a) 2.3c
(b) 4.3c
(c) 3.2c
(d) 2.0c
Answer : A

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

Question. At the moment dew formation starts on a cool night, the air
(a) Must loose all water vapour
(b) Must remain unsaturated
(c) Must get mixed up with some ot
(d) Must become saturated
Answer : D

Question. In the visible spectrum the colour having the shortest wavelength is
(a) Green
(b) Red
(c) Violet
(d) Blue
Answer : C

Question. Rainbow is caused due to
(a) Reflection of sun light air
(b) Dispersion of sun light from water drops
(c) Refraction of sun light from water drops
(d) Diffraction of sun rays from water drops
Answer : B

Question. A piece of cloth looks red in sun light. It is held in the blue portion of a solar spectrum it will appear
(a) red
(b) black
(c) blue
(d) white
Answer : B

Question. A near sighted person cannot see distinctly beyond 50 cm from his eye. The power in diopter of spectacle lenses which will enable him to see distant objects clearly is
(a) +50
(b) -50
(c) +2
(d) -2
Answer : D

Question. The size of the pupil of the eye is adjusted by
(a) cornea
(b) retina
(c) iris
(d) blind spot
Answer : C

Fill in the Blank

Question. Phenomenon of splitting of white light into its constituent colours is ..........
Answer : dispersion

Question. The screen on which the image is formed by the lens system of the human eye is called ..........
Answer : Retina

Question. The closest distance at which the eye can focus clearly is called the ..........
Answer : Near point

Question. For young adult with normal vision, least distance of distinct vision = ..........
Answer : 25 cm.

Question. .......... is a dark muscular diaphragm that controls the size of the pupil
Answer : Iris

Question. ........... is the ability of the eye to adjust its focal length.
Answer : Accommodation of eye

Question. The coloured diaphragm between the cornea and the lens is ..........
Answer : Iris

Question. For a normal eye, the range of vision is from ..........
Answer : 25 cm to infinity

Question. Sunlight comprises .......... colours.
Answer : 7

Question. Light enters the eye through a thin membrane called as ...........
Answer : cornea

Question. .......... regulates and controls the amount of light entering the eye.
Answer : Pupil

True or False

Question. the solar spectrum in general is an absorption spectrum.
Answer : True

Question. Lens which is used for correcting the presbyopia defect of the eye is concave.
Answer : False

Question. In Myopia the image of distant objects is focussed before the retina.
Answer : True

Question. The colour that deviates maximum while passing through a glass prism is violet.
Answer : True

Question. The sun looks red at sunset because most of the blue light in sunrays is scattered leaving behind red yellow lights.
Answer : True

Question. A person suffering from myopia cannot see distant objects clearly.
Answer : True

Question. Hypermetropia is corrected by using a convex lens of suitable power.
Answer : True

Question. A dentist uses a convex mirror to view the inner parts of a patient’s mouth.
Answer : False

Question. The eye which can see near object clearly is said to suffer from hypermetropia.
Answer : False

Assertion and Reason

DIRECTION : In the following questions, a statement of assertion (A) is followed by a statement of reason (R). Mark the correct choice as:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.
(e) Both Assertion and Reason are false.

Question. Assertion : Blue colour of sky appears due to scattering of blue colour.
Reason : Blue colour has shortest wave length in visible spectrum.
Answer : A

Question. Assertion : A rainbow is sometimes seen in the sky in rainy season only when observer’s back is towards the sun.
Reason : Internal reflection in the water droplets
cause dispersion and the final rays are in backward direction.
Answer : A

Question. Assertion : Light from a distant object arriving at the eye lens may get converged at a point in front of the retina.
Reason : The eye is producing too much divergence in the incident beam.
Answer : C

Question. Assertion : The light of violet colour deviates the most and the light of red colour the least, while passing through a prism.
Reason : For a prism material, refractive index is highest for red light and lowest for the violet light.
Answer : B

Question. Assertion : When we see an object, the image formed On the retina is real and inverted.
Reason : If the magnification of a system is less than one, then the image formed is inverted.
Answer : C

Question. Assertion : Myopia is the defect of the eye in which only nearer objects are seen by the eye.
Reason : The eye ball is elongated.
Answer : A

Question. Assertion : Secondary rainbow is fainter than primary rainbow.
Reason : Secondary rainbow formation is three step process and hence, the intensity of light is reduced at the second reflection inside the rain drop.
Answer : A

Question. Assertion : Sunlight reaches us without dispersion in the form of white light and not as its components.
Reason : Dispersion takes place due to variation of refractive index for different wavelength but in vacuum the speed of light is independent of wavelength and hence vacuum is a non-dispersive medium.
Answer : A

Question. Assertion : In case of rainbow, light at the inner surface of the water drop gets internally reflected.
Reason : The angle between the refracted ray and normal to the drop surface is greater than the critircal angle.
Answer : A

Question. Assertion : The stars twinkle while the planet do not.
Reason : The stars are much bigger in size than the planets.
Answer : B

Question. Assertion : Danger signals are made of red colour.
Reason : Velocity of red light in air is maximum, so signals are visible even in dark.
Answer : C

Question. Assertion : The focal length of the objective lens of the telescope is larger than that of eyepiece.
Reason : The resolving power of telescope increase when the aperture of objective lens is small.
Answer : C

Question. Assertion : There exists two angles of incidence for the same magnitude of deviation (except minimum deviation) by a prism kept in air.
Reason : In a prism kept in air, a ray is incident on first surface and emerges out of second surface. Now if another ray is incident on second surface (of prism) along the previous emergent ray, then this ray emerges out of first surface along the previous incident ray. This particle is called principle of reversibility of light.
Answer : A

The splitting of a beam of white light into seven constituent colours, when it passes through a glass prism, is called the dispersion of light. When a beam of white light enters a prism, it gets refracted and splits into its seven constituent colours, viz. violet, indigo, blue, green, yellow, orange and red. This splitting of the light ray occurs because of the different angles of bending for each colour. Hence, each colour while passing through the prism bends at different angles with respect to the incident beam. This gives rise to the formation of the coloured spectrum.

Question. White light is a mixture of 
(a) Seven colours
(b) Nine colours
(c) Fourteen colours
(d) Fifteen colours
Answer : A

Question. By which optical phenomenon does the splitting of white light into seven constituent colours occur? 
(a) Refraction
(b) Interference
(c) Dispersion
(d) Reflection
Answer : C

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

Question. Which colour of light deviates maximum in the dispersion of white light by prism? 
(a) Green
(b) Violet
(c) Red
(d) Blue
Answer : B

Question. Which colour of the white light suffers least deviation when a beam of white light is passed through the prism? 
(a) Violet
(b) Green
(c) Red
(d) Yellow
Answer : C

Very Short Answer Type Questions

Question. What is Tyndall effect?
Answer : The phenomenon of scattering of light by the colloidal particles is called Tyndall effect.

Question. Name the atmospheric phenomenon due to which the sun can be seen above the horizon about two minutes before actual sunrise.
Answer : Atmospheric refraction.

Question. Give an example of a phenomenon where Tyndall effect can be observed.
Answer : When a fine beam of sunlight enters a room containing suspended particles of dust, the path of the beam of light is visible. It is due to the scattering of light (Tyndall effect).

Question. Why do different components of white light deviate through different angles when passing through a triangular glass prism?
Answer : Due to change in refractive index offered by the medium.

Question. The sky appears dark instead of blue to an astronaut. State its reason.
Answer : The sky appears dark to the astronaut as scattering does not take place at very high altitude due to the absence of atmosphere.

Question. Name the type of particles which acts as a prism in the formation of rainbow in the sky.
Answer : Water droplets present in the atmosphere.

Question. What is the cause of dispersion of white light on passing through a prism?
Answer : The refractive index of the material of a prism is different for different colours of light as different colours have different speeds in the material of a prism. Also, prism has non-parallel surfaces.

Question. Which phenomenon is responsible for making the path of light visible?
Answer : Tyndall effect.

Question. Why is red colour selected for danger signal lights?
Answer : Wavelength of red colour is more and so, it is least scattered. It can be easily seen through a large distance.

Question. What will be the colour of the sky, when it is observed from a place in the absence of any atmosphere? Why?
Answer : Sky appears dark.
Reason: In the absence of atmosphere, there would have been no scattering of sunlight at all.

Question. Give an example of optical phenomena which occurs in nature due to atmospheric refraction.
Answer : Twinkling of stars.

Question. What would have been the colour of the sky if there had not been any atmosphere around the earth?
Answer : black. 

Question. For dispersion of light through a prism which colour has maximum deviation?
Answer : violet  

Short Answer Type Questions

Question. A star appears slightly higher (above) than its actual position in the sky. Illustrate it with the help of a labelled diagram.
Answer : The gradual change in the refractive index of different layers of the atmosphere due to the varying conditions of it causes atmospheric refraction. When starlight enters the atmosphere, it gets refracted continuously. The higher level of air acts as a rarer medium while the dense air near the surface of earth acts as a denser medium. So, the atmosphere bends the starlight towards the normal. As a result, the apparent position of star is slightly different from its actual position.
Thus, star appears slightly higher (above) than its actual position in the sky.

Question. (a) What is meant by scattering of light?
(b) State the factors on which the colour of scattered light perceived by us depends.
Answer : (a) Scattering of Light: The phenomenon of the change in the direction of propagation of light caused by 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.
(b) The colour of the scattered light perceived by us depends on the size of the particles, i.e.
(i) Very fine particles scatter mainly blue colour.
(ii) Large-sized particles scatter the light of longer wavelengths.
(iii) Enough larger particles scatter the light which may appear white.

Question. What is meant by spectrum of white light? How can we recombine the components of white light after a prism has separated them? Draw a diagram to illustrate it.
Answer : The coloured pattern VIBGYOR formed by a prism by splitting the incident white light is called a spectrum. By having two prisms, inverted to each other, one can recombine the light to get white light again.

Question. A star sometimes appears brighter and some other times fainter. What is this effect called? State the reason for this effect.
Answer : This effect is called Twinkling effect.
Reason: It is due to atmospheric refraction of starlight. The atmosphere has varying layers. The gradual change in the refractive index of different layers of atmosphere causes atmospheric refraction and starlight undergoes multiple refraction. So, the fluctuation in the position of star occurs continuously due to the changing amount of light entering the eye. The star sometimes appears brighter and at some other times, it appears fainter. This causes twinkling of star.

Question. A person needs a lens of power −5.5 dioptres for correcting his distant vision.
For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
Answer : For distant vision = −0.181 m, for near vision = 0.667 m
The power P of a lens of focal length f is given by the relation P= 1 f (in metres )
(i) Power of the lens used for correcting distant vision = −5.5 D Focal length of the required lens, f=1/p=1/-5.5= - 0.181 m
The focal length of the lens for correcting distant vision is −0.181 m.
(ii) Power of the lens used for correcting near vision = +1.5 D Focal length of the required lens, f=1/P=1/1.5=0.667m
The focal length of the lens for correcting near vision is 0.667 m.

Question. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Answer : The person is suffering from an eye defect called myopia. In this defect, the image is formed in front of the retina. Hence, a concave lens is used to correct this defect of vision.
Object distance, u = infinity
Image distance, v = −80 cm
Focal length = f
According to the lens formula, 1/v-1/u=1/f 1/-
80-1/α=1/f
1/f=1/-
80 f=- 0.8m
P=1/f=-1/-0.8=-1.25D
A concave lens of power −1.25 D is required by the person to correct his defec

Question. Give reasons:
(i) The extent of deviation of a ray of light on passing through a glass prism depends on its colour.
(ii) Lights of red colour are used for danger signals.
Answer : (i) Refractive index of a medium is different for different colours of light.
(ii) Due to large wavelength, red colour is least scattered and travel to large distance.

Question. What is a spectrum? Why do different coloured rays deviate differently on passing through a glass prism?
Answer : nce is given by the acronym V I B G Y O R — Violet, Indigo, Blue, Green, Yellow, Orange and Red.
The speed of light of different colours in a medium like glass is different. Varying speeds for different colours lead to different refractive indices for different colours. It has been observed that the refractive index of glass for violet colour is more than that for red colour. All the colours
present in white light refract through different angles and hence, emerge out from the prism with different directions and become distinct.

Question. Draw a ray diagram to show the refraction of light through a glass prism. Mark on it (a) the incident ray. (b) the emergent ray and (c) the angle of deviation.
Answer : i-incident angle, r – refraction angle, δ – angle of deviation, e – angle of emergence.

Refraction of light through a glass prism

Question. Define the term dispersion of white light. State the colour which bends (i) the least and (ii) the most while passing through a glass prism.
Answer : The splitting up of white light into its component colours is called dispersion. The colour which bends (i) the least is red, and (ii) the most is violet, while passing / through a glass prism.

Question. What is the colour of the clear sky during day time? Give reason for it.
Answer : Clear sky appears blue.
Reason: When sunlight passes through the atmosphere having the molecules of air and other fine particles, whose size is smaller than the wavelength of visible light, these molecules and particles scatter the blue colour more strongly than the other colours of spectrum as the wavelength of blue colour is more. This scattered blue light enters our eye. So, the colour of sky appears blue to us during day time.
The band of coloured component of a light beam is called its spectrum. The colour seque

Question. Explain why do the planets not twinkle but the stars twinkle.
Answer : Planets being of larger size can be taken as a collection of large number of point-sized objects/sources of light, which nullify the twinkling effect of each other.
Due to varying conditions of atmosphere, starlight undergoes multiple refraction and its path varying slightly while passing through the atmosphere. Therefore, the apparent position of star fluctuates and amount of light entering the eye changing continuously. The star sometimes appear brighter and some other time, it appears fainter. This causes twinkling of star.

Question. Four friends went to a forest for a picnic. When they were moving around the forest, they saw that the light was filtering from the leaves of the trees. The path of the light beam becomes visible to all of them. They wondered how this happen so. Suddenly, they saw two hunters. They immediately went to the nearest police station to complain about them. Police immediately with their force went to the forest to arrest those people.
(a) Name the effect they observed in the canopy of the dense forest and explain it.
(b) Where can you see this effect other than the forest?
(c) What can you predict about the nature and awareness of these four friends?
Answer : (a) They observed Tyndall effect in the canopy of the dense forest. The phenomenon of scattering of light by colloidal particles, present in the medium due to which the path of the beam of the light becomes visible, is called Tyndall effect.
(b) When a beam of sunlight enters a dusty room through a small hole, then its path becomes visible to us.
(c) Concern for nature, concern and affection with forest animals, helping nature.

Question. A person wears eye glass of focal length 70 cm what is the far point of the person?
Answer : 1/f=1/v-1/u
v=?
f=-70cm
u=-∞
1/-70=1/v-1/-∞
1/v=1/-70 v=-70cm. 

Question. If your eye glasses have focal length 60cm what is your near point?
Answer : 1/f=1/v-1/u
1/60=1/v+1/25
v=-43cm. 

Question. Why do we observe random wavering or flicking of the objects near a fire or on a very hot day?
Answer : Area above the fire is hot, and its density and hence refractive index changes frequently, therefore apparent image of the object also changes. 

Question. Why are we not able to see the things clearly when we come out of a darkroom?
Answer : When we are in dark, pupil size is bigger. As we come out of dark room,its size needs to become smaller. For that time-interval person is unable to see. 

Question. A certain person has minimum distance of distinct vision of 150cm . He wishes to read at a distance of 25cm. What focal length glass should he use? What is the nature of eye defect?
Answer : U=-25cm V=-150cm.
1/f=1/V-1/U
1/f=1/(-150) - 1/(-25)
f=30cm.
f being +ve, lense used is convex lens.
Hypermetropic

Long Answer Type Question

Question. (a) What is dispersion of white light? What is the cause of such dispersion? Draw a diagram to show the dispersion of white light by a glass prism.
(b) A glass prism is able to produce a spectrum when white light passes through it but a glass slab does not produce any spectrum. Explain why is it so?
Answer : (a) The splitting up of white light into its constituent colours is called dispersion. The colour sequence is given by the acronym V I B G Y O R – Violet, Indigo, Blue, Green, Yellow, Orange and Red. This colour pattern is called a spectrum.

Dispersion takes place because the speed of light of different colours through a glass prism is different and so, refractive index, therefore, each colour bends (refracts) through different angles with respect to incident ray as they pass through a prism. The red colour has maximum speed in glass prism. So, it is least deviated while the violet colour has minimum speed so its deviation is maximum. Thus, the ray of each colour emerges along different paths and becomes distinct.
(b) For dispersion, the two refracting surfaces must be inclined to each other as in case of prism. In rectangular glass slab, the refracting surfaces are parallel to each other. So, dispersion cannot occur. This is due to fact that the rectangular glass slab can be considered as equivalent of two
identical prisms in inverted position placed in an inverted position with respect to each other. The deviation and dispersion produced by the second inverted prism is equal and opposite to that produced by the first prism. Therefore, there will neither be dispersion, nor deviation, Le. second inverted prism recombines the , colour to give a white light parallel to the incident ray again and will undergo only lateral displacement. Hence, rectangular glass slab cannot produce any spectrum.

Question. (i) Draw a figure which shows the arrangement for observing the phenomenon of scattering of light in the laboratory.
(ii) What colours would you observe in the experiment? Why?
Answer : (i) An arrangement for observing the scattering of light in the laboratory is as shown below.
(ii) (a) On the screen, first orange red colour and then bright crimson red colour patch is observed.

(b) From the other three sides of colloidal solution of sulphur in a glass tank (T), blue colour is observed.
This is because the very fine colloidal sulphur particles scatter away the blue colour from the path of beam and only red colour (least scattered) of the beam of white light reaches the screen through the solution.