Please refer to below Exam Question for Class 10 Science Chapter 11 Human Eyes and Colourful World. These questions and answers have been prepared by expert Class 10 Science teachers based on the latest NCERT Book for Class 10 Science and examination guidelines issued by CBSE, NCERT, and KVS. We have provided Class 10 Science exam questions for all chapters in your textbooks. You will be able to easily learn problems and solutions which are expected to come in the upcoming class tests and exams for standard 10th.

Chapter 11 Human Eyes and Colourful World Class 10 Science Exam Question

All questions and answers provided below for Exam Question Class 10 Science Chapter 10 Human Eyes and Colourful World are very important and should be revised daily.

Exam Question Class 10 Science Chapter 11 Human Eyes and Colourful World

Very Short Answer Type Questions
Question: On what factor does colour of scattered light depend? 
Answer: The colour of scattered light depends on the size of the scattering particle. Very fine particles scatter mainly blue light whereas particles of larger size scatter light of longer wavelengths.

Question: Why is the sun visible to us 2 minutes before actual sunrise and 2 minutes after actual sunset?
Answer: We are able to see the sun 2 minutes before actual sunrise and 2 minutes after actual sunset because sunrays undergo atmospheric refraction as they travel from an optically rarer medium to an optically denser medium when they enter the earth’s atmosphere due to which they are refracted downwards and hence appear to be above the horizon.

Question: What would have been the colour of sky if the earth had no atmosphere?
Answer: The sky would have looked dark if the earth had no atmosphere as there would not have been any scattering.

Question: Define power of a lens.
Answer: Power (P), of a lens is the reciprocal of its focal length. Power = 1/ f, where f is the focal length of the lens.

Question: On the basis of which observation did Newton conclude that sunlight is made up of seven colours?
Answer: The observation on the basis of which Newton concluded that sunlight is made up of seven colours was the emergence of white light from the other side of a second identical and inverted prism placed alongside first prism and through which all colours of the spectrum were passed.

Short Answer Type Questions

Question: State and explain three phenomena observed in nature subsequent to scattering of light.
Answer: Phenomena observed in nature subsequent to scattering of light:
(1) Color of the sky during Sunrise and Sunset:
During sunset and sunrise, the sunlight travels the maximum distance through the atmosphere and through thicker layers of air. Most of the blue light and shorter wavelength are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelength.
This gives rise to the reddish appearance of the sun. Whereas at noon, the sun appear white as only a little of blue and violet colours are scattered.
(2) Formation of rainbow: After it rains, small particles of water vapor still persist in the atmosphere. When sunlight passes through these particles (the raindrops), they refract the light, disperse it, then internally reflect it and finally refract it (3) Danger signals are usually red as red,having the longest wavelength, is scattered the least.

Question: How will you use two identical glass prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw and label the ray diagram.
Answer: Two identical prisms can be used to get a narrow beam of white light incident on one prism emerges out of second prism as white light.
A triangular glass prism P, is placed on its base P1 and another similar prism P₂ is placed alongside it in the inverted position on its vertex, let’s say . A with respect to the first prism P₁ so that its refracting surfaces are in opposite direction to the refracting surfaces of first prism. 

Explanation: When a beam of white light is allowed to fall on first prism P1, it disperses white light into seven colours. The second prism P₂ receives these seven colours and recombines them into original white light and a patch of ordinary white light is obtained on the screen placed behind the second prism P₂.
Newton explained these observations.
The first prism P₁ splits the white light into seven coloured rays.
The second glass prism P₂ recombines all the seven coloured rays from the first one and falls on the screen as white light. This observation gave Newton an idea that the sunlight is made up of seven colours.
This is due to the fact that the refraction (bending) produced by the second prism P₂ is equal and opposite to the refraction (bending) produced by the first prism.
The emergent white light from the second prism P₂ may or may not be in the direction of white light incident on first prism P₁.

Question: State one effect produced by the scattering of light by the atmosphere.
Answer: There are two effects produced due to scattering of light in the atmosphere: tyndall effect and appearance of the blue colour of the sky.

Question: Why is Tyndall effect shown by colloidal particles? State four instances of observing the Tyndall effect. 
Answer: Tyndall effect is shown by colloidal particles as they scatter light.
Scattering of light is the change in the direction of light on striking an obstacle (atom, molecule, dust particles, smoke, water droplets etc.) When beam of light strikes such fine particles, the path of light becomes visible. The phenomenon of scattering of light by colloidal particles is Tyndall effect.
Four instances where we can observe Tyndall’s effect are:
(1) When a fine beam of sunlight enters a smoke filled room, smoke particles become visible due to scattering of light by these particles.
(2) When sunlight passes through a canopy of a dense forest, tiny water droplets in the water scatter light.
(3) Milk is a colloid that contains globules of fat and protein. When a beam of light is directed at a glass of milk the light is scattered. That is a great example of the Tyndall effect. 

(4) Blue colour of clear sky: The scattering of blue component of white an light by air molecules present in the atmosphere causes the blue colour of the sky.
Explanation: As the size of the scatterer is much smaller than the wavelength of light,light of smaller wavelength such as blue is scattered the most as compared to light of longer wavelength.

Question: A person needs a lens of power –5.5 dioptre for correcting his distinct vision. For correcting his near vision, he needs a lens +1.5 dioptre.What is the focal length of the lens required for correcting (A) distinct vision, and (B) near vision?
Answer: Given: P = +1.5 D
To find focal length, f =?
P =1/ f
1
+1.5 =1/ f
f = 1/+1.5=+0.67m
The +ve sign indicates that the lens is convex.
(A) Given:
Power = –5.5 D
To find: focal length, f =?
P =1/ f
–5.5=1/ f
So, f = 1/5.5 =-0.18 m
Since the focal length is –v e, it shows that the lens used is concave.

Question: Draw a labelled diagram to show (A) reddish appearance of the sun at the sunrise or the sunset and (B) white appearance of the sun at noon when it is overhead
Answer: 

Question: Why is the colour of the clear sky blue?
Answer: The blue colour of sky is due to the scattering of sunlight by the large number of molecules such as fine dust particles, gases, water vapour etc present in the earth’s atmosphere.
As the size of the scatterer is much smaller than the wavelength of light, light of smaller wavelength such as blue is scattered the most as compared to light of longer wavelength.

Question: Differentiate between a glass slab and a glass prism. What happens when a narrow beam of (A) a monochromatic light, and (B) white light passes through (i) glass slab and (ii) glass prism? 
Answer: Differences between a glass slab and a glass prism. 

(A) (i) When a narrow beam monochromatic light passes through a a glass slab,
it deviates from the actual path but the direction of incident ray and the emergent ray are parallel to one another.
(ii) light passes through a glass prism, it deviates from the actual path but the direction of incident say and the emergent ray of light are not parallel to each other.
(B) (i) When a narrow beam of white light passes through a glass slab, the white light does not split it into its constituent colours. The direction of incident ray and the emergent ray of light is parallel to each other.
(ii) When a narrow beam of white light passes through a glass prism, the white light splits into its constituents seven colours. The incident ray and the emergent ray of light is not parallel to each other.
40. An astronaut in space finds sky to be dark. Explain reason for this observation.
Answer: There is no sky in space. Hence, the light coming from the sky does not scatter. For the sky to appear bright, the light has to scatter so that we can see the sky clearly. As a result, the sky appears dark to the astronaut in space.

Question: (A) What is visible spectrum?
(B) Why is red used as the stopping light at traffc signals?
(C) Two triangular glass prisms are kept together connected through their rectangular side. A light beam is passed through one side of the combination. Will there be any dispersion? Justify your answer.
Answer: (A) Visible spectrum is the band of coloured components of a white light beam . (B) Red light is scattered the least by air molecules and has longer wavelength. It travels the long distance.
(C) The given setup will behave like a glass slab, resulting in recombination of the seven colours produce white light

Question: In the figure given below, a narrow beam of white light is shown to pass through a 3 triangular glass prism. After passing through the prism, it produces a spectrum X Y on the screen.

(A) Name the phenomenon.
(B) State the colours seen at X and Y.
(C) Why do different colours of white light bend at different angles through a prism?
Answer: (A) The phenomenon is called dispersion.
(B) X- Violet Y- Red
(C) Different colours of white light bend through different angles with respect to the incident beam light due to difference in speed of light of different wavelengths. 

Long Answer Type Questions: 

Question: (A) A person suffering from myopia (near-sightedness) was advised to wear corrective lens of power – 2.5 D. A spherical lens of same focal length was taken in the laboratory. At what distance should a student place an object from this lens so that it forms an image at a distance of 10 cm from the lens?
(B) Draw a ray diagram to show the position and nature of the image formed in the above case. 
Answer: (A) P = – 2.5 D
f = ?
P = 1/f or f=1/p=1/-2.5D
= – 0.4 m Or – 40 cm
f = – 40 cm
v = – 10 cm
u = ?

1/f=1/v-1/u

1/u=1/v-1/f
 
=1/10-1/-40=1/-10+1/40

=-4+1/40=-3/40

u=-40/3 or -13.33 cm

The object should be placed – 13.33 cm from the lens so that A forms an image at a distance of 10 cm from the lens.
(B) 

Question: Use scattering of light? phenomenon to explain why (A) the Sun appears reddish at sun-rise, and (B) the clear sky appears blue.
Answer: Scattering of light is the phenomenon of change in the direction of light on striking an obstacle like an atom, a molecule, dust particle, water droplet etc.
(A) At the time of sunrise, the sun is near the horizon due to which sunrays have to travel much larger part of the atmosphere. Most of the blue light is thus scattered away. The red colour having the longest wavelength is scattered the least and hence enters our eyes.
(B) The blue colour of sky is due to the scattering of light by the large number of molecules and fine dust particles present in the earth’s atmosphere which mostly scatter light of shorter wavelength such as blue as the size of the scatterer is much smaller than the wavelength of light.

Question: (A) Draw a labelled ray diagram to show the path of a ray of light incident obliquely on one face of a glass slab.
(B) Calculate the refractive index of the material of a glass slab. Given that the speed of light through the glass slab is 2 × 108 m/s and in air is 3 × 108 m/s.
(C) Calculate the focal length of a lens, if its power is – 2.5 D. 
Answer: (A) 

(B) Speed of light in glass slab (v) = 2 x 10⁸ m/s
Speed of light in air (c) = 3 x10⁸ m/s
Refractive index of glass (n) = ?
         n = c/v
         n=3×10⁸/2×10⁸=1.5

Hence, refractive index of material of glass slab is 1.5.
(C) Power P = – 2.5 D
Total length f = ?
P = 1/f
f=1/p  or 1/-2.5 or 100cm/-2.5
= – 4.0 cm Or – 04 m
The focal length of a lens of power – 2.5 D is – 0.4 m Or – 40 cm

Question: What is the cause of dispersion of white light through a glass prism?
Draw a ray diagram to show the path of light when two identical glass prisms are arranged together in inverted position with respect to each other and a narrow beam of white light is allowed to fall obliquely on one of the faces of the prisms. 
Answer: Cause of dispersion of white light by prism:
(1) Light rays of different colours travel with different speeds in glass and as the refractive index of glass is largest for violet colour and least for red colour, violet colour is dispersed the maximum and red colour the least.
(2) The two refracting surfaces are inclined at an angle because of which colours separated by the first surface continue to travel along different paths from the second interface.
Ray diagram to show recombination of white light: 

Question: Why does the Sun appear reddish early in the morning? Explain with the help of a labelled diagram. 
Answer: The sun appears reddish early in the morning at the time of sunrise as the sun is near the  horizon due to which sunrays have to travel much larger part of the atmosphere. Most of the light of shorter wavelength, such as blue light, is scattered away. The red colour having the longest wavelength is scattered the least and hence enters our eyes.
Labelled diagram to explain this process is drawn below:

Question: What is a rainbow? Draw a labelled diagram to show the formation of a rainbow.
Answer: Rainbow: A rainbow is the natural spectrum formed in the sky after a rain shower due to the dispersion of sunlight by the tiny droplets of water present in the atmosphere. The tiny droplets of water act like small prisms which refract and disperse the incident sunlight, then reflect the light internally and finally refract it again when it comes out of the raindrop. Different colours reach the observers eyes due to the dispersion of light and internal 

Question: Draw a ray diagram to explain the dispersion of white light. With the help of a labelled ray diagram, show how a spectrum of white light may be recombined.
Answer: The splitting of light into its component colours is called dispersion of light. Ray diagram showing dispersion of white light by a glass prism is drawn below: 

When a second identical prism is kept in an inverted position with respect to the first prism,all the colours of the spectrum passed through the second prism and white light emerged from the other side of the second prism.Ray diagram to show recombination of white light:

Question: What is the cause of dispersion of white light through a glass prism?
Draw a ray diagram to show the path of light when two identical glass prisms are arranged together in inverted position with respect to each other and a narrow beam of white light is allowed to fall obliquely on one of the faces of the prisms. 
Answer: Cause of dispersion of white light by prism:
(1) Light rays of different colours travel with different speeds in glass and as the refractive index of glass is largest for violet colour and least for red colour, violet colour is dispersed the maximum and red colour the least.
(2) The two refracting surfaces are inclined at an angle because of which colours separated by the first surface continue to travel along different paths from the second interface.
Ray diagram to show recombination of white light: 

Question: Describe an activity that demonstrates the scattering of light.
Answer: An activity to demonstrate scattering of light is described below: 

(1) Place a source of white light S such as an electric bulb at the focus of a converging lens L₁. This lens provides a parallel beam of light.
(2) Allow the light beam to pass through a transparent glass beaker B of capacity 500 mL containing clear water.
(3) Allow the light beam to pass through a circular hole made in cardboard C.
(4) Obtain a sharp image of the hole on a screen MN using another convex lens L₂.
(5) Dissolve about 200 g of sodium thiosulphate (hypo) in water taken in the beaker B. Add about 1 to 2 mL of concentrated sulphuric acid to the water in the beaker containing hypo solution.
(6) We will notice fine microscopic sulphur particles precipitating in about 2 to 3 seconds.
(7) As the sulphur particles begin to form, we observe the blue light from the three sides of the beaker. This is due to scattering of short wavelengths by minute colloidal sulphur particles.
(8) Observe the colour of the transmitted light from the fourth side of the glass beaker facing the circular hole. We first observe orange red colour and then bright crimson red colour on the screen.
This activity demonstrates the scattering phenomenon of light that explains the bluish colour of the sky in general and the reddish appearance of the sun at the sunrise or the sunset.

Question: Study the following ray diagram and list two mistakes committed by the student while tracing it. Rectify these mistakes by drawing the correct ray diagram to show the real position and size of the image corresponding to the position of the object AB.

Answer: The mistakes committed by the student while tracing the ray diagram are:
(1) The distance from optical center to F₁ is not equal to distance from optical center to F₂. Both distances OF1 and OF2 should be equal.
(2) OF₁ = 1/2 2OF₁ and OF₂=1/2 2OF₂  which
shows that F1 and F2 are not at the centre of 2OF₁ and 2OF₂ respectively.
(3) Since the object lies between F₁ and 2F₁,image should be magnified and lie beyond 2F₂.
Correct ray diagram is drawn below:  

Question: Explain in brief the reason for each of the following:
(A) Advanced sun-rise
(B) Delayed sun-set
(C) Twinkling of stars 
Answer: (A) Advance sunrise: When the sun is slightly below the horizon light rays coming from the sun travel from the rarer to denser medium layers of air because of atmospheric refraction of light, light appears to come from a higher position above the horizon.
Thus the sun appears earlier than actual sun rise.

(B) Delayed sun set: Same reason as similar atmospheric refraction occurs. 

(C) Twinkling of Stars: The light coming from the stars gets refracted several times before reaching the observers eye. Due to change in physical condition of the atmosphere the light sometimes reaches the observer and sometimes it doesn’t, hence they appear to twinkle.

Question: (A) Draw a ray diagram to explain the term angle of deviation.
(B) Why do the component colours of incident white light split into a spectrum while
passing through a glass prism, explain.
Answer: (A) Ray diagram showing refraction of light through a prism:

Angle of deviation is the angle between the direction of incident ray and the emergent ray.
(B) When white light is incident on a glass prism, a band of seven colours, known as spectrum, is observed on a white screen kept on the other side of the prism. This splitting or dispersion occurs because light rays of different colours travel with different speeds in a refractive medium. The refractive index of glass is largest for violet colour and least for red colour due to which violet colour is dispersed the maximum and red colour the least.

Question: (A) With the help of diagram explain Isaac Newton’s experiment that led to the idea that the sunlight is made up of seven colours.
(B) What is atmospheric refraction? List two natural phenomena based on atmospheric refraction.
Answer: (A) Issac Newton was the first to use a glass prism to obtain the spectrum of white light. He tried to split various colours of the spectrum of white light by using another similar prism, he could not get any more colours. Thus he proved that sunlight is made of seven colours. 

Question: Draw a labeled diagram to explain the formation of rainbow in the sky.
Answer: A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets,present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms.
They refract and disperse the incident sunlight,then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection,different colours reach the observer’s eye. We can also see a rainbow on a sunny day when we look at the sky through a waterfall or through a water fountain, with the Sun behind us. 

Question: What is spectrum of white light? Why do different coloured rays deviate at different angles on passing through a glass prism?
Explain the method of recombining the different components of white light.
Answer: The band of the coloured components of a light beam (VIBGYOR) obtained by the dispersion of white light is known as its spectrum.
Splitting or dispersion occurs when light passes through a glass prism because light rays of different colours travel with different speeds in glass. The refractive index of glass is largest for violet colour and least for red colour due to which angle of deviation is maximum for violet colour and minimum for red colour.
When a second identical prism is kept in an inverted position with respect to the first prism,all the colours of the spectrum passed through the second prism and white light emerged from the other side of the second prism.
Ray diagram to show recombination of white light:

Question: Answer questions (A)-(B) on the basis of your undersrandling of the following paragraph and related studied concepts.
A rainbow is formed on a sunny day when we look at the sky through a waterfall of through a water foundation with the sun behind us.
A rainbow is always fomed in a direction opposite to that of the sun. Sometime, a secondary rainbow (double rainbow) is also seen. it is caused by each ray of light reflecting twice on the inside of each droplet before it leaves. The second reflection causes the colours on the secondary rainbow to be
reversed.

(A) The colour at position marked 3 and 5 are similar to the colour of the sky and the colour of gold metal respectively. Is the above statement made by the student correct or incorrect? Justify.
(B) Which of the above shown positions corres-ponds approximately to the colour of:
(i) a solution of potassium permanganate?
(ii) danger or stop signal lights?
Answer: A beam of white light falling on a glass prism gets split up into seven colours as shown in the diagram:
(A) The colours at position marked 3 and 5 are yellow and blue respectively. On, the other hand, student has identified them as blue and yellow. Hence the statement made by the student is incorrect. 

The spliting of light into its component colours is called dispersion. The band of the coloured components of a light beam is called its spectrum. Different colours of light bend through different angles with respect to the incident , as they pass through the prism. The red light bends the least while the violet the most.
(B) (i) Position 7 is the position of violet colour,  which corresponds to the colour of a solution of potassium permanganate.
(ii) Position 1 is the position of red colour, which corresponds to the colour of ‘danger’ or stop signal lights.

Question: (A) Explain the following term used in relation to defects in vision and correction provided
by them:
(i) Myopia
(ii) Astigmatism
(iii) Bifocal lenses
(iv) Far-sightedness.
(B) Why is the normal eye unable to focus on an object placed within 10 cm from the eye?
(C) If due to some disease or injury, the cornea of an eye is clouded, then the vision is impaired and the person may become blind. This type of blindness may be cured by replacing the defective cornea with the cornea of the donated eye. Suggest some measures for motivating people to donate their eyes after death. 
Answer: (A) (i) Myopia: In this defect a person can see nearby ob􀅉ects but not far away objects. It is corrected by using concave lens 
(ii) Astigmatism: In this defect the eye cannot focus objects both horizontally and vertically at the same time. It is corrected by using cylindrical lenses.
(iii) Bifocal lenses: These are lenses which have both concave and convex structure. These are used when a person is suffering from presbyopia and myopia.
(iv) Far-sightedness: When a person can see faraway objects but not nearby objects. It is called far-sightedness. This defect is corrected by using convex lenses.
(B) To see an object placed at a distance of less than 25cm, the ciliary muscles have to make the focal length still smaller, Which puts strain on the ciliary muscles and they cannot make the focal length so small. Hence, it is not possible to see a distance lens then 25 cm.
(C) Make people aware that donating an eye after death will give vision to two people and reduce their sufferings

Question: What is atmospheric refraction? Use this phenomenon to explain the advanced sunrise and delayed sunset.
Answer: Atmospheric refraction: The density of the atmosphere, as we know goes on decreasing as
the distance above the sea level increases. The refractive index of a layer of air at a particular level goes on changing. Due to this change in density, the refraction takes place when light passes through the Earth’s atmosphere.
This phenomenon is known as atmospheric refraction.
Advanced sunrise and delayed sunset:
The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.
By actual sunrise, we mean the actual crossing of the horizon by the Sun. The time difference between actual sunset and the apparent sunset is about 2 minutes. The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the same phenomenon. 

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 atmospheric layers is due to the varying conditions of it causing atmospheric refraction. Do look at the diagram above:
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 the star is slightly different from its actual position.
Thus, the star appears slightly higher (above) than its actual position in the sky.