NCERT Solutions For Class 10 Science Chapter 11 Human Eye And Colourful World

NCERT Solutions For Class 10 Science Chapter 11 Human Eye And Colourful World Long Question And Answers

Question 1. A myopic person has been using spectacles of power- 1.00 D for clear vision. During old age he also needs to use separate reading glasses of power + 2.00 D. Explain what may have happened to Lois’s eyesight.
Answer: When he was using lenses only for myopic defect, his near point was normal i.e., equal to 25 cm.

His far-off off-point was 100 cm according to the power oflens he was using. His eye lens had the normal power of accommodation. Due to old age, his ciliary muscles became weaker and he did not have a normal power of accommodation.

So his near point of vision also increased and reached up to 50 cm as shown below and he needed a lens of 2.00 D power for that purpose.

p=+2.00D of \(f=\frac{1}{\mathrm{P}}=\frac{1}{2}=0.5 \mathrm{~m}=50 \mathrm{~cm}, u=-25 \mathrm{~cm}\)

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Using the lens formula, we have

⇒ \(\begin{aligned}
\frac{1}{v} & =\frac{1}{f}+\frac{1}{u} \\
& =\frac{1}{50}+\frac{1}{-25} \\
& =\frac{1}{-50}
\end{aligned}\)

v=50cm Thus, the near point of this person is 50 cm away from his eyes.

Question 2. The point of a myopic person is 100 cm. Calculate the power of the lens required to enable him to see distant objects. For which colours, is the refractive index n of prism material

1. Minimum
2. Maximum.

Answer: Required focal length, f=- 100 cm =- 1 m or -1m

⇒ \(\begin{aligned}
P & =\frac{1}{f} \\
& =\frac{1}{-1 \mathrm{~m}} \\
& =-1 \mathrm{D}
\end{aligned}\)

1. N is the minimum for red colour.
2. N is the maximum for violet colour.

Question 3. What is astigmatism? How is it caused? How is it corrected? Or What is astigmatism and how is this vision defect counteracted?
Answer: Astigmatism: It is a defect of vision in which a person cannot simultaneously see both the horizontal and vertical views of an object with the same clarity.

Cause of astigmatism:

This defect occurs when the cornea of the eye is not perfectly spherical. This results in objects in one direction being well focussed while those in a perpendicular direction are not well focussed.

Correction of astigmatism: Astigmatism can be corrected by using cylindrical lenses.

They have different curvatures in horizontal and vertical directions so they can be oriented suitably to compensate for the irregularities in the cornea, as shown in the figure.

Question 4. A person with a defective eye-vision is unable to see objects nearer than 1.5 m. He wants to read books at a distance of 30 cm. Find the nature, focal length and power of the lens he needs in his spectacles.
Answer: This person suffers from the defect of hypermetropia.

For him u =- 30 cm, v =- 1.5 m =- 150 cm

Therefore, the focal length of the corrective lens to be used by us

⇒ \(\begin{aligned}
\frac{1}{f} & =\frac{1}{v}-\frac{1}{u} \\
& =\frac{1}{-150}-\frac{1}{-30} \\
& =\frac{4}{150} \text { of } f=37.5 \mathrm{~cm}
\end{aligned}\)

The positive sign shows that the lens needed is a convex lens with a focal length of 37.5 cm. Hence, the power oflens needed

⇒ \(\begin{aligned}
\mathrm{P} & =\frac{1}{f} \\
& =\frac{100}{37.5} \\
& =2.67 \mathrm{D}
\end{aligned}\)

Question 5. Give some points of similarity and dissimilarity between a camera and the human eye.
Answer: Points of similarity:

Points Of Dissimilarity: 

Question 6. In a beautiful valley, there was a village. Train passes from the village, the whistle and the sound of train mixed with the sound of waterfall seemed to be very pleasant to everyone.

Children of that village loved to play near the railway track. One very light foggy day, a group of children found that a fishplate was missing from the track. Knowing this all of the villagers became tense. Aditya, one ofthe kids, suddenly put his ear to the line and tried to know whether the train was coming or not and he knew that the train was coming. He asked his friend to inform the railway cabin crew and he himselfput off his red shirt and started running towards the train, waving his red shirt. Timely, the driver and cabin man got information and a major accident had not taken place.

1. Name the two physical phenomena of science used by Aditya.
2. Why did Aditya use a red shirt instead of other colours?
3. What moral values were shown by Aditya1?

Answer: Sound travels through a medium.

1. Scattering of light.
2. Red is least scattered by fog or smoke so it can be seen from a large distance.
3. Proper knowledge and its application.
4. Concern for each other

Question 7. What is the 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.

A glassprism can produce a spectrum when white light passes through it but a
glass slab does notproduce any spectrum. Explain why it is so

Dispersion: The splitting of light into its constituent seven colours is called dispersion of light.

Cause: Ordinary white light is a superposition of waves of wavelengths extending throughout the visible spectrum. The speed of light in a vacuum is the same for wavelengths, but the speed in a material light substance is different for different wavelengths.

As a result, different colours are deviated by different angles when they pass through a prism. This is called the dispersion of light.

In a prism, the refraction oflight takes place at the two slant surfaces. The dispersion of white light occurs at the first surface of the prism where its constituent colours are deviated through different angles.

At the second surface, these split colours suffer only refraction and they get further separated. But in a rectangular glass block, the refraction of light takes place at the two parallel surfaces.

At the first surface, although the white light splits into its constituent colours on refraction, these split colours suffering refraction at the second surface emerge in the form of a parallel beam, which gives an impression of white light.

Question 8. Discuss the refraction of light through a prism. What are the angle of deviation and angle of emergence? How are these angles related to the angle of incidence and the angle prism? or Draw a ray diagram to show the path of the light ray that enters the glassprism obliquely. Label it the angle of incidence and angle of deviation.

Answer: Refraction through a glass prism: The figure shows the principle section ABC of a glass prism.

Aray PQ is incident on face AB. As it enters the denser medium (glass), it bends away from the normal along path QR.

The ray QR again suffers refraction at face AC; bending away from the normal, it emerges along. The angle between the incident ray and the emergent ray is Incident called the angle of deviation.

The angle e made by the emergent ray with the p normal to the refracting face AC is called the angle of emergency. A light ray bends towards the base ofthe prism as it suffers refraction through the prism. It is seen that ray i+e=A+=D

In other words, Angle of incidence + Angle of emergence = Angle of prism + Angle of deviation

Question 9. The sun near the horizon appears flattened at sunset and sunrise. Explain why?
Answer: Apparent flattening of the sun’s disc at sunrise and sunset: This is due to atmospheric refraction. The density and the refractive index ofthe atmosphere decrease with altitude, so the rays from the top and bottom portion ofthe sun on the horizon are refracted by different degrees.

This causes the apparent flattening of the sun. But the rays from the sides ofthe sun on a horizontal plane are generally refracted by the same amount, so the sun still appears circular along its sides.

Question 10. The sun is visible to us about two minutes before the actual sunrise and about two minutes after the actual sunset. Give reason.
Answer: Advance sunrise and delayed sunset.

The apparent shift in the position of the sun at sunrise and sunset: The sun is visible before actual sunrise and after actual sunset, because of atmospheric refraction. With altitude, the density and hence refractive index of the air-layer decreases.

As shown in the figure, the light rays starting from the sun travel from rarer to denser layers. They bend more and more towards the normal. To an obese on the earth, light rays appear to come from position S’.

The sun which is actually in position S below the horizon, appears in position S’ above the horizon. Thus the river sun appears to rise early by about two minutes and for the same reason, it appears to set late by about two minutes. This increases the length of the day by About Four Minutes

Question 11. Draw a neat labelled diagram of the structure ofthe human eye and mention the functions of any four parts.
Answer: Cornea: Thin transparent membrane which allows the light to enter and refracts
the light to focus it on the crystalline lens.

Crystalline lens: Double convex lens changes its focal length with the help of ciliary
muscles and the suspensory ligaments and enables the light ray to fall on the retina.

Iris: Coloured portion ofthe eye regulates the size of the pupil and controls the amount
oflight entering the eye.

Retina: Consists oflight light-sensitive cells (rod and cone cells) which enable us to see in dim and bright light.

Question 12. What is the Tyndall effect? What is its cause? Name two phenomena observed in daily life which are based on the Tyndall effect.
Answer: Tyndal effect: When a beam oflight is passed through a colloidal solution, and placed in a dark room, the path of the beam becomes illuminated (or visible), when observed through a microscope placed perpendicular to the path oflight.

This effect is called the Tyndall effect. On the other hand, the path of a beam oflight is not visible through a true solution, as shown in the figure

Cause of Tyndall effect: The size ofthe colloidal particle is relatively larger than the solute particle of a true solution. The colloidal particles first absorb energy from the incident light and then scatter a part of this energy from their surfaces. Thus Tyndall effects due to the scattering oflight by the colloidal particles and the colloidal particles are seen as points oflight moving against a dark background.

Some daily life phenomena based on the Tyndall effect are as follows:

When a fine beam of sunlight enters a smoke-filled room through a small hole, the smoke particles become visible due to the scattering flight. When sunlight passes through the canopy of a dense forest, the tiny water droplets in the mist scatter light and become visible.

Question 13. Explain the structure and functioning of the human eye. How are we able to see nearby as well as distant objects?
Answer: The structure and functioning human eye are as given below: Cornea: Cornea is transparent. It is responsible for most ofthe refraction oflight rays towards the retina.

Iris: Iris is a dark, circular muscular diaphragm behind the cornea. It gives the eye its colour. It also controls the pupil size and thus controls the amount oflight entering the eye.

Pupil: Pupil is surrounded by the iris. It regulates and controls the amount of light
entering the eye.

Retina: The retina is the innermost layer of the eye. It contains an outer pigmented layer and an inner nervous layer which contains the photoreceptors (rods and cones). Cone cells are cone-shaped and rod cells are rod-shaped.

Optic nerve: The optic nerve connects the eye to the brain. The nerve fibres carry impulses from the retina to the visual cortex.

Eye lens: The eye lens is made up of fibrous, jelly-like material. It is transparent and has a biconvex structure. It forms an inverted real image of the object on the retina and also separates the aqueous and the vitreous humour.

Vitreous humour: Vitreous humour is clear and semi-solid, supporting the eyeball. The eye lens forms an inverted real image of the object on the retina, which sends image electric signals to the brain via the optic nerves. The brain reconstructs erect images of objects and makes us see them as they are.

Question 14. When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eyes can be corrected.
Answer: Myopic: This term is used to define short-sightedness. Light from a distant object forms an image before it reaches the retina. This could be because the eye is too long or the cornea or crystalline lens is too strong. A myopic person has clear vision when looking at objects close to them but distant objects will appear blurred.

To correct this defect, a concave lens (minus powered) is placed in front of a myopic eye, moving the image back to the retina and clarifying the image.

Hypermetropic: It means long-sightedness, in which the image of a nearby object is formed behind the retina. This could be because the eye is too short or the cornea or crystalline lens does not refract the light enough.

A hypermetropic person may have blurred vision when looking at objects close to them and clearer vision when looking at objects in the distance.

To correct this defect, a convex lens (plus powered) is placed in front of a hypermetropic eye, and the image is moved forward and focuses correctly on the retina.

Question 15. Explain the refraction oflight through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.
Answer: The angle of deviation is the angle D between the incident ray and the emergent ray when a ray oflight passes through a glass prism. Aray oflight PE is entering from air to glass at the first surface AB.

The lightrayEF on refraction has bent towards the normal. At the second surface AC, the light ray FS has entered from glass to air and bent away from normal. The angle made by extending the incident ray with the emergent ray is called the angle of deviation.

Question 16. How can we explain the reddish appearance of the sun at sunrise or sunset? Why does it not appear red at noon?
Answer: The reddish appearance of the sun at sunrise or sunset is due to the scattering oflight by the molecules of air and other fine particles in the atmosphere that have a size smaller than the wavelength of visible light from the sun near the horizon.

It passes through thicker layers of air and a larger distance in the earth’s atmosphere before reaching our eyes and most ofthe blue light and shorter wavelengths are scattered away by the particles.

So, only red light, being of a higher wavelength, reaches us which gives the reddish appearance of the sun at sunrise or sunset.

At noon, the sun appears white, not red, as only a little of the blue and violet colours are scattered as light from the sun overhead would travel relatively shorter distances.

Question 17. Explain the phenomenon of dispersion of white light through a glassprism, using a suitable ray diagram. When white light such as sunlight is passed through a prism it is reduced to a narrow beam.

This narrow beam is made to fall on a triangular glass prism. When a white screen is held on the other side of the prism, we observe a band of seven colours on the screen.

The colour sequence obtained on the screen from its lower end is given by the famous acronym VIBGYOR, where V stands for violet, I stands for Indigo, B stands for Blue, G stands for Green, Y stands for Yellow, O stands for Orange and R stands for Red.

The red colour bends the least on passing through the prism and the violet colour bends through the maximum angle on passing through the prism.

This is why red is at the top and violet is at the bottom. Thus, the phenomenon of splitting white light into its constituent seven colours on passing through a glass prism is known as the dispersion of light.

Question 18. How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
Answer: The refraction of light by the earth’s atmosphere is due to the gradually changing refractive index of air layers.

The physical conditions ofthe refracting medium (air) are not stationary and the apparent position of the object as seen through the hot air fluctuates. This wavering is thus an effect of atmospheric refraction on a small scale in our local environment.

Since, the atmosphere bends light from the astronomical bodies viz, stars towards the normal, the apparent position of the astronomical bodies is slightly different from its actual position. The astronomical bodies appear slightly higher (above) than their actual position when viewed near the horizon.

The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth.

The path of rays oflight coming from the distant star goes on varying slightly, the apparent position ofthe star fluctuates and the amount of starlight entering the eye flickers such that the star sometimes appears brighter and at some other times fainter which gives us the twinkling effect.

Question 19. News spread in the school that a student of class IX-A had died due to a heart attack, but he donated his beautiful eyes to one of his friends.

All the members of the school felt very sad about his untimely death, but on the other hand, they were overwhelmed upon hearing the donation of his eyes to his friend, who would now be able to see this beautiful nature. Answer the following questions based on the situation given above.

Do you think that the student who expired had done a good job? Is it worth to donate
vital organs?

1. What values are promoted here?
2. What other organs can be donated after dying?

Answer: Yes. Donating vital organs can make the life of a living person easier.

1. Social welfare, kind-hearted, selfless, friendship.
2. Kidneys, heart, liver, etc.

Question 20. ‘Sonia and Soya’ are best friends and study in grade 4. Recently, Sonia has been facing difficulty in reading the blackboard text from the last desk. Soya is a little uncomfortable and wonders why Sonia complains about sitting at the last desk.

On observation, she found that Sonia often carries junk food in her lunch. Soya has started sharing her lunch full of green vegetables and fruits with her. Sonia is now better and has also started taking a ‘balanced diet’.

Answer the following questions based on the above situation:

Name the eye defect Sonia is suffering from. What are the two possible reasons related
to her eye defect?

1. What value is displayed by Soya and Sonia?
2. How can this value be promoted? Suggest one activity.

Answer: Myopia/short-sightedness. Lens defect (increased thinness), eyeball defect
(elongation).

Friendship, concern for each other, and appreciation for a balanced diet.

Sharing common things; Helping each other in studies.

Question 21. While boating with parents a child saw a beautiful fish in the lake. He tried to catch it, thinking that it was very close to the boat. His parents told him that the fish were deep in the water and he should not try to catch them. But he did not listen to them and the situation came when he fell into the lake instead of catching it.

Answer the following:

1. Why did the fish appear closer to the boat when it was deep in the water?
2. What values are neglected by the child?

Answer:

This is due to the phenomenon of refraction. Light reflected from the fish travels through the water towards the eye.

As it passes from the water into the air, it refracts away from the normal because water is denser than air and thus, the fish appears closer. Obeying elders, being careful, and understanding reasons for natural phenomena.