Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon? Give reason to justify your answer.

In the early morning, the Sun is low on the horizon. The sunlight has to travel a longer distance through the Earth’s atmosphere to reach us. The atmosphere contains air molecules and dust particles which scatter light. The shorter wavelengths like blue and violet are scattered away, while the longer wavelengths like red and orange reach our eyes. That is why the Sun appears reddish during sunrise. Will this phenomenon be observed by an astronaut on the Moon?No, this will not happen on the Moon. The Moon does not have an atmosphere, so there are no air molecules or dust particles to scatter the sunlight. Since there is no scattering of light, the Sun appears white on the Moon and not reddish.

State the difference in colours of the sun observed during sunrise/sunset and noon. Give explanation for each.

Difference in colours of the Sun during sunrise/sunset and noon: During sunrise and sunset: The Sun appears reddish. This is because the Sun is near the horizon and its light has to travel a longer distance through the Earth's atmosphere. The shorter wavelengths (like blue and violet) get scattered away. The longer wavelengths (like red and orange) reach our eyes, so the Sun looks red. During noon: The Sun appears white or yellowish. This is because the Sun is overhead, and its light travels a shorter distance through the atmosphere. Very little scattering occurs, so all colours of sunlight reach our eyes nearly equally. As a result, the Sun looks white or slightly yellow.

Why is a convex lens called a converging lens?

A convex lens is called a converging lens because it brings parallel rays of light together after they pass through the lens. When parallel rays of light fall on a convex lens, they are bent inwards and meet at a single point on the other side of the lens. This point is called the focus. Because the lens converges (joins) the light rays at one point, it is called a converging lens.

What is hypermetropia?

Hypermetropia is a common eye problem where a person can see distant objects clearly but has difficulty seeing nearby objects clearly. This happens because the eye lens focuses the image behind the retina instead of on it, usually because the eyeball is too short or the lens is not curved enough. People with hypermetropia need a convex lens to help focus the image correctly on the retina so they can see nearby objects clearly.

What is the role of the ciliary muscles?

The ciliary muscles help in changing the shape of the eye lens to focus on objects at different distances. This process is called accommodation. When we look at a nearby object, the ciliary muscles contract, making the lens thicker to increase its converging power. When we look at a distant object, the ciliary muscles relax, making the lens thinner to decrease its converging power.

What is the function of the optic nerve in the human eye?

The optic nerve carries the visual information (images) from the retina to the brain. When light falls on the retina, it creates signals. These signals travel through the optic nerve to the brain. The brain then interprets these signals to form the images we see. So, the optic nerve acts like a messenger that sends the image information from the eye to the brain.

Can visible light be scattered by atoms/molecules in the earth’s atmosphere?

Yes, visible light can be scattered by atoms and molecules in the Earth's atmosphere. This scattering is called Rayleigh scattering. It happens because the tiny particles in the air scatter shorter wavelengths of light (like blue) more than longer wavelengths, which is why the sky looks blue.

Name the three common defects of vision. What are their causes? Name the type of lens used to correct each of them.

1. Myopia (Nearsightedness) Cause: The eyeball is too long or the lens is too curved. Light focuses in front of the retina. Correction: Concave lens (diverging lens) 2. Hypermetropia (Farsightedness) Cause: The eyeball is too short or the lens is too flat. Light focuses behind the retina. Correction: Convex lens (converging lens) 3. Presbyopia Cause: Loss of elasticity of the eye lens due to aging. The eye cannot focus on nearby objects. Correction: Bifocal lens or progressive lens

A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.

The student is suffering from myopia (nearsightedness). Cause: The eyeball is too long or the eye lens is too curved. As a result, light from distant objects focuses in front of the retina. Correction: Myopia is corrected using a concave lens (diverging lens). The concave lens spreads out the light rays so they focus properly on the retina, allowing clear vision of distant objects.

State the role of the eye lenses in the human eye?

Role of the eye lens in the human eye: The eye lens focuses the light coming from objects onto the retina to form a clear image. It is a transparent convex lens. With the help of ciliary muscles, the eye lens can change its shape to focus on objects that are near or far. This is called accommodation. It makes sure that a sharp image is formed on the retina so we can see clearly.

Why are we not able to see things clearly when we come out of a dark room?

When we come out of a dark room, we cannot see things clearly because our eyes need time to adjust to the bright light outside. In the dark, our eyes make a special chemical called rhodopsin that helps us see in low light. When we suddenly go into bright light, this chemical gets destroyed quickly. It takes some time for our eyes to adjust to the bright light, so during that time, our vision is blurry or unclear. This process is called light adaptation.

What is a spectrum? How can we recombine the components of white light after a glass prism has separated them?

A spectrum is the band of seven colours (Violet, Indigo, Blue, Green, Yellow, Orange, Red – VIBGYOR) formed when white light passes through a prism and splits into its component colours. Recombining the components of white light:The components of white light can be recombined by passing the spectrum through a second glass prism placed in an inverted position (opposite to the first prism). This second prism bends the light rays in such a way that they combine again to form white light.

Why do different colours deviate through different angles on passing through a prism?

Different colours bend by different amounts when passing through a prism because each colour has a different wavelength. Shorter wavelengths (like violet) bend more, and longer wavelengths (like red) bend less. This makes the colours spread out at different angles.

Why do we observe random wavering or flicking of the objects near a fire or on a very hot day?

We observe random wavering or flickering of objects near a fire or on a very hot day because of the refraction of light in the hot air. When air gets heated, it becomes less dense and its refractive index changes. Hot air rises and mixes with cooler air, creating layers of air with different temperatures and densities. Light rays bend differently as they pass through these varying layers, causing the image of objects to appear to move or flicker.

Write the function of retina in human eye.

The retina in the human eye catches the light that comes into the eye and changes it into signals. These signals are sent to the brain, which helps us see pictures.

List two causes of hypermetropia.

Here are two causes of hypermetropia: The eyeball is too short from front to back. The eye’s lens or cornea is not curved enough to focus light properly on the retina. Because of these, light focuses behind the retina, making close objects appear blurry.

List the parts of the human eye that control the amount of light entering into it. Explain how they perform this function?

The parts of the eye that control how much light comes in are the iris and the pupil. The iris is the colored part of your eye. It has muscles that make the pupil bigger or smaller. The pupil is the black circle in the middle of the iris. It lets light into the eye. When it’s very bright, the iris makes the pupil smaller to let less light in and protect your eye. When it’s dark, the iris makes the pupil bigger to let more light in so you can see better.

NCERT solutions for Class 10th Science The Human Eye and the Colourful World

Q: 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

The correct answer is (c) ciliary muscles. The ciliary muscles control the change in the focal length of the eye lens.

Q: What is the far point and near point of the human eye with normal vision?

For a human eye with normal vision: Far point is infinity. This means a normal eye can see distant objects clearly, no matter how far they are. Near point is about 25 centimeters from the eye. This is the minimum distance at which the eye can see objects clearly without strain.

Updated on June 1, 2025 | By Learnzy Academy

Q1. What is meant by power of accommodation of the eye?

Power of accommodation of the eye is the ability of the eye to adjust the focal length of its lens to see both nearby and distant objects clearly.

This is done by the ciliary muscles, which change the shape of the eye lens:

For distant objects, the lens becomes thin.
For nearby objects, the lens becomes thick.

A normal human eye can see objects clearly from about 25 cm to infinity.

Q2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

A person with a myopic eye cannot see objects beyond 1.2 m clearly. To correct this defect, a concave lens should be used.

A concave lens helps to diverge the light rays so that they appear to come from the person's far point (1.2 m) and can be focused properly on the retina.

Q3. What is the far point and near point of the human eye with normal vision?

For a human eye with normal vision:

Far point is infinity. This means a normal eye can see distant objects clearly, no matter how far they are.
Near point is about 25 centimeters from the eye.This is the minimum distance at which the eye can see objects clearly without strain.

Q4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

If a student has difficulty reading the blackboard while sitting in the last row, the student is suffering from myopia (short-sightedness).

In myopia, distant objects appear blurry because their images are formed in front of the retina. This defect can be corrected by using a concave lens, which helps to focus the image on the retina and makes distant objects appear clear.

Q5. The human eye can focus on 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

The correct answer is: (b) accommodation

The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This ability is called accommodation.

Q6. The human eye forms the image of an object at its (a) cornea. (b) iris. (c) pupil. (d) retina.

The correct answer is: (d) retina

The human eye forms the image of an object on the retina.

Q7. 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

The correct answer is (c) 25 cm.

The least distance of distinct vision for a young adult with normal vision is about 25 centimeters.

Q8. 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

The correct answer is (c) ciliary muscles.

The ciliary muscles control the change in the focal length of the eye lens.

Q9. 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?

Given -
Power for distant vision = –5.5 dioptres
Power for near vision = +1.5 dioptres

Using the formula:
Power (P) = 100 / focal length (f in cm)
So, focal length (f) = 100 / Power (P)

(i) For distant vision:
f = 100 / (–5.5) = –18.18 cm

(ii) For near vision:
f = 100 / (+1.5) = 66.67 cm

Hence the focal length of the lens required for correcting:

Distant vision = –18.18 cm & Near vision = +66.67 cm

Q10. 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?

The far point of the myopic person is 80 cm.
To correct myopia, a concave lens is used.
Power of the lens = –100 / 80 = –1.25 dioptres.

Hence, the lens needed is a concave lens with power –1.25 dioptres.

Q11. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

A normal eye cannot see objects clearly if they are closer than 25 cm because the eye lens cannot become thick enough to focus the image on the retina. This 25 cm is called the near point, which is the shortest distance at which the eye can see clearly. Objects closer than this appear blurry.

Q12. What happens to the image distance in the eye when we increase the distance of an object from the eye?

When we increase the distance of an object from the eye, the image distance inside the eye stays almost the same because the retina is fixed. The eye changes the shape of the lens to focus the image properly on the retina.

Q13. Why do stars twinkle?

Stars twinkle because their light passes through the Earth’s atmosphere. The atmosphere has layers of air with different temperatures and densities, which bend the light in different directions. This makes the stars look like they are changing brightness and position, causing them to twinkle.

Q14. Explain why the planets do not twinkle.

Planets do not twinkle because they are closer to the Earth and appear as small discs, not points of light like stars. When their light passes through the atmosphere, the bending of light is averaged over the whole disc, so their brightness and position stay steady. That is why planets shine with a steady light and do not twinkle.

Q15. Why does the sky appear dark instead of blue to an astronaut?

The sky appears dark to an astronaut because there is no atmosphere in space to scatter sunlight. On Earth, the atmosphere scatters blue light in all directions, making the sky look blue. But in space, without air or atmosphere, there is no scattering, so the sky looks black or dark.

Q16. Why is Tyndall effect shown by colloidal particles? State four instance of observing the Tyndall effect.

Colloidal particles are large enough to scatter light passing through the mixture. This scattering of light makes the path of light visible, which is known as the Tyndall effect.

Four instances of observing the Tyndall effect:

Sunlight passing through the canopy of a dense forest filled with mist.
A beam of torch light visible in a foggy room.
Sunlight entering a dark room through a window and becoming visible due to dust.
Light passing through a glass of diluted milk.

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