The Human Eye and the Colourful World – Notes, MCQs, Quiz & Worksheet

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.

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.

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

The correct answer is (c) 25 cm.

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

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.

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.

The ciliary muscleshelp 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.

The parts of the eye that control how much light comes in are the iris and the pupil.

1. The iris is the colored part of your eye. It has muscles that make the pupil bigger or smaller.
2. 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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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:

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

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.

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.

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.

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.

Here are two causes of hypermetropia:

1. The eyeball is too short from front to back.
2. 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.

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.

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.

The correct answer is: (d) retina

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

The correct answer is (c) ciliary muscles.

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

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.

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.

For a human eye with normal vision:

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