The Human Eye and the Colourful World : Class 10

Why does a distant mountain appear clear to the naked eye, yet a nearby book becomes difficult to read if you hold it too close? Why does the sky look blue, while the Sun often appears reddish at sunrise and sunset? These everyday observations are closely connected with the way our human eye works and the way light behaves in the atmosphere.

The Class 10 Science chapter The Human Eye and the Colourful World brings together two important ideas: the structure and functioning of the eye, and optical phenomena such as refraction, dispersion, atmospheric refraction and scattering of light. The current NCERT Class 10 Science textbook lists this as Chapter 10.

The Human Eye

The human eye is a natural optical instrument that enables us to see objects by detecting light coming from them. It works somewhat like a camera, but its structure and functioning are much more sophisticated.

Light entering the eye is focused on the retina, where light-sensitive cells convert the optical information into electrical signals. These signals are transmitted to the brain through the optic nerve, allowing us to perceive an image.

Main Parts of the Human Eye

The important parts of the eye and their functions are:

Part	Function
Cornea	Transparent curved front surface through which most light enters the eye
Iris	Coloured muscular part that controls the size of the pupil
Pupil	Opening through which light enters the eye
Eye lens	Transparent convex lens that focuses light on the retina
Ciliary muscles	Change the curvature and therefore the focal length of the eye lens
Retina	Light-sensitive screen on which the image is formed
Optic nerve	Carries electrical signals from the retina to the brain
Aqueous humour	Transparent fluid between the cornea and lens
Vitreous humour	Transparent jelly-like substance behind the lens

Cornea

The cornea is the transparent curved outer covering at the front of the eye.

A significant part of the refraction of light entering the eye occurs at the air–cornea interface. The cornea therefore plays an important role in bringing incoming light toward the retina.

Iris and Pupil

The iris is the coloured muscular structure surrounding the pupil.

The pupil is the small opening in the centre of the iris through which light enters the eye.

The iris automatically regulates the amount of light entering the eye:

In bright light, the pupil becomes smaller.
In dim light, the pupil becomes larger.

This protects the retina from excessive light and allows sufficient light to enter when illumination is low.

Eye Lens

The eye lens is a transparent, flexible, crystalline convex lens.

Its principal function is to focus light from an object onto the retina.

Unlike an ordinary glass lens, the eye lens can change its shape because of the action of the ciliary muscles. This ability allows the eye to focus on objects at different distances.

Retina

The retina is a light-sensitive layer at the back of the eye.

The image formed on the retina is:

real
inverted
generally diminished in size

The retina contains light-sensitive cells that respond to light and initiate electrical signals.

Optic Nerve

The optic nerve carries the signals generated by the retina to the brain.

The brain processes these signals and enables us to perceive the objects around us.

How Do We See an Object?

We see an object when light from that object enters our eyes.

For a luminous object such as a bulb or the Sun, the object itself emits light.

For a non-luminous object such as a book or chair, light from an external source falls on the object and is reflected toward our eyes.

The basic sequence is:

Object → Light enters eye → Refraction by eye → Lens focuses light → Image on retina → Signals through optic nerve → Brain interprets image

Thus, vision is not produced by the eye alone. The eye detects the optical information, while the brain interprets the signals.

Power of Accommodation

One of the most important concepts in this chapter is accommodation.

Definition

Accommodation is the ability of the eye lens to change its focal length by changing its curvature so that objects at different distances can be focused sharply on the retina.

The change is produced by the action of the ciliary muscles.

When viewing a distant object

When the object is far away:

ciliary muscles are relatively relaxed,
the eye lens becomes relatively thin,
its focal length increases,
its converging power decreases.

When viewing a nearby object

When the object is brought closer:

ciliary muscles contract,
the eye lens becomes thicker and more curved,
its focal length decreases,
its converging power increases.

The eye therefore adjusts its optical power according to the distance of the object.

Near Point and Far Point

Two important terms related to accommodation are the near point and far point.

Near Point

The near point is the minimum distance at which an object can be seen clearly and comfortably by a normal eye.

For a normal adult eye, the near point is approximately 25 cm from the eye.

It is also called the least distance of distinct vision.

Far Point

The far point is the farthest point that can be seen clearly by a normal eye without accommodation.

For a normal eye, the far point is considered to be infinity.

Therefore:

Point	Normal eye
Near point	Approximately 25 cm
Far point	Infinity

Defects of Vision

Sometimes the eye cannot focus an object sharply on the retina. Such conditions are called defects of vision or refractive errors.

The important defects discussed at this level are:

Myopia
Hypermetropia
Presbyopia

Cataract is another important eye condition, but it is different from these refractive errors.

Myopia — Near-Sightedness

Myopia is a defect of vision in which a person can see nearby objects clearly but has difficulty seeing distant objects clearly.

It is commonly called short-sightedness or near-sightedness.

What happens in myopia?

For a distant object, the parallel rays entering the eye are focused in front of the retina instead of directly on the retina.

This may happen because:

the eyeball is too long, or
the converging power of the eye lens is too high.

Correction of Myopia

Myopia is corrected using a concave lens of suitable power.

A concave lens diverges the incoming rays before they enter the eye. The eye lens can then focus them correctly on the retina.

Myopia → Concave lens

Important memory point

A myopic eye sees near objects clearly but distant objects appear blurred.

Hypermetropia — Far-Sightedness

Hypermetropia is a defect of vision in which a person can see distant objects clearly but has difficulty seeing nearby objects clearly.

It is commonly called long-sightedness or far-sightedness.

What happens in hypermetropia?

Light from a nearby object would be focused behind the retina if the eye did not accommodate sufficiently.

This may happen because:

the eyeball is too short, or
the converging power of the eye lens is insufficient.

Correction of Hypermetropia

Hypermetropia is corrected using a convex lens of suitable power.

A convex lens converges the incoming rays before they enter the eye, allowing the eye to form the image on the retina.

Hypermetropia → Convex lens

Myopia and Hypermetropia Compared

Feature	Myopia	Hypermetropia
Common name	Near-sightedness	Far-sightedness
Nearby objects	Seen clearly	Difficult to see clearly
Distant objects	Difficult to see clearly	Usually seen clearly
Image tendency	In front of retina	Behind retina
Correcting lens	Concave	Convex
Lens action needed	Diverging	Converging

A very useful memory rule is:

Myopia → Concave

Hypermetropia → Convex

Presbyopia

Presbyopia is an age-related defect of vision.

As a person becomes older:

the ciliary muscles may gradually lose their effectiveness,
the eye lens may become less flexible,
the accommodation ability decreases.

As a result, the person may have difficulty seeing nearby objects clearly.

Correction of Presbyopia

Depending on the visual problem, presbyopia may be corrected using:

a convex lens for difficulty with near vision,
bifocal lenses when both near and distant vision require correction,
or other suitable corrective lenses prescribed according to the person’s vision.

Bifocal Lens

A bifocal lens has two different optical powers in different portions of the lens.

Traditionally, one portion is used for distant vision and the other for near vision.

Presbyopia, Myopia and Hypermetropia

Defect	Main problem	Common correction
Myopia	Distant objects unclear	Concave lens
Hypermetropia	Nearby objects unclear	Convex lens
Presbyopia	Reduced accommodation with age	Suitable convex/bifocal or other prescribed correction

A person may have more than one refractive problem. Therefore, the appropriate corrective lens depends on the actual visual condition.

Cataract

A cataract occurs when the eye lens becomes cloudy or opaque, causing vision to become blurred.

It is not simply another form of myopia or hypermetropia.

In significant cataract, the cloudy lens may be surgically replaced with an artificial intraocular lens.

Why Do We Have Two Eyes?

Humans have two eyes partly because binocular vision provides important advantages.

The two eyes view an object from slightly different positions. The brain combines the two views to help estimate:

depth,
relative distance,
and three-dimensional relationships.

This is known as binocular vision.

Having two eyes also provides a wider field of view than using only one eye.

Refraction of Light Through a Prism

A prism is a transparent optical element with plane, polished surfaces that are not parallel to each other.

When light passes through a prism, it undergoes refraction.

The direction of light changes because the speed of light changes when it moves from one transparent medium to another.

A prism can also separate white light into its component colours.

$refraction through glass prism$

Dispersion of White Light

The splitting of white light into its constituent colours when it passes through a prism is called dispersion of light.

The familiar sequence of colours is:

Violet → Indigo → Blue → Green → Yellow → Orange → Red

It is commonly remembered as VIBGYOR.

The band of colours produced is called the spectrum.

Why does dispersion occur?

White light contains different wavelengths.

When light passes through a prism, different colours undergo different amounts of refraction.

Consequently, they emerge in different directions and become separated.

Which colour deviates most?

Violet deviates the most.

Which colour deviates least?

Red deviates the least.

Thus:

Maximum deviation → Violet

Minimum deviation → Red

Spectrum

A spectrum is the band of colours obtained when white light is dispersed.

The visible spectrum consists approximately of:

Violet, Indigo, Blue, Green, Yellow, Orange and Red.

The colours do not all have the same wavelength.

Among the visible colours:

red has the longest wavelength,
violet has the shortest wavelength.

This difference is closely related to their different degrees of refraction and scattering.

Why Does a Prism Produce Colours?

A common misconception is that a prism creates colours.

It does not create the colours.

White light already contains the different components of visible light. The prism separates them because different wavelengths undergo different amounts of refraction.

This phenomenon is called dispersion.

Formation of a Rainbow

A rainbow is a natural optical phenomenon associated with sunlight and water droplets in the atmosphere.

A raindrop can act approximately like a tiny prism.

When sunlight enters a water droplet:

Light is refracted as it enters the droplet.
White light undergoes dispersion into different colours.
Light undergoes internal reflection inside the droplet.
The reflected light is refracted again as it emerges from the droplet.

A large number of water droplets together can produce the coloured arc observed as a rainbow.

The order of colours in a primary rainbow is generally:

Red on the outer side → Violet on the inner side

Atmospheric Refraction

The atmosphere is not optically uniform.

Air near the Earth’s surface generally has greater density than air at higher altitudes, and therefore its optical properties vary gradually with height.

As light travels through these layers, it can undergo continuous bending. This phenomenon is called atmospheric refraction.

It explains several interesting observations.

Twinkling of Stars

Stars appear to twinkle because their light passes through continuously changing layers of the atmosphere.

The atmosphere contains moving regions with different temperatures and densities. Therefore, the refractive conditions along the light path keep changing.

As a result:

the apparent position of the star changes slightly,
the apparent brightness also fluctuates.

This produces the familiar twinkling effect.

Why do planets usually not twinkle as much?

Stars are extremely distant and appear approximately as point sources of light.

Planets, however, are much closer and appear as small extended sources. Light reaches us from different parts of their apparent discs, so fluctuations from different paths tend to average out.

Therefore, planets generally show much less noticeable twinkling than stars.

Advanced Sunrise and Delayed Sunset

Due to atmospheric refraction, we can sometimes see the Sun slightly before it has actually risen above the horizon.

Similarly, the Sun can remain visible for a short time after it has actually gone below the horizon.

This happens because sunlight is bent by the atmosphere.

Apparent Position of the Sun

Because of atmospheric refraction, the Sun appears slightly higher than its actual geometric position when it is close to the horizon.

Therefore:

apparent sunrise occurs slightly earlier,
apparent sunset occurs slightly later.

This effect contributes to the slightly longer apparent duration of daylight.

Why Does the Sky Appear Blue?

The blue colour of the daytime sky is explained by the scattering of light by molecules and very small particles in the atmosphere.

Sunlight contains different wavelengths of visible light.

Shorter wavelengths, particularly in the blue-violet region, are scattered more strongly than longer wavelengths such as red.

Although violet is scattered strongly too, the colour we perceive as the sky’s dominant short-wavelength colour is generally described as blue because of the combined effects of the spectrum, solar intensity and the sensitivity of human vision.

Thus:

Greater scattering of shorter wavelengths → blue appearance of the sky

Scattering of Light

Scattering of light is the phenomenon in which light is redirected in different directions when it interacts with small particles or molecules.

The amount of scattering depends on factors including wavelength and particle size.

For very small particles compared with the wavelength of light, shorter wavelengths are scattered much more strongly than longer wavelengths.

This wavelength dependence helps explain the blue sky and the reddish appearance of the Sun near the horizon.

Why Does the Sun Appear Red at Sunrise and Sunset?

At sunrise and sunset, sunlight travels through a longer path in the atmosphere before reaching our eyes.

During this longer journey, shorter wavelengths such as blue are scattered away from the direct path more strongly.

The remaining direct sunlight reaching the observer contains a relatively larger proportion of longer wavelengths, especially red and orange.

Therefore, the Sun can appear:

reddish
orange
or sometimes deep red

near sunrise and sunset.

Key idea

Longer atmospheric path → greater scattering of shorter wavelengths → red/orange appearance of the Sun

Why Is the Sky Not Usually Red at Noon?

Around midday, when the Sun is relatively high in the sky, sunlight travels through a comparatively shorter atmospheric path before reaching the observer.

Less of the blue component is removed from the direct sunlight by scattering compared with sunrise or sunset.

The scattered blue light reaching our eyes from different directions makes the overhead sky appear predominantly blue.

Atmospheric Refraction vs Scattering

Students often confuse these two phenomena.

Feature	Atmospheric Refraction	Scattering of Light
Basic idea	Bending of light due to changing optical conditions	Redirection of light by particles/molecules
Main cause	Gradual variation in refractive index of atmospheric layers	Interaction of light with particles/molecules
Important example	Twinkling of stars	Blue sky
Also explains	Apparent early sunrise and delayed sunset	Reddish Sun at sunrise/sunset

Important Optical Phenomena at a Glance

Phenomenon	Main explanation
Accommodation	Change in focal length of eye lens
Myopia	Corrected using a concave lens
Hypermetropia	Corrected using a convex lens
Presbyopia	Age-related reduction in accommodation
Dispersion	Splitting of white light into colours
Rainbow	Refraction + dispersion + internal reflection in water droplets
Twinkling of stars	Atmospheric refraction
Early sunrise	Atmospheric refraction
Delayed sunset	Atmospheric refraction
Blue sky	Scattering of shorter wavelengths
Red/orange Sun at sunrise/sunset	Greater scattering during a longer atmospheric path

Important Differences Students Must Remember

Dispersion vs Scattering

Dispersion	Scattering
Separation of different wavelengths/colours	Redirection of light by particles
Commonly demonstrated using a prism	Commonly observed in the atmosphere
Produces a spectrum from white light	Helps explain the blue sky
Different colours refract by different amounts	Shorter wavelengths are scattered more strongly by very small atmospheric particles

Refraction vs Dispersion

Refraction is the change in direction of light when it passes from one medium to another due to a change in its speed.

Dispersion is the separation of different wavelengths because they undergo different amounts of refraction.

Therefore, dispersion involves refraction, but ordinary refraction does not necessarily produce visible colour separation.

Important Formulas Related to the Eye and Lenses

The chapter is closely connected with the earlier concepts of lens power and the lens formula.

Power of a Lens

The power of a lens is: $P = \frac{1}{f}$

where:

P = power of lens in dioptre (D)
f = focal length in metre (m)

For a convex lens, power is positive.

For a concave lens, power is negative.

Example

Suppose a concave lens has focal length: $f=-0.5\,m$

Then: $P=\frac{1}{-0.5}=-2D$

Therefore, its power is: $\boxed{-2D}$

The negative sign indicates a concave lens.

Lens Formula

For a thin lens, the relationship between object distance u, image distance v, and focal length f is:

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

The signs of u, v, and f must be assigned according to the adopted Cartesian sign convention.

Students should not substitute numerical values without first applying the correct signs.

Common Misconceptions

Misconception 1: The pupil is a lens.

Incorrect.

The pupil is an opening through which light enters the eye. The eye lens performs the focusing.

Misconception 2: The iris forms the image.

Incorrect.

The iris regulates the amount of light entering through the pupil. The eye lens helps focus light on the retina.

Misconception 3: Myopia is corrected using a convex lens.

Incorrect.

Myopia is corrected using a concave lens.

Misconception 4: Hypermetropia is corrected using a concave lens.

Incorrect.

Hypermetropia is corrected using a convex lens.

Misconception 5: A prism creates the colours of white light.

Incorrect.

The different colours are already components of white light. The prism separates them through dispersion.

Misconception 6: Stars twinkle because they produce changing amounts of light.

Incorrect.

Their apparent brightness fluctuates mainly because their light passes through changing atmospheric conditions.

Misconception 7: The blue sky is caused by blue light being produced by the atmosphere.

Incorrect.

The atmosphere does not generate the blue colour. It scatters shorter-wavelength components of sunlight more strongly.

Misconception 8: All eye defects are caused by the same problem.

Incorrect.

Different defects can arise from different changes in the eyeball, lens power, or accommodation ability and therefore require different corrections.

Examination-Oriented Key Points

Accommodation is the ability of the eye lens to change its focal length to focus objects at different distances.
The normal near point of an adult eye is approximately 25 cm.
The far point of a normal eye is infinity.
Myopia is corrected by a concave lens.
Hypermetropia is corrected by a convex lens.
Presbyopia is associated with ageing and reduced accommodation.
A bifocal lens can provide different powers for near and distant vision.
Dispersion is the splitting of white light into its component colours.
Violet deviates the most through a prism.
Red deviates the least through a prism.
A rainbow involves refraction, dispersion and internal reflection of sunlight in water droplets.
Twinkling of stars is due to atmospheric refraction.
Early sunrise and delayed sunset are consequences of atmospheric refraction.
The blue appearance of the sky is explained by scattering of light.
The reddish appearance of the Sun near sunrise and sunset is associated with stronger scattering of shorter wavelengths over a longer atmospheric path.

Quick Revision Notes

The human eye is a natural optical instrument that detects light and enables vision.
The cornea is the transparent curved front surface of the eye.
The iris controls the size of the pupil and therefore regulates the amount of entering light.
The eye lens focuses light on the retina.
The retina is the light-sensitive layer where the image is formed.
The optic nerve carries signals from the retina to the brain.
Accommodation is the ability of the eye lens to change its focal length.
Normal near point = 25 cm approximately.
Normal far point = infinity.
Myopia → Concave lens.
Hypermetropia → Convex lens.
Presbyopia → reduced accommodation due to ageing.
Dispersion separates white light into different colours.
The colour sequence is VIBGYOR.
Violet deviates most and red deviates least through a prism.
A rainbow results from refraction, dispersion and internal reflection in water droplets.
Twinkling of stars → atmospheric refraction.
Early sunrise and delayed sunset → atmospheric refraction.
Blue sky → scattering of shorter wavelengths.
Red/orange Sun near sunrise and sunset → longer atmospheric path and stronger scattering of shorter wavelengths.
Convex lens → positive power.
Concave lens → negative power.
Lens power is given by $P=1/f$, with $f$ measured in metres.
Refraction changes the direction of light; dispersion separates wavelengths; scattering redirects light in different directions.

Practice MCQs

1. Which part of the human eye controls the amount of light entering the eye?

A. Retina
B. Iris
C. Optic nerve
D. Eye lens

Correct Answer: B. Iris

Explanation: The iris controls the size of the pupil and therefore regulates the amount of light entering the eye.

2. The image formed on the retina of a normal eye is:

A. Virtual and erect
B. Virtual and inverted
C. Real and inverted
D. Real and erect

Correct Answer: C. Real and inverted

Explanation: The eye lens forms a real, inverted image on the retina. The brain processes the signals so that we perceive the scene correctly.

3. The ability of the eye lens to change its focal length is called:

A. Accommodation
B. Dispersion
C. Persistence
D. Scattering

Correct Answer: A. Accommodation

Explanation: Accommodation enables the eye to focus sharply on objects at different distances by changing the curvature of its lens.

4. The least distance of distinct vision for a normal adult eye is approximately:

A. 2.5 cm
B. 25 cm
C. 2.5 m
D. 250 cm

Correct Answer: B. 25 cm

Explanation: The conventional near point of a normal adult eye is approximately 25 cm.

5. Myopia is corrected using a:

A. Convex mirror
B. Convex lens
C. Plane mirror
D. Concave lens

Correct Answer: D. Concave lens

Explanation: A concave lens diverges incoming rays so that the eye can focus them correctly on the retina.

6. In a myopic eye, light from a distant object tends to be focused:

A. In front of the retina
B. Exactly on the retina
C. Behind the retina
D. On the cornea

Correct Answer: A. In front of the retina

Explanation: In myopia, the eye has excessive converging power or excessive axial length, causing distant-object images to form in front of the retina.

7. Hypermetropia is generally corrected using a:

A. Concave lens
B. Convex lens
C. Plane glass plate
D. Concave mirror

Correct Answer: B. Convex lens

Explanation: A convex lens provides additional convergence, helping the eye form the image of a nearby object on the retina.

8. Which condition is mainly associated with reduced accommodation due to ageing?

A. Myopia
B. Hypermetropia
C. Presbyopia
D. Dispersion

Correct Answer: C. Presbyopia

Explanation: Presbyopia develops with age as the accommodation ability of the eye decreases.

9. Which colour deviates the most when white light passes through a prism?

A. Red
B. Yellow
C. Green
D. Violet

Correct Answer: D. Violet

Explanation: Violet has the shortest wavelength among the visible colours and undergoes the greatest deviation in a prism.

10. Which colour deviates the least through a prism?

A. Red
B. Violet
C. Blue
D. Indigo

Correct Answer: A. Red

Explanation: Red light undergoes the least deviation among the visible colours.

11. The splitting of white light into its constituent colours is called:

A. Reflection
B. Dispersion
C. Accommodation
D. Diffraction

Correct Answer: B. Dispersion

Explanation: Dispersion occurs because different wavelengths of light undergo different amounts of refraction.

12. The twinkling of stars is primarily caused by:

A. Dispersion by clouds
B. Reflection from the Moon
C. Atmospheric refraction
D. Scattering by the retina

Correct Answer: C. Atmospheric refraction

Explanation: Changing atmospheric conditions continuously alter the refraction of starlight, causing fluctuations in apparent position and brightness.

13. Planets generally appear to twinkle less than stars because planets:

A. Do not emit or reflect light
B. Are closer and appear as extended sources
C. are always below the atmosphere
D. have no apparent angular size

Correct Answer: B. Are closer and appear as extended sources

Explanation: Light from different parts of the apparent disc of a planet undergoes atmospheric fluctuations that tend to average out.

14. The blue appearance of the sky is mainly due to:

A. Reflection of blue oceans
B. Dispersion by the Moon
C. Scattering of shorter wavelengths
D. Refraction by clouds only

Correct Answer: C. Scattering of shorter wavelengths

Explanation: Atmospheric molecules scatter shorter wavelengths of sunlight more strongly, producing the characteristic blue appearance of the sky.

15. The Sun often appears reddish at sunrise and sunset because:

A. The Sun becomes physically cooler
B. Red light is completely absorbed by the atmosphere
C. The atmosphere produces red light
D. Shorter wavelengths are scattered more strongly over the longer path

Correct Answer: D. Shorter wavelengths are scattered more strongly over the longer path

Explanation: Near the horizon, sunlight travels through a greater thickness of atmosphere, causing stronger scattering of shorter wavelengths and leaving relatively more red/orange light in the direct beam.

16. Which combination is involved in the formation of a rainbow?

A. Reflection only
B. Refraction, dispersion and internal reflection
C. Scattering only
D. Diffraction and absorption only

Correct Answer: B. Refraction, dispersion and internal reflection

Explanation: Sunlight entering a raindrop is refracted and dispersed, undergoes internal reflection, and is refracted again as it leaves.

17. If the focal length of a lens is $-0.5\,m$, its power is:

A. $+2D$
B. $-0.5D$
C. $-2D$
D. $+0.5D$

Correct Answer: C. $-2D$

Explanation: Using $P=1/f$, $P=1/(-0.5)=-2D$. The negative sign indicates a concave lens.

18. Which statement correctly distinguishes refraction from dispersion?

A. Refraction always produces a rainbow
B. Dispersion is the bending of only red light
C. Refraction changes the direction of light, while dispersion separates different wavelengths
D. Dispersion occurs only in mirrors

Correct Answer: C. Refraction changes the direction of light, while dispersion separates different wavelengths

Explanation: Dispersion results from the different degrees of refraction experienced by different wavelengths.

19. Which statement about the normal human eye is correct?

A. Its far point is approximately 25 cm
B. Its near point is infinity
C. Its near point is approximately 25 cm and far point is infinity
D. Both near and far points are 25 cm

Correct Answer: C. Its near point is approximately 25 cm and far point is infinity

Explanation: For a normal adult eye, the conventional near point is about 25 cm, while the far point is at infinity.

20. A student says, “A prism creates the colours present in white light.” Which is the best correction?

A. A prism absorbs all colours except red
B. A prism separates the different wavelengths already present in white light
C. A prism converts infrared radiation into visible light
D. A prism produces colours through reflection only

Correct Answer: B. A prism separates the different wavelengths already present in white light

Explanation: White light already contains different visible wavelengths. A prism separates them because they undergo different amounts of refraction.

Frequently Asked Questions (FAQs)

1. What is accommodation of the eye?

Accommodation is the ability of the eye lens to change its focal length so that objects at different distances can be focused clearly on the retina. This is achieved by changes in the curvature of the lens due to the action of the ciliary muscles.

2. What is the normal near point of the human eye?

The near point of a normal adult eye is approximately 25 cm. An object placed closer than this generally cannot be focused sharply and comfortably by a normal eye.

3. Why is myopia corrected by a concave lens?

In myopia, light from a distant object tends to focus in front of the retina. A concave lens diverges the incoming light slightly before it enters the eye, shifting the final focus backward onto the retina.

4. Why is hypermetropia corrected by a convex lens?

In hypermetropia, the image of a nearby object tends to form behind the retina. A convex lens provides additional convergence, helping the eye bring the image onto the retina.

5. Why do stars twinkle but planets usually do not?

Stars are extremely distant and appear almost like point sources. Atmospheric conditions along their light paths fluctuate, causing noticeable changes in apparent brightness. Planets appear as extended sources, so fluctuations from different parts tend to average out.

6. Why does the sky appear blue?

The atmosphere scatters shorter wavelengths of sunlight more strongly than longer wavelengths. The scattered light reaching our eyes from different directions therefore contains a strong short-wavelength component, giving the daytime sky its characteristic blue appearance.

7. Why does the Sun look red during sunrise and sunset?

At these times, sunlight travels through a longer atmospheric path. Shorter wavelengths are scattered away more strongly, so the direct sunlight reaching the observer is relatively richer in longer wavelengths such as red and orange.

8. What is the difference between dispersion and scattering?

Dispersion separates different wavelengths of light, such as when white light passes through a prism. Scattering redirects light in different directions when it interacts with particles or molecules.

9. Why does violet deviate more than red in a prism?

Different wavelengths have different refractive indices in a transparent material. Violet light is refracted more strongly than red light, so violet undergoes greater deviation through the prism.

10. What causes presbyopia?

Presbyopia is primarily associated with ageing and a reduction in the eye’s accommodation ability. The eye lens becomes less flexible and the ciliary muscles may become less effective, making near vision difficult.

Conclusion

The chapter The Human Eye and the Colourful World connects the biology of vision with some of the most familiar optical phenomena around us. The eye focuses light on the retina through the process of accommodation, while defects such as myopia, hypermetropia and presbyopia explain why different people may need different corrective lenses.

The second part of the chapter shows how light behaves in the atmosphere. Dispersion explains the separation of colours, atmospheric refraction explains phenomena such as the twinkling of stars and apparent changes in sunrise and sunset, while scattering of light helps explain the blue sky and reddish Sun near the horizon.

For effective revision, remember the central connections: Myopia → concave lens, Hypermetropia → convex lens, twinkling → atmospheric refraction, blue sky → scattering, and rainbow → refraction + dispersion + internal reflection. Understanding these relationships is far more useful than memorising the facts as isolated statements.