The Human Eye

The human eye is like a camera.
Its lens system forms an image on a light-sensitive screen called the retina.
The human eye works on the refraction of light through a natural convex lens made of transparent living material and enables us to see things around us.

Construction of the eye

Cornea

The front part of the eye is called the cornea.
It is made of a transparent substance and is bulging outwards.
The outer surface of the cornea is convex in shape.
Light coming from objects enters the eye through the cornea.

Iris

Just behind the cornea is the iris (or coloured diaphragm).
Iris is a flat, coloured, ring-shaped membrane behind the cornea of the eye.
There is a hole in the middle of the iris called the pupil.
Thus, the pupil is a hole in the middle of the iris. The pupil appears black because no light is reflected from it.

Eye Lens

The eye lens is a convex lens made of a transparent, soft and flexible material like a jelly made of proteins.
Being flexible, the eye lens can change its shape (it can become thin or thick) to focus light on the retina.
The eye lens is held in position by suspensory ligaments.
One end of the suspensory ligaments is attached to the eye lens and the other end is attached to the ciliary muscles.

Ciliary Muscles

Ciliary muscles change the thickness of the eye lens while focusing.
In other words, the focal length of the eye lens (and hence its converging power) can be changed by changing its shape through the action of ciliary muscles.

Retina

The screen on which the image is formed in the eye is called the retina.
The retina is behind the eye lens and at the back part of the eye.
The retina of an eye is just like the film in a camera.
It is a delicate membrane having a large number of light-sensitive cells called:
Rods – respond to the intensity of light.
Cones – respond to the colour of objects.
These cells generate electrical signals.

Blind Spot

At the junction of the optic nerve and retina, there are no light-sensitive cells (no rods or cones).
Therefore, no vision is possible at that spot.
This point is called the blind spot.
Thus, the blind spot is a small area of the retina insensitive to light, where the optic nerve leaves the eye.
If the image of an object falls on the blind spot, it cannot be seen.

Aqueous Humour

The space between the cornea and the eye lens is filled with a watery liquid called the aqueous humour.

Vitreous Humour

The space between the eye lens and the retina is filled with a transparent jelly-like substance called the vitreous humour.
The vitreous humour supports the back of the eye.

Working of the eye

The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.
The light rays coming from an object enter the eye through the cornea, pass through the pupil, and fall on the eye lens. The eye lens is a convex lens, so it converges the light rays and forms a real and inverted image of the object on the retina.
Actually, the outer surface of the cornea also acts as a convex lens and converges most of the light entering the eye. The eye lens performs only the final convergence to focus the image exactly on the retina.
The image formed on the retina is conveyed to the brain through the optic nerve, producing the sensation of vision.
The retina contains a large number of light-sensitive cells. When the image falls on the retina, these cells become active and generate electrical signals. These signals are transmitted to the brain through the optic nerve. Although the image formed on the retina is inverted, the brain interprets it as an erect image.

Function of Iris and Pupil

The iris controls the amount of light entering the eye by adjusting the size of the pupil according to the intensity of light.
When the light entering the eye is bright (during daytime), the iris contracts the pupil, making it small, so that less light enters the eye.
When the light is dim (at night or in a dark room), the iris expands the pupil, making it large, so that more light enters the eye.

Adjustment of the Pupil

The adjustment of the pupil does not occur instantly; it takes some time.

From bright light to darkness:

Initially, we cannot see clearly because the pupil is small.
After a short time, the pupil expands, allowing more light to enter, and vision improves.

From darkness to bright light:

Initially, we experience glare because the pupil is large.
Gradually, the pupil contracts, reducing the amount of light entering the eye and protecting it from bright light.

Power of Accomodation

Accommodation is the ability of the human eye to focus both distant and nearby objects on the retina by changing the focal length (or converging power) of the eye lens.

When looking at a distant object (at infinity)

Ciliary muscles are fully relaxed.
Suspensory ligaments become tight.
Eye lens is stretched and becomes thin (less convex).
Thin lens has large focal length and Small converging power
It focuses parallel rays from distant objects on the retina.
In this condition, the eye is said to be unaccommodated.

When looking at a nearby object

Ciliary muscles contract.
Suspensory ligaments become loose.
Eye lens bulges due to its elasticity and becomes thick (more convex).
Thick lens has short focal length and large converging power
It focuses diverging rays from nearby objects on the retina.
In this condition, the eye is said to be accommodated.

The focal length of the eye lens cannot be decreased below a certain minimum limit.

The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision.

It is also called the near point of the eye.

For a young adult with normal vision, the near point is about 25 cm.

The farthest point upto which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.

A normal eye can see objects clearly that are between 25 cm and infinity.

Defects of Vision and their Correction

There are three common defects of vision (or defects of eye). These are:

Myopia (Short-sightedness or Near-sightedness)
Hypermetropia (Long-sightedness or Far-sightedness)
Presbyopia

Myopia

Myopia is also known as near – sightedness.
A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
A person with this defect has the far point nearer than infinity.
In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
This defect may arise due to excessive curvature of the eye lens, or elongation of the eyeball.

This defect can be corrected by using a concave lens of suitable power.
Please note that the concave lens used for correcting myopia should be of such a focal length (or power) that it produces a virtual image of the distant object (lying at infinity) at the far point of the myopic eye.
The whole purpose of using a concave lens is to reduce the converging power of the eye-lens
The concave lens used here decreases the converging power of the eye-lens and helps in forming the image of the distant object on the retina of the myopic eye.

Hypermetropia

Hypermetropia is also known as far-sightedness.
A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
The near point, for the person, is farther away from the normal near point (25 cm).
This is because the light rays from a closeby object are focussed at a point behind the retina.
This defect arises either because the focal length of the eye lens is too long, or the eyeball has become too small.

This defect can be corrected by using a convex lens of appropriate power.
Please note that the convex lens used for correcting hypermetropia (or long-sightedness) should be of such a focal length (or power) that it forms a virtual image of the object (placed at the normal near point N of 25 cm), at the near point N’ of the hypermetropic eye.
The whole purpose of using a convex lens here is to increase the converging power of the eye-lens.
The convex lens used in spectacles increases the converging power of the eye-lens and helps in forming the image of a nearby object on the retina of the eye.

Presbyopia

In old age, due to ciliary muscles becoming weak and the eye-lens becoming inflexible (or rigid), the eye loses its power of accommodation.
Because of this, an old person cannot see the nearby objects clearly. This leads to the defect called presbyopia.
Presbyopia is that defect of vision due to which an old person cannot see the nearby objects clearly due to loss of power of accommodation of the eye.
A person suffering from myopia as well as hypermetropia uses spectacles having bifocal lenses, in which:
The upper part consists of a concave lens (to correct myopia) used for distant vision.
The lower part consists of a convex lens (to correct hypermetropia) used for reading purposes

Refraction of light through a Prism

Here PE is the incident ray, EF is the refracted ray and FS is the
emergent ray.
ray of light is entering from air to glass at the first surface AB.
The light ray on refraction has bent towards the normal.
At the second surface AC, the light ray has entered from
glass to air. Hence it has bent away from normal.
The peculiar shape of the prism makes the emergent ray bend at an
angle to the direction of the incident ray. This angle is called the angle of
deviation. In this case ∠D is the angle of deviation.

Dispersion of White light by a glass prism

The splitting up of white light into seven colours on passing through a transparent medium such as a glass prism is called dispersion of light.
The band of seven colours formed on a white screen when a beam of white light is passed through a glass prism is called the spectrum of white light.
The seven colours of the spectrum are:

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

Explanation of Dispersion of light

The dispersion of white light occurs because the colours of white light travel at different speeds through the glass prism.
The amount of refraction, or bending, depends on the speed of coloured light in glass.
Since the different colours travel at different speeds, they are refracted or bent by different angles on passing through the glass prism. Some colours are bent less, whereas others are bent more.
Therefore, when white light consisting of seven colours falls on a glass prism, each colour is refracted or deviated by a different angle. As a result, the seven colours spread out to form a spectrum.
The red colour has the maximum speed in the glass prism, so it is deviated the least.
The violet colour has the minimum speed in the glass prism, so it is deviated the most.
Due to this, the red colour forms the upper part of the spectrum, while the violet colour forms the lower part.

Recombination of the Spectrum of white light

The first glass prism PQR disperses (splits) the white light into seven coloured rays. The second prism P’Q’R’ receives all the seven coloured rays from the first prism and recombines them into the original white beam of light which falls on the screen S.

The recombination of the seven colours, produced by the first prism, is due to the fact that the second prism has been placed in the reversed position due to which the refraction produced by the second prism is equal and opposite to that produced by the first prism.

Rainbow formation

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

Atmospheric Refraction

When light goes from one medium to another medium having different optical densities, then refraction of light rays (or bending of light rays) takes place.
Now, in the atmosphere, we have air everywhere. But all the air in the atmosphere is not at the same temperature.
Some of the air layers of the atmosphere are cold whereas other air layers of the atmosphere are comparatively warm (or hotter).
The cooler air layers of the atmosphere behave as optically denser medium for light rays whereas the warmer air layers (or hotter air layers) of the atmosphere behave as optically rarer medium for the light rays.
When light rays pass through the atmosphere having air layers of different optical densities, then refraction of light takes place.

If we look at objects through the hot air over a fire, the objects appear to be moving slightly. This can be explained as follows:

The air just above the fire becomes hotter (than the air further up)
This hotter air is optically rarer but the colder air further up is optically denser.
So, when we see the objects by the light coming from them through hot and cold air layers having different optical densities, then refraction of light takes place randomly due to which the objects appear to be moving slightly.

The Stars seem higher than they actually are

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 atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position.
The star appears slightly higher (above) than its actual position when viewed near the horizon

Twinkling of Stars

When the light coming from a star enters the earth’s atmosphere, it undergoes refraction due to the varying optical densities of air at various altitudes.
The atmosphere is continuously changing (due to which the optical densities of air at different levels in the atmosphere keep on changing). The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next.
When the atmosphere refracts more star-light towards us, the star appears to be bright and when the atmosphere refracts less star-light, then the star appears to be dim.
In this way, the star-light reaching our eyes increases and decreases continuously due to atmospheric refraction. And the star appears to twinkle at night.

Why don’t the planets twinkle ?

The planets are much closer to the earth, and are thus seen as extended sources
A planet can be considered to be a collection of a very large number of point sources of light.
The dimming effect produced by some of the point sources of light in one part of the planet is nullified by the brighter effect produced by the point sources of light in its other part.
Thus, on the whole, the brightness of a planet always remains the same and hence it does not appear to twinkle.

Advance 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.
The actual sunrise takes place when the sun is just above the horizon. But due to refraction of sunlight caused by the atmosphere, we can see the rising sun about 2 minutes before it is actually above the horizon.
When the sun is slightly below the horizon, then the sun’s light coming from less dense air to more dense air is refracted downwards as it passes through the atmosphere.
Because of this atmospheric refraction, the sun appears to be raised above the horizon when actually it is slightly below the horizon.
It is also due to atmospheric refraction that we can still see the sun for about 2 minutes even after the sun has set below the horizon.

Scattering of Light

Scattering of light means to throw light in various random directions.
Light is scattered when it falls on various types of suspended particles in its path.
Depending on the size of particles, the scattering can be of white sunlight as such or of the coloured lights which make up the white sunlight.

Tyndall Effect

The earth’s atmosphere is a heterogeneous mixture of minute particles.
These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air.
When a beam of light strikes such fine particles, the path of the beam becomes visible.
The light reaches us, after being reflected diffusely by these particles.
The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect.
This phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole. Thus, scattering of light makes the particles visible.
Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light.

The colour of the scattered light depends on the size of the scattering particles.
Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths.
If the size of the scattering particles is large enough, then, the scattered light may even appear white.

Why is the colour of the clear Sky Blue?

The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light.
These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end.
The red light has a wavelength about 1.8 times greater than blue light.
Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes.

If the earth had no atmosphere, there would not have been any scattering.
Then, the sky would have looked dark.
The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights.

Red Colour of Sun at Sunrise and Sunset

At the time of sunrise and sunset, the light from sun has to travel the longest distance of atmosphere to reach the observer.
Since the blue light of short wavelength is scattered more, much of it is lost, while the red light of long wavelength is scattered a little, so it is not much lost.
Thus blue light is almost absent in sunlight reaching the observer and only the red (white − blue = red) light reaches us.
As a result, the sun appears red at sunrise and sunset.

Use of Red Light for the Danger Signal

In the visible light, the wavelength of red light is longest, therefore the light of red colour is scattered least by the air molecules of the atmosphere.
Hence the light of red colour as compared to the light of other colours can penetrate to a longer distance without becoming weak.
Thus red light can be seen from the farthest distance in comparison to the light of other colours having the same intensity.
Hence red light is used for danger signal so that the signal may be visible from the far distance even in fog, etc.

You can also read: Light – Reflection and Refraction

Table of Contents

PHY

The Human Eye and the Colourful World