Human eye and colourful world:

• Human eye is one of the valuable and sensitive sense organ among the five sense organs present in the human body.
• It acts as a window for us to enter into this beautiful and colourful world.
• Human eye is a specialized sense organ capable of receiving visual images, which are then carried to the brain.
• It allows light perception, color vision and depth perception.
• Every sense organ is involved in different life processes such as nutrition, respiration, transportation and excretion.

Structure of human eye:

Part of eye	Features	Function
Retina	It is a light sensitive screen on which image formation takes place.It contains colour sensitive cells i.e. rod cells and cone cells.	It generates electrical signals which are carried to brain through optic nerves.
Cornea	Thin membrane present in front part of eye and covers the eye-ball.	Light enters the eye through cornea. Most of the refraction of light takes place through it.
Eye lens	Lens lies just behind the pupil. It is a transparent, biconvex structure in the eye and is made up of protein.	Lens provides finer adjustment of focal length to focus nearby or distant objects.It projects the real and inverted image on retina.
Iris	Dark muscular diaphragm between the cornea and the lens.Colour of iris (eye colour) may be black, brown, blue and green.	It controls the amount of light entering the eye involuntarily.
Pupil	An opening between the iris through which light enters the eye.	It is a part of the iris which adjusts its size to regulate the amount of light entering the eye.
Ciliary Muscle	These muscles are attached to ciliary muscles.	By adjusting its shape eye adjusts its focal length.
Optic nerves	They are present on the back of eye.	It carries visual information from retina to brain.

Power of accommodation of human eye:

• The eye lens is made up of fibrous and jelly like material.
• Its curvature can be adjusted by ciliary muscles to some extent which thus changes its focal length.
• When the muscles relaxed, the lens becomes thin which causes increase in focal length. We can clearly see the distant objects.
• When ciliary muscles contract, the lens become thicker which causes decrease in focal length (upto a certain limit) so that we can see the nearer objects We can see objects upto 25 cm clearly.
• The ability of the eye lens to adjust its focal length to near and distant objects is said to be accommodation of eye.
• The minimum distance at which we can see the object clearly without strain in eye is said to be least distance for distinct vision. It is also called near point of eye.
• The far distant point at which we can see the objects clearly is called far point of the eye.
• The crystalline lens of people at old age becomes milky and cloudy which causes partial or complete loss of vision is said to be Cataract.

How does eye work?

The working principle of camera is based on the working of human eye. We can understand it more clearly by knowing about that particular. However we have provided the mechanism of working of human eye.

• Light enters the eye through the cornea and is reflected (sent forward) towards the pupil.
• From the pupil, the reflected light then passes through the crystalline lens.
• The lens has a main role to project a real inverted and small image on the retina.
• Rods and cones cell present in the retina converts the visual information to electrical signals and after that, it is transmitted to the brain via optic nerves.
• The brain interprets the information thus orders accordingly.
• However, we study human eye and colourful world in a manner to understand its mechanism.

Defects of vision:

Sometimes people cannot see the objects clearly and comfortably. They have blurred vision due to refractive defects of the eye.

The common defects observed in human eye are:

• Myopia
• Hypermetropia
• Presbyopia

1.    Myopia or short-sightedness :

• The person who can see the nearer objects clearly but cannot see the distant objects distinctly is said to be myopia.
• They have far point nearer than the infinity.
• The image of distant objects is formed in front of retina (not on the retina).

Causes of myopia:

1. Excessive curvature of the eye lens,
2. Elongation of the eyeball.

Correction:

• Myopia can be corrected by using concave lens having suitable power what is needed.

2.    Hypermetropia or long-sightedness:

• The person who can see the far objects clearly but cannot see the nearer objects distinctly is said to be hypermetropia.
• The near point of eye shifts far from the normal near point that is at 25 cm.
• The image formed is behind the retina. So to see clearly one has to keep objects much beyond the near point (25cm).

Cause of hypermetropia:

1. Either the focal length of the eye lens is too long,
2. Or the eyeball has become too small.

Correction:

• This eye defect can be corrected using convex lens with the required appropriate power.

3.      Presbyopia:

• When human becomes older (human ageing) the accommodation power of eye starts decreasing.
• The image of the near point formed far behind the lens thus make it difficult to see nearby as well as the far objects clearly.

Cause of presbyopia:

1. Human ageing.
2. Due to weakening of ciliary muscles.
3. Reduced lens flexibility.

Correction:

• Such a defect can be corrected using bi-focal lens. Bifocal lens contains both concave lens and convex lens. Concave in the upper portion and convex in the lower portion of the spectacles.
• It can also be corrected by surgery by using suitable contact lens.

Refraction of light through prism:

• The glass prism is a solid figure which consists of three rectangular faces and two triangular faces.
• The rectangular faces are inclined on each other as lateral surfaces.
• Opposite surfaces of the prism are equal and parallel.
• The angle between two lateral faces is called angle of prism.

Dispersion of light:

• The phenomenon of splitting of visible light into its constituent colours after it being passed through a prism or like transparent substance is said to be dispersion of light.
• The white light is splitted into seven constituent colours, they are Voilet, Indigo, Blue, Green, Yellow, Orange, Red.
• When visible light passes through prism after refraction, the different colours bends with different angles with respect to the incident ray.
• Each colour takes different bending angle. Red light deviates the least while violet light the most.

Colour spectrum:

• When a beam of white light is passes through a glass prism, the band of seven colours formed is called spectrum of white light or colour spectrum.
• Isaac Newton was the first man to use a glass prism to obtain the spectrum of sunlight.

Recombination of colour spectrum:

• The colour spectrum of seven colours can be combined to get back the white colour.
• This can be done by using two prisms side by side but the position of other prism must be in inverted position with respect to the first.
• When white light passes through the first prism it disperses into seven colours but the inverted prism recombines the same and the seven colours after passing through it gives white light back.

Formation of rainbow:

• Our atmosphere has a best example of dispersion of light i.e. case of rainbow formation.
• Rainbow is natural spectrum which appears in the sky after rain.
• We often observe the rainbow when it is raining in the sunny day.
• The phenomenon is that when white sunlight falls on the rain drops it disperses into 7 constituent colours. Thus forms the rainbow.
• Here rain drops act as a prism.
• Rainbow is always formed in opposite direction to that of the Sun.

Atmospheric refraction:

• The refraction caused due to earth’s atmosphere is said to be atmospheric refraction.
• Our atmosphere contains different layers of air at different positions. i.e hot air becomes lighter and moves up.
• There comes change in refractive indices of hotter and cooler air at different heights of atmosphere.
• Thus the physical conditions of the refractive medium is not stable. It fluctuates everytime.

There are certain phenomenon observed in atmospheric refraction. They are:

Twinkling of stars:

• The main reason behind twinkling of stars is atmospheric refraction.
• When starlight enters the earth’s atmosphere it undergoes refraction continuously in the gradually changing refractive indices before it reaches the earth.
• The atmosphere always bends starlight towards normal, so the visual apparent position of the star different from its actual position.
• This visual apparent position of the star is not stable as it keeps on changing slightly. Thus the amount of starlight entering the eye flickers.
• The star sometimes appears bright sometimes dim which is called twinkling effect.
• This is another aspect of human eye and colourful world.

Advance sunrise and delayed sunset:

• Due to atmospheric refraction, the Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset.
• Actual sunrise: Actual crossing of the horizon by the Sun.
• The same phenomenon of atmospheric refraction explains the same.

Scattering of light:

• The phenomenon by which a beam of light is redirected in many different directions when it interacts with a particle of matter.
• There are several phenomena related to scattering of light, they are tyndall effect, blue colour of sky, colour of water in deep sea, the reddening of the sun at sunrise and the sunset etc.
• Thus human eye and colourful world makes beautiful combinations.

Tyndall effect:

• The phenomenon of scattering of light through colloidal solution is called tyndall effect.
• Scattering of light makes the particles visible, this phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole.
• Scattering is also the important topic of human eye and colourful world.

Why is the colour of the clear Sky Blue?

The colour of the clear Sky is Blue because of the following reasons:

• Atmosphere contains fine particles which are much smaller in size as compared to the wavelength of visible light.
• These finer particles effectively scatters blue light rather than red light as red light has wavelength of 8 times greater than that of blue light.
• As sunlight passes through the atmosphere, the fine particles in air scatter the blue colour.
• If there would be no atmosphere the sky would have looked dark.

Why the danger signal lights are red in colour?

The danger signal lights are red in colour because the red light having high wavelength is least scattered by fog or smoke. Therefore, it can be seen in the same colour at a distance. This is the interpretation of human eye and colourful world.

Colour of the Sun at Sunrise and Sunset:

• Light from the Sun near the horizon passes through thicker layers of air (in morning) and larger distance in the earth’s atmosphere before reaching our eyes. So red colour having larger wavelength reaches our eye for few minutes.
• Sunlight at noon (overhead sun) travel relatively shorter distance so appears the same.

To practice questions and answers related to this chapter Click here  Q/A on Human Eye and The Colourful World