The Human Eye And The Colourful World

The least distance of distinct vision of a normal human eye is 25 cm, it means that an adult person with normal eyesight can view objects distinctly at a distance of 25 cm from the eye and if we reduce the distance between the object and eye than this, then the vision will become blurry and will experience headache.

The two properties of the image formed are:–

1. The image formed is real and inverted.

2. The magnification of the image is always less than 1 because of least distance of distinct vision which is 25 cm (u) and the image distance is 23 cm (v) diameter of the human eye.

Concave lens also called as diverging lens is used in the type of defects in which the eye is unable to see nearby objects because of the image formation in front of the retina rather than on the retina, hence the defect is Myopia or near sightedness.

When a person reads a book he keeps it near to it’s near point of distinct vision, so the person is not suffering from myopia or near sightedness and as the person can’t read a number at a distance of 5m far, so the person definitely has Hypermetropia which is also called far sightedness because he is not able to see far objects and to correct this type of defect convex lens is needed or converging lens is needed because the image is formed behind the retina.

The phenomena is called refraction of light. Due to change in refractive index(property of the medium) of the atmosphere of earth the light bends when it goes from one medium to another medium because the change in speed of light in different mediums and due to this it bends and we see the sun before and after actual sunrise and sunset.

The defects in eyes are caused by ageing and due to size changing of eyeball and due to the changing of curvature of eye lens.

i. Due to the excessive curvature of eye lens the focal length of the lens decreases and hence the image, of objects far from the eye, forms in front of retina rather than on the retina, so the defect is called near sightedness or Myopia because the person can only see nearby objects distinctly.

ii. The power of accommodation of eye decreases with time or ageing due to the weakness of ciliary muscles and due to their weakness the focal length of the eye lens cannot be changed properly, hence the defect is called Presbyopia.

In daytime the colour of the sky is Blue because of the phenomena called scattering of light. Tiny particles present in the atmosphere which have size smaller than the wavelength of light scatter those colour of white light which has short wavelength which is blue in the visible range of light.

ADDITIONAL – Red has maximum wavelength in visible range and violet has minimum.

This is the diagram of formation of rainbow due to dispersion of light.

i. Dispersion occurs at the point ‘A’ because the splitting of colours of white light occurs at this point.

ii. Internal reflection occurs at the point(s) B and B’ because the light gets divided into several colours or beams and gets reflected.

The person cannot see nearby objects and is able to see far objects clearly, so he is suffering from Myopia also called near sightedness also his least point of distinct vision is far than that of normal eye. To correct this type of defect concave lens or diverging lens are needed because image is formed in front of retina rather than on retina.

i. The ray diagram for the defect of vision of the eye is

The image is formed in front of retina.

ii. The ray diagram for correction of defect of vision is

By correction, the image is formed on the retina, for correction we use concave lens or diverging lens.

In order to see objects by the eye the image of the object should form on the retina, exactly on the retina. When a person is suffering from Myopia or near sightedness, the near point of distinct vision gets increases and an object placed at a distance less than this new near point has its image formed in front of retina.

If a person can’t see the object clearly beyond 2m then he should use concave or diverging lens in order to correct his defect.

When there is sunrise or sunset the sun is near the horizon, so the light rays have to cover large distance and hence the blue colour scatters away and far from earth but the red colour scatters near the earth and lesser than blue colour because red has maximum wavelength.

At noon the sun is at least possible distance from earth, so the blue colour scatters very less due to that distance and hence we see sun as white in colour, and as blue has minimum wavelength, so the rest of the colours would also not get scattered.

The splitting of colours of white light is due to dispersion.

a) The colour of X is Violet and the colour of Y is Red.

b) Due to the difference in wavelength and speed of each colour they deviate at different angles.

I. CORNEA – The cornea acts as protective layer through which light enters our eye, it is transparent. Most of the refraction occurs here.

II. IRIS – It is dark and behind the cornea. It controls the size of pupil according to the light entering with intensity. It determines eye colour.

III. PUPIL – It checks the amount of light entering the eye and changes it’s size with the help of iris accordingly.

IV. RETINA – The function of retina is to receive the light focused by the eye lens or crystalline lens and convert it to electrical signals that will be interpreted by the brain through optic nerve. It has enormous number of light-sensitive cells.