# JAC Class 10 Science Notes Chapter 11 Human Eye and Colourful World

Students must go through these JAC Class 10 Science Notes Chapter 11 Human Eye and Colourful World to get a clear insight into all the important concepts.

### JAC Board Class 10 Science Notes Chapter 11 Human Eye and Colourful World

→ Human eye (eyeball) : The human eye is one of the most valuable and sensitive sense organs in our body. It enables us to see the wonderful world and colours around us. The human eye is like a camera. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.

The main parts of the human eye are as follows:

• Cornea
• Iris
• Pupil
• Crystalline lens
• Ciliary muscles
• Retina
• Optic nerves
• Aqueous humour and
• Vitreous humour

→ The ability of the eye lens to adjust its focal length, so that image of a nearby as well as distant objects can be focussed on the retina and hence seen comfortably and distinctly is called the power of accommodation of the eye.

→ The minimum distance at which an object can be seen most distinctly without strain is called the least distance of distinct vision or the near point of the human eye. For a young adult with normal vision, the near point is about 25 cm.

→ The farthest point up to which the eye can see an object clearly is called the far point of the eye. It is infinity for a normal eye.

→ A person with normal vision can see objects clearly that are between 25 cm and infinity from the eye.
Ciliary muscles hold the eye lens in position and help in changing (modifying) the curvature of the eye lens and hence the focal length of the eye lens.

→ Defects of vision and their correction:
(i) Myopia or near-sightedness: A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. This defect may arise due to (1) excessive curvature of the eye lens or (2) elongation of the eyeball. This defect is corrected by using a concare lens of suitable power.

(ii) Hypermetropia or far-sightedness : A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. This defect arises due to (1) the focal length of the eye lens is increased due to least curvature of the eye lens, or (2) the length of the eyeball is shortened (becomes shorter than normal). This defect is corrected by using a convex lens of suitable power.

(iii) Presbyopia (also called old age hyper-metropia) : Presbyopia is the defect of vision in which aged people find it difficult to see the nearby objects comfortably and distinctly without spectacles.

It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. This defect can be corrected by using a convex lens of suitable power. Sometimes, a person may suffer from both myopia and hypermetropia. Such persons have to use bifocal lenses.

(4) Cataract: The crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract. It is possible to restore vision through a cataract surgery.

→ Refraction of light through a prism: When light is refracted through a triangular glass prism, the emergent ray makes an angle with the incident ray. This angle is called the angle of deviation. It depends on the angle of incidence, the refractive index of the glass and the angle of the prism.

→ Dispersion of light: The splitting of light into its component colours is called dispersion. The phenomenon of a splitting of white light into its seven constituent colours is called dispersion of white light.

Prism disperses white light into its constituent colours. The sequence of colour from bottom to top is violet, indigo, blue, green, yellow, orange and red. (Remember: vibgyor)

The band of the coloured components of a light beam is called its spectrum. The band of seven colours obtained from the splitting of white light is called spectrum of white light.

→ Reason for dispersion of light: Actually white light is composed of seven colours as VIBGYOR. Now different colours of white light have different wavelengths. They travel with the same speed in vacuum and air but in any other medium they travel with different speeds.

The absolute refractive index of a medium is nm = $$\frac { c }{ v }$$ As $$v_{\text {violet }}<v_{\text {red }}, n_{\text {violet }}>n_{\text {red }}$$. Hence, the violet light bends the most while the red light bends the least.

As the refractive index n of glass of prism for different colours is different, they deviate through different angles with respect to incident ray on passing through prism.

→ Rainbow: A rainbow is a natural spectrum appearing in the sky after a rain shower. Rainbow is caused by dispersion of sunlight on refraction by tiny water droplets, present in the atmosphere. It is always formed in the direction opposite to that of the Sun. In this case there is dispersion, internal reflection and refraction of light.

→ Atmospheric refraction : Atmospheric refraction is the phenomenon of bending of light passing through the earth’s atmosphere.

Some phenomena based on atmospheric refraction are :

• Twinkling of stars
• Delayed sunset
• The stars seem higher than they actually are (i.e., the displacement of stars).
• The Sun appears oval (or flattened) at sunrise and sunset, but appears circular at noon, (i.e., the apparent change in the shape of the sun)

→ Scattering of light: The deflection of light by minute particles and molecules / atoms in all directions is known as scattering of light. The amount of scattering (i.e., extent of deflection) of light depends on the frequency (colour) of light and the size of the particles responsible for scattering.

Very fine particles scatter mainly blue light. Bigger particles scatter light of longer wavelength. If the scattering particles are very large, the scattered light appears white as light of all wavelengths in the visible region is scattered.

Some examples of scattering of light are:

• Tyndall effect
• Blue colour of clear sky
• The signal lights for danger are red in colour.
• Reddish colour of the Sun at sunrise and sunset.
• Reddish appearance of the full moon at the time of rising and setting.