Physics important topics:VISION AND THE WORLD OF COLOURS

Eye and Vision:
• Near point is the nearest point at which the objects can be seen distinctly.
The near point of an eye with healthy vision is 25 cm.
Far point is the farthest point at which the objects can be seen distinctly.
The far point of an eye with healthy vision is at infinity.

The ability of the eye to form an image on the retina by adjusting the focal length of the lens in the eye, by varying the curvature of the lens, irrespective of the position of the object, is the power of accommodation.
Long sightedness:
Since the image is not formed at the retina, instead of being formed at the retina, nearer objects cannot be seen clearly even though distant objects are clearly seen. This defect of the eye is the long sightedness. The near point of the eye of such a person will be at a distance of more than 25 cm. The light rays from a close by object are focussed at a point behind the
This defect arises either because (i) the focal length of the eye lens is too long,
(ii) the eyeball has become too small. • This defect can be corrected by using a convex lens of appropriate power.
For some persons, even though nearby objects can be seen clearly, they may not be able to see distant objects clearly. This defect is the
nearsightedness. The near point of such persons will not be at infinity. It will be at a definite distance from the eye.
• The image of a distant object is formed Infront of the retina.

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.
For elderly people the distance to the near point is greater than 25 cm. This is due to the diminishing ability of the ciliary muscles. For such people the power of accommodation will be less. This is presbyopia.
Eye Donation
Eye donation is one of the noblest donations.
Dispersion of light
• Any light that is composed of more than one color is a composite light.
• Dispersion is the phenomenon of splitting up of a composite light into its
constituent colours. • The regular array of colours formed by dispersion is the visible spectrum.
• Dispersion of light caused by the water droplets in the atmosphere causes
rainbow. Sunlight, when it passes through water droplets, undergoes refraction and internal reflection. The light ray emerging from the water droplets which make the same angle with the line of vision have the same colour. These droplets appear in the form of an arc of a particular colour. Thus, there is red colour at the upper edge and violet colour at the lower edge. All the
other colours are seen in between, depending on their wavelengths. • When the position of the sun is near the horizon, the rainbow appears to be
When seen from an aeroplane, the rainbow is seen as a circle.
When the sun is much above the horizon, the rainbow disappears.

Recombination of colours:
Pass white light through a prism obtain the constituent colours on the
screen. A prism similar to the first is placed in inverted position, adjacent to the first, obtain white light on the screen.
Persistence of vision:
When an object is viewed by a person, its image remains in the retina of the eye for a time interval 0.0625s after seeing it. This phenomenon is called persistence of vision. If more than one scene is viewed within 0.0625s, the effect of all these
scenes will be felt by the eye simultaneously.
Scattering of Light:
Scattering is the change in direction brought out by the irregular and partial reflection of light when it hits the particles of the medium
Scattering and Wavelength:
Rate of scattering and the size of the particles are interrelated.
As the size of the particle increases, the rate of scattering also increases.
If the size of the particles is greater than the wavelength of light, then the
scattering is same for all colours.
Colours of the raising and the setting sun:
During sunrise and sunset, light reaching us from the horizon has to travel long distances through the atmosphere. During this long journey, colours of shorter wavelength would be almost fully lost due to scattering. Then, the red light which undergoes only less amount of scattering decides the colour of the horizon. That is why the sun appears red during sunset
and sunrise.

Tyndal Effect:
• When rays of light pass through a colloidal fluid or suspension, the tiny
particles get illuminated due to scattering. Because of this, the path of light is made visible. This phenomenon is Tyndal Effect.
The intensity of scattering depends on the size of particles in the colloid. • As the size increases, the intensity of scattering increases.
Light Pollution:
• The use of light in excess in a non-judicious manner is referred to as light
The consequences of light pollution:
• The life cycle of living beings will be affected adversely. • Sky watching becomes impossible due to diminished sky vision. • The light from tall flats misleads the migrating birds. It affects the accuracy
of their judgement of direction. • The excess light from the high beam of headlight in vehicles causes a
hindrance to the vision of others and can cause accidents.
The excessive use of lighting devices increases energy consumption and creates energy crisis.

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