# Class 8 Light Notes

Class 8 Light Notes

## Light:-

Light is a form of energy that produces sensation of vision in our eyes.

## Reflection of Light:-

The phenomenon of bouncing back of a ray of light after striking on a highly polished (reflecting) surface is called reflection of light.

Incident ray:- The light ray, which strikes any surface, is called the incident ray.

Reflected ray:- The ray that comes back from the surface after reflection is known as the reflected ray

Angle of incidence (∠i):- The angle between the normal and incident ray is
called the angle of incidence.

Angle of reflection (∠r):- The angle between the normal and the
reflected ray is known as the angle of reflection.

## Laws of Reflection:-

• First law:- The angle of incidence is equal to the angle of reflection. i.e., ∠i = ∠r
• Second Law:- The incident ray, the normal at the point of incidence and the reflected ray, all lie in the same plane.

#### Lateral Inversion:-

The phenomenon of left side appearing right side and right side appearing left side in the image formed by a mirror is called lateral inversion.

## Regular Reflection:-

The reflection from a smooth surface such that all the parallel rays reflected are also parallel is called regular reflection.

## Irregular/Diffused Reflection:-

The reflection from an irregular surface such that all the parallel rays reflected are not parallel is called irregular or diffused reflection.

• Images are formed by regular reflection.
• Reflected light can be reflected again.
• The devices based on multiple reflection of light are periscope and kaleidoscope.
• Periscopes are used in submarines, tanks and also by soldiers in bunkers to see things outside.
• Kaleidoscopes are used by designers of wallpapers and fabrics and artists to get ideas for new patterns.
• Sunlight is referred to as white light that consists of seven colours i.e., violet, indigo, blue, green, yellow, orange and red.

## Refraction of Light:-

The phenomenon of bending of a ray of light when it passes from one medium to another medium is called refraction of light .

• In going from rarer to denser medium, the ray of light bends towards normal and in going from denser to rarer medium, the ray of light bends away from normal.

## Human Eye:-

• Human eye ball is approximately spherical in shape with diameter of about 2.3 cm.
• Light enters the eye through a thin membrane called the cornea.
• Iris controls the size of pupil.
• Pupil regulates the amount of light entering the eye.
• Image is formed on retina & its nature is real & inverted.
• The retina has two types of light sensitives cells i.e., rods & cones.
• Rods are responsible for vision at low light.
• Cones are active at higher light levels & is responsible for colour vision.
• Ciliary muscles help to change the focal length of the eye lens.

Blind spot:- The junction of retina and optic nerve where no light sensitive cells are present is called blind spot.

• Vision is not possible at blind spot.

Persistence of vision:- The effect of an image stays on retina for about 1/16 of a second even after the object is removed. This is called persistence of vision.

## Care of Eyes:-

Some tips to take care of the eyes are as follows:-

• Do not look at the sun or a powerful light directly.
• If advised, use suitable spectacles.
• Too little or too much light is bad for eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can injure the retina.
• Never rub your eyes. If particles of dust go into your eyes, wash your eyes with clean water. If there is no improvement go to a doctor.
• Wash your eyes frequently with clean water.
• Always read at the normal distance for vision. Do not read by bringing your book too close to your eyes or keeping it too far.