Class VIII
Light

• Reflection of light
• (Laws of reflection)
• Image formation by the plane mirror
• Irregular and regular surfaces
• Human eye

• The light makes things visible when light form an object enters our eyes that we see object. The light may have been emitted by the object, or reflected by it. A mirror changes the direction of light that fall on it.

Reflection of light
The light ray, which strikes any surface, is called the incident ray. The ray that comes back from the surface after reflection is known as the reflected ray as shown in figure (!)


Fig (1)
(reflection of light)


When the ray of light incident on a surface, the ray comes back from the surface is known as reflection of light.

• The angle between the normal and incident ray is called angle of incidence (< i). The angle between the normal and reflected ray is known as the angle of reflection (r) shown in figure (1).
• There are the laws of reflection
• The angle of incidence is always equal to the angle of reflection i.e. I = r.
• The incidence ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
• The image formed by a plane mirror is virtual, exert and lateral inversion it is defined as, in an image formed by a mirror the left of the object appears on the right and the right appears on the left .

In figure,
PQ = mirror
E = Eye
At I = Virtual image
At O = object

Fig (2)
(image formation by plane mirror)

• When all the parallel rays reflected from a place surface are not parallel, the reflection is known as diffused or irregular reflection. It is due to the irregularities in the reflecting surface as shown in figure (3)

Fig (3)
(irregular surfaces)

• The reflection from smooth surface like that of a mirror is called regular reflection as shown in figure (4). Images are formed by regular reflection.

Fig (4)
(regular reflection)

• The objects which shine in the light other objects are called illuminated objects e.g. moon receives light from the sun and reflects it. That's how we see the moon.
• The object which emit their own light are known as luminous objects. E.g. sun, fire, flame of a candle and an electric lamp.
• The sun light is referred to as white light. It consists of seven colours. The splitting of light by a prism (mirror and water form a prism) in to its colours is known as dispersion of light. Rainbow is a natural phenomenon showing dispersion.

Fig (5)
(Dispersion of light)

• Human eye has a roughly spherical shape. The outer coat of the eye is white. Its transparent front part is called cornea, behind the cornea a dark muscular structure find. It is called iris. In the iris there is a small opening called the pupil. The size of the pupil is controlled by the iris. The iris is that part of eye which gives it its distinctive colour.

Fig (6)

• In figure the lens focuses light on the back of the eye, on a layer called retina. It is consist of se nerve cells.
  
  There are two kinds of cells.
  
  1. Cones, which are sensitive to bright light and
  2. Rods, which are sensitive to dim light.
• Cones sense colour. At the of the optic nerve and retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.
• The impression of an image does not vanish immediately from the retina. It persists there for about 1/16^th of a second. So, if still images of a moving object are flashed on the eye at the ate faster than 16 per second, then the eye perceiues this object as moving.
• The minimum distance at which the eye can see objects distinctly varies with age. The most comfortable distance at which one can read with a normal eye is about 25 cm.
• The most popular resource for visually challenged person is Braille.