Physics Notes

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8th physics light reflection refraction eyes defect and dispersion The ray of light which falls on the mirror surface is called incident ray. The point at which the incident ray strikes the mirror is called the point of incidence. The ray of light which is sent back by the mirror is called the reflected ray. The ‘normal’ is a line drawn at right angles to the mirror surface at the point of incidence. The angle between incident ray and normal is called the angle of incidence. The angle between reflected ray and normal is called the angle of reflection.Laws of reflection of light:  The incident ray, the reflected ray, and the normal {at the point of incidence}, all lie in the same plane.  The angle of reflection is always equal to the angle of incidence. Regular Reflection and Diffuse Reflection of Light

Regular Reflection

In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction. •Regular reflection of light occurs from smooth surfaces like that of a plain mirror. • Images are formed by regular reflection of light.

Diffused Reflection

In Diffuse reflection, a parallel beam of incident light is reflected in different directions.  The diffuse reflection of light takes place from rough surfaces  A sheet of paper produces diffuse reflection of light. No image is formed in diffuse reflection of light. Characteristics of image formed by a plane mirror

 The image formed by a plane mirror is virtual & Erect.
 The image formed in a plane mirror is the same distance behind the mirror as the object is in front of it.  The image formed in a plane mirror is of the same size as the object.  The image in a plane mirror is laterally inverted.

Periscope : A Periscope is a device through which a person can se the objects that are out of the direct line of sight. For example, by using a periscope, we can see the objects on the other side of a high wall which cannot be seen by us directly. Periscope has two plane mirrors arranged parallel to one another. Each plane mirror, however, makes an angle of 450 with the side of the tube. Some of the uses of periscopes are given below:

 Periscope is used to see over the heads of a crowd
 A Periscope is used by soldiers sitting in bunker to observe the enemy activities outside {over the ground}.
A Periscope is used by a navy officer sitting in a submarine to see ships over the surface of water in the sea.

Multiple Images: When two plane mirrors are kept inclined at an angle, they can form multiple images of an object. If two plane mirrors are inclined at an angle , then the number of images formed in them is given by the formula: No. of images formed = (3600/ ) - 1 Kaleidoscope : The kaleidoscope is an instrument which produce multiple reflections of coloured glass pieces {or coloured plastic pieces} and create beautiful patterns.

Construction: The Kaleidoscope consists of three long and narrow strips of plane mirrors inclined at 600 to one another forming a hollow prism, and fitted into a cardboard tube. One end of the cardboard tube is closed by an opaque disc {cardboard disc} having a small hole at its centre. The other end of cardboard tube is closed with circular discs of glass. The inner disc being of transparent glass {clear glass } and the outer disc of ground glass {translucent glass}. A number of small pieces of different coloured glass {or plastic } and having different shapes are kept between the two glass discs {which can move around freely in the space between the two glass discs}. When we hold the kaleidoscope tube towards light and look inside it through the small hole, we see beautiful patterns of coloured glass.

Patterns formed by Kaleidoscope
Dispersion of light
The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light. The seven colours of the spectrum of white light are:...
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