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## Class 10 Study Material

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## Course Material – Class X Physics

Topic: Reflection, Laws of Reflection and Plane Mirrors

• #### Learning Objectives

Understanding Properties of Light

Reflection and Refraction

Fermat Principle

• #### Concept Explanation

Introduction: Optics is the study of propagation of light. It is a radiant form of energy that is, emitted due to excitation of atoms or molecules that causes sensation of vision in a normal human eye.

Optics is mainly classified into two branches, geometrical optics and physical optics.

• #### Early Introduction of Concepts

Wherever possible and permissible, we introduce basics of concept in early classes so as to give the students a Holistic Understanding of a Concept

• #### CBSE and International Curriculum Integration

Reflection of light: When light rays strike the boundary of two media such as air and glass, a part of light is turned back into the same medium. This is called reflection of light.

Regular reflection: When the reflection takes place from a perfect plane surface it is called regular reflection. In this case, the reflected light has large intensity in one direction and negligibly small intensity in other directions.

Diffused reflection:  When the surface is rough, we do not get regular behaviour of light. Although at each point, light ray gets reflected irrespective of the overall nature of surface, difference is observed because even in a narrow beam of light there are many rays which are reflected from different points of surface. It is quite possible that these rays may move in different directions due to irregularity of the surface. This process enables us to see an object from any position. Such a reflection is called as diffused reflection. For example, reflection from a wall, from a newspaper etc. That is why you cannot see your face in the newspaper and in the wall.

Laws of reflection: It has been found experimentally that rays undergoing reflection follow the laws of reflection:
i) The incident ray, the reflected ray and the normal at the point of incidence lie in the same plane. This plane is called the plane of incidence (or plane of reflection).
ii) The angle of incidence (the angle between normal and the incident ray) and the angle of reflection (the angle between the reflected ray and the normal) are equal, i.e., i  r .

• #### Practical Application

1. An incident ray makes an angles of 550 with the surface of a plane mirror. What is the angle of reflection?

Solution: In order to find out the angle of reflection, we should first know the angle of incidence. In this case, the incident ray makes an angle of 550 with the surface of the mirror, so the angle of incidence is not 550. The angle of incidence is the angle between incident ray and normal. So, in this case, the angle of incidence will be 900 – 550 = 350. Since the angle of incidence is 35 degrees, therefore, the angle of reflection is also 35 degrees.

2. A light ray reflects from a plane mirror as shown in the figure. What is the angle of deviation  ?
Solution:

• #### Revise

1. Light is a form of energy which make things visible when it falls on them (stimulates the sensation of vision).

2. A substance through which light can more or less pass is called optical medium.

3. When light travelling through homogeneous transparent medium is incident on a surface which separates this medium from another,then a part of the light comes back into the first medium in a definite direction.This is called reflection of light.The surface from which reflection takes place is called a reflector.The amount of light reflected depends on (i) the angle of incidence and (ii) the nature of the media.

4. Laws of reflection:

i)The incident ray,the normal to the reflecting surface at the point of incidence and the reflected ray,all lie in one plane.

ii)The angle of incidence is equal to the angle of reflection.

5. The angle between the incident ray and the plane mirror is known as angle of glancing.

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