CBSE Class 12 Physics Reflection from Plane Surfaces Theory and Examples

CBSE Class 12 Physics Reflection from Plane Surfaces Theory and Examples. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

1. DEFINITION
When a light ray strikes the surface separating two media, a part of it gets reflected, i.e., returns back in the initial medium. It is known as reflection.

2. TERMS RELATED TO REFLECTION

2.1 RAY :
A ray of light is the straight line path of transfer of light energy. It is represented by a straight line with an arrow - head indicating the direction of propagation.

2.2 MIRROR :
It is a highly polished smooth surface from which most of theSmooth side incident light gets reflected. It is represented by a line with hatches in the reverse side of the smooth surface.

2.3 OBJECT :

A. Point from which incident ray actuallydiverge is called real object. Or point at which incident rays appear to converge is called virtual object.

B. object is defined on the basis of incident ray.

C. Minimum two rays are required to show the position of object

2.4 IMAGE :

A. Point at which reflected or refracted rays actually converge is called real image. Or point from which reflected or refracted rays appear to diverge is called virtual image.

B. Minimum two reflected or refracted rays are required to determine the image position.

3. BASIC LAWS

3.1. Law Of Rectilinear Propagation Of Light states that light propagates in straight lines in homogeneous media.
3.2. Law Of Independence Of Light Rays states that rays do not disturb each other upon intersection.
3.3. Law Of Reversibility Of Light Rays states that rays retrace their path when their direction is reversed.
3.4. Laws Of Reflection :

A. The incident - ray, reflected ray and normal to the reflecting surface at the point of incidence all lie in the same plane.
B. The angle of reflection is equal to the angle of incidence, i.e. ∠i = ∠r .

Note :
1. These two laws are also valid for curved surfaces.
2. If ∠i = 0 then ∠r = 0 , i.e., if a ray is incident normally on a boundary, after reflection it retraces its path.
3. None of frequency, wavelength and speed changes. However intensity and hence amplitude usually decreases since I α A2
4. There is a phase change of π if reflection takes place from denser medium.

Laws of reflection

Ex.1 The plane of a mirror makes an angle of 30º with horizontal. If a vertical ray is incident on a mirror, then what is the angle between mirror and reflected ray ?
Sol. Angle betwen incident ray and mirror = 90º − 30º = 60º

By law of reflection ∠i = ∠r So angle of reflection ∠r = 30º, Hence angle between mirror and reflected ray = 60º

4. CHARACTERISTICS OF REFLECTION AT PLANE MIRROR

4.1. The image is always of same size and at same distance behind the mirror as the object in front of it.

4.2. When the object is real virtual image is formed and vice - versa.

4.3. The image is always erect when the object is in front of the mirror but inverted when the object is above the mirror.

4.4. The image is laterally inverted. The mirror actually reverses front and back in three dimensions (and not left to right ) i.e. only x-direction is reversed resulting in the change of left into right or vice - versa. A plane mirror changes right handed co-ordinate system to left handed one

4.5. As every part of mirror forms complete image of an extended object and due to superposition of images brightness will depend on the light reflecting area of the mirror. [It can be explained by Haygen's principle.]

4.6. Though every part of a mirror forms complete image of an object , we usually see only that part of image from which light after reflection from the mirror reaches our eye.

4.7. Deviation δ is defined as the angle between directions of incident ray and emergent ray. So if light is incident at an angle of incidence i, δ = π – ( i + r ) = π –2i

4.8. If keeping the incident ray fixed, the mirror is rotated by an angle θ , about an axis in the plane of mirror , The reflected ray is rotated through an angle 2θ.

4.9 Most of the smooth surfaces works as a good reflector when light falls at grazing incidence (∠i → 900 )

4.10 From smooth reflecting surface we get parallel beams of light which produces glare. However from rough surface we get diffused radiation.

4.11 If object moves towards the plane mirror at speed v, the image moves towards the plane mirror at speed – v. So , the image speed w.r.t object is – v – v = – 2v

4.12 If mirror is moved towards ( or away from) the object with speed v the image will move towards ( or away from ) the object with a speed 2v. 

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