Laws of Reflection

It is found that the reflection of light, from a Surface, always take place in a well defined manner according to the rules. We call these two rules as the two laws of reflection. We must, however, first know some terms to understand and explain the two laws of reflection.

(i) The ‘Point of incidence’ and the ‘Normal’: The Point of incidence is that point on the Surface of the mirror where the incident ray falls. A line Perpendicular to the Surface, at the Point of incidence, is known as the ‘Normal.’

(ii) The incident ray and the Reflected ray:The ray of light, coming towards the mirror, is called the incident ray. Similarly, the ray of light, which turns back after ‘reflection’ from the mirror Surface, is called the reflected ray.

(iii) Angle of incidence and Angle of Reflection:The angle between the incident ray and the normal at the point of incidence, is known as the angle of incidence $(angle i)$ . Similarly, the angle between the normal and the reflected ray, at the point of incidence, is called angle of Reflection $(angle r)$ .

(iv) The Plane of incidence and the Plane of Reflection: The Plane, defined by the incident ray and the normal, at the point of incidence, is called the Plane of incidence. Similarly, the plane defined by the reflected ray and the normal, at the point of incidence, is called the plane of reflection.

First Law:

The incident ray, the normal at the point of incidence and the reflected ray, all lie in the same Plane.

Second Law:

The angle of incidence and the angle of reflection are equal to each other.

$angle i:=;angle r$

Illustration: The incident ray makes an angle of 30 degree with the normal. Find the angle of reflection.

Solution: Given, the angle of incidence $large angle i:=30^0$

According to the laws of reflection $angle i:=;angle r$

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Sample Questions

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Question : 1

According to the law of reflection, _____________
A. Sin i+ sin r = constant	B. $frac{Sin i}{Sin r}$ =constant	C. i = r	D. $frac{i}{r}constant$
Right Option : C
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Question : 2

Which of the following are correct ? (a) The ray of light, which turns back after ‘reflection’ from the mirror surface, is called the incidence ray . (b) A line perpendicular to the surface, at the point of incidence, is known as the Normal.
A. Statement (a) is correct .	B. Statement (b) is correct .	C. Both (a) and (b) is correct .	D. None of these .
Right Option : B
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Question : 3

The incident ray makes an angle of 25 degree with the normal. What is the angle of reflection ?
A. 25 degree	B. 50 degree	C. 35 degree	D. 10 degree
Right Option : A
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