Laws of Reflection

Concept Explanation

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 $angle i:=;angle r$

Therefore $angle r:=;angle i=30^0$

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

Solution: Given, the ray makes an angle of 30 degree with the surface $angle MOA =30^0$

And we know the normal makes an angle of $90^0$ with the surface.

From the figure:

$angle MOA +angle AON=90^0$

$30^0+angle i =90^0$

angle of incidence $large angle i:=90^0-30^0= 60^0$

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

Therefore $angle r:=;angle i=60^0$

Note: When the ray of light falls normally or perpendicular on the surface of a plane mirror.

Then the angle of incidence is $0^0$

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

So $angle r:=;angle i=0^0$

Hence the ray which is incident normally on a mirror is reflected back along the same path.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

The angle of reflection is between the reflected ray and the_______________
A. Normal	B. Incident ray	C. Surface of Mirror	D. Horizontal surface
Right Option : A
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Question : 2

The incident ray, the reflected ray and the normal lie on the same________________
A. Line	B. Paper	C. Plane	D. Planet
Right Option : C
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Question : 3

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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