Prove with the help of an activity that angle of incidence is always equal to the angle of reflection
Answers
There are two laws of reflection
First law :- When a light ray strikes a plane mirror, the light ray reflects off the mirror. Reflection involves a change in direction of the light ray.The angle of incidence is the angle between this normal line and the incident ray; the angle of reflection is the angle between this normal line and the reflected ray. According to the law of reflection, the angle of incidence equals the angle of reflection.
Second Law :- Incident ray, Reflected ray and the normal at the point of incidence lie in the same plane.
Experiment
Objective :- To prove the law of reflection through a plane mirror.
Things required:- Soft board, white sheet of paper , optical pins , mirror , pencil, protactor and ruler .
Method :-
1- Place the paper on the board and fix
2- Place the mirror vertically on the white sheet of paper and trace its edge.
3- Draw a line at right angles to the edge of the edge of the mirror to act as the normal- ON .
4- Stating with angle i as 30 degree , draw an incident ray and place two pins, P and Q along it as shown
5- With your eyes at position shown, place two other pins R and S to coincide with the images of P and Q as seen in the mirror
6- Remove pins R and S and join the dots left with a straight line
7-Measure and record angle r.
8- Repeat procedure 4,5,6 and 7for angles i = 35 degree, 40 degree, 45 degree, 50 degree and 55 degree.
Record the results in a table.
On carefully observing the result on will clearly see that :-
1-The angle of incidence equals the angle of reflection.
2- Incident ray, Reflected ray and the normal at the point of incidence lie in the same plane.
Hence the laws of reflection prooved :)
The laws of reflection governs the reflection of light rays off smooth conducting surfaces such as polished metal or metal-coated glass mirrors. It state that:
1. The incident ray, the reflected ray and the normal to the surface of the mirror all lie in the same plane.
2. The angle of incidence <i is equal to the angle of reflection <r, both angles are measured with respect to the normal to the mirror.
The experiment to verify these laws is as follows:
Apparatus required: a drawing board, a white sheet of paper, a few common pins and a plane mirror.
Procedure: Pin the white sheet of paper firmly on the drawing board. Place the plane mirror on it and trace its outline on the paper. Then remove the mirror and draw the 'normal. ' The normal is the 90 degrees vertical line drawn to the outline of the mirror.
Now place the mirror again on the outline. The normal will be reflected clearly on the mirror.
Next place two pins in a straight line on one side of the normal on the white sheet of paper.
Next place two pins on the other side of the normal in such a way that these two pins are in a straight line with the reflection of the two pins on the other side of the normal.
Now remove the mirror and the pins and join the pin marks to the normal.
It will be seen that the angles which the lines make on both sides of the normal will be equal (refer to the attached image). So, if one line represents the incident ray - the ray which is travelling from the source of light - and the other line represents the reflected ray - the ray of light which has been reflected - it is proved that the angle which the incident ray makes to the normal is always equal to the angle which the reflected ray makes to the normal i. e. <i=<r
Since the lines representing the normal and the incident and reflected rays are all represented on the sheet of plain paper, the incident ray, the reflected ray and the normal are coplanar.
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Answer:
The incident ray, the reflected ray and the normal at the point of incidence, lie in the same plane.
(ii) The angle of incidence and angle of reflection are equal i.e. ∠i = ∠r.
Verification
Take a wooden drawing board and fix a white sheet of paper on it. In the middle of paper draw a straight line KK\ Mark a point B on it. Draw a perpendicular BN. Place a mirror XX’ on line KK’ such that polished side of mirror is along the line. Hold-the mirror in the mirror holder.
Fix two steel pins P and Q on the straight line AB at least 10 cm apart. Look for the images of the pins P and Q and fix two pins P Q’ such that P’, Q’ and images of P and Q are all in the same straight line. Remove the pins and draw small circles around the pin pricks.
Remove the mirror also. Join P’Q’ and produce the straight line to meet at B.
Measure ∠ABN = i and ∠CBN = r.
It is found that ∠i = ∠r. This proves that Angle of Incidence is equal to Angle of Reflection.
As the incident ray, the reflected ray and the normal lie in the plane of paper, therefore, they lie in the same plane.