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Laws of Reflection

We are able to see objects because they reflect light. We are familiar with the phenomenon of reflection of light. The surfaces of many bodies act like mirrors, while many others do reflect light but are not like mirrors.
 

 

The line MM' represents the position of the plane mirror. The ray of light AO, falling on the mirror is called incident ray. The point O where the incident ray meets the mirror is called the point of incidence. The line ON drawn perpendicular to the mirror at the point of incidence is called the normal. The ray OB which returns after reflection in the mirror is called the reflected ray. The angle AON between the incident ray and the normal at the point of incidence is called the angle of incidence (i). The angle BON between the reflected ray and the normal at the point of incidence is called the angle of reflection (r).


Activity:

Things Required: Plane mirror strip, a sheet of white paper, protractor, ruler, pencil, a drawing board, drawing pins, and some common pins.

 

Method: Fix the paper on the drawing board with the help of drawing pins. Draw a line PQ on the sheet of paper and mark a point O at the mid point of PQ. At O, draw ON perpendicular to PQ. ON is the normal. With your protractor, draw AO making any desired angle AON. AO will be the incident ray. Place the plane mirror strip with its back surface along the line PQ. Fix two pins at A and B on AO (at least 5 cm apart). Locate the images of pins A and B in the mirror. Keep one eye at the level of the paper and line up the images of the pins A and B. Insert pins C and D in line with these images. The images of the pins at A and B and the pins at C and D, must be exactly in the same line and overlapping each other. Mark the positions of all the pins. Remove the pins and the mirror and draw the reflected ray OD. Mark the directions of the incident and reflected rays with the arrows. Measure the angle AON and DON.
 

   


The angle of incidence is equal to the angle of reflection. The incident ray, the normal and the reflected ray all lie on the same plane, the paper.

The conclusions of the above activity can be stated in the form of two laws, called the laws of reflection.


Laws of Reflection
(i) When a ray of light is incident on a plane mirror, it is reflected in such a way that the angle of incidence is always equal to the angle of reflection.

(ii) The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
 

Did you Know?
You can see yourself when you stand in front of a mirror, but you cannot see yourself when you look at a pin-up board. Why?

 

    

 Mirror surface is smooth and polished. Hence we can see our self, when we stand in front of a mirror, because of the reflected light which can reach our eyes.

This type of reflection is called regular reflection.

Surface of the pin up board is rough. Hence we cannot see our self, when we stand in front of a pin up board, even though the reflected light reaches our eyes.

This type of reflection is called irregular reflection.





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