A short note on the Michelson’s Method for the velocity of the light.

                         A short note on the Michelson’s Method for the velocity of the light.

It is the precise method to find the velocity of the light. An octagonal mirror M1 is mounted on the shaft of the variable speed of the motor .The Light from a source S at an angle 45 degree 0n one of the faces of mirror M1 after passing through the slit S1. The reflected light falls on the mirror M2 .In figure M3 is a plane mirror and with the help of this mirror M3 placed at the centre of curvature of mirror M2 the bean of light is returned back and falls on face3 of the mirror M1 again at an angle of 45 degree .The reflected light by this mirror is then collected by a telescope T and the eye at the position .At the rest position of the mirror M1 an image of the light source can be observed on the telescope.

If the mirror M1 is rotated, the light returning to it from the mirror M2 will not be incident at an angle of 45 degree ,and hence will not enter the telescope .When the speed of the rotation of mirror M1 is so fitted  that the face 2 of mirror occupies absolutely the same position as was occupied by face 3 earlier when

the time light moves from M1toM2 and back to M1,Then the image of source will again appear .

If d be the distance between the mirror M1 &M2 and c be the velocity of the light ,then the time taken by the light to teavel from M1 to M2 and back to M1 is

                                                                              T=2d / c

                      If F is the number of the revolution per second of mirror M1 and m is the number of faces of this mirroe then the angle rotated by the mirror during the time is ,

                                                 @=2p   m

On solving the above equation we get that ,C=2pmfd

Which is the required expression for the velocity of the light .In the Michelson’s method the real speed of the motor is 500rev per second and the distance the mirror and the source is about 35 kilometer. Hence it is the michelson’s method to measure the velocity of light . If the value of the right side value in the above equation is known then easily the velocity of light can be determined.