# Bunsen’s Photometer | Physics Grade XI Reference Note

### Photometer

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# Bunsen’s Photometer

Bunsen’s Photometer

A Bunsen’s grease spot photometer is used to compare the luminous intensities of the two sources. If the luminous power of the one source is known, then that of the second source can be calculated.

It consists of a piece of white paper with grease spot at its center and two sources of light whose luminous intensities are to be compared are placed on the two sides of the grease spot of the paper. These two sources are placed in such a way the grease spot becomes indistinguishable from the paper when the luminous flux per unit area coming from the white paper would be equal to that from the greased spot as shown in the figure. The grease spot transmits a fraction of incident light.

Suppose x represents the fraction of light transmitted by the grease spot. Then the fraction of light reflected by it is (1-x). The light energy coming per second from a unit area of grease spot to the left side of the paper is I1 (1-x) + xI2 where I1 and I2 are the intensity of illumination of the left side and right side of the paper due to S1 and S2 respectively. Similarly, the light coming per second from the unit area of the grease spot on the right side is I1x + (1-x)I2.

For the greased-spot to be indistinguishable from the paper,
(1-x) + xI2 = I1x + (1-x)I2

on solving above equation, we get I1 = I2
we know I1 = L1/ r12 and I2 = L2/ r22

Then, we have,
L1/ r12 = L/ r22
or, L1/ L2 = r12 / r22

Thus, by measuring r1 and r2 the luminous intensities of the two sources can be compared. If L1 is known, L2 can be calculated.

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