**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 I_{1} (1-x) + xI_{2} where I_{1} and I_{2} are the intensity of illumination of the left side and right side of the paper due to S_{1} and S_{2} respectively. Similarly, the light coming per second from the unit area of the grease spot on the right side is I_{1}x + (1-x)I_{2}.

_{2}= I

_{1}x + (1-x)I

_{2}

_{1}= I

_{2}

_{1}= L

_{1}/ r

_{1}

^{2}and I

_{2}= L

_{2}/ r

_{2}

^{2}

_{1}/ r

_{1}

^{2}= L

_{2 }/ r

_{2}

^{2}

_{1}/ L

_{2}= r

_{1}

^{2}/ r

_{2}

^{2}

Thus, by measuring r_{1} and r_{2} the luminous intensities of the two sources can be compared. If L_{1} is known, L_{2} can be calculated.

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