A circular planar wire loop is dipped in a so | vedclass

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(C) When a soap film is held vertically,the thickness of the film increases from top to bottom due to gravity. At the very top,the thickness $t$ is mu

Vedclass · Accepted Answer

The top portion appears dark while the first colour to be observed as one moves down is violet.

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(C) When a soap film is held vertically,the thickness of the film increases from top to bottom due to gravity. At the very top,the thickness $t$ is much smaller than the wavelength of visible light $(t \ll \lambda)$. For reflected light from a thin film,the condition for destructive interference is $2\mu t = n\lambda$ for $n = 0, 1, 2, ...$. At the top,where $t \approx 0$,the path difference is effectively $\lambda/2$ due to the phase change of $\pi$ upon reflection from the denser medium. This results in destructive interference for all visible wavelengths,making the top portion appear dark. As we move down,the thickness $t$ increases. The first colour to appear is the one that satisfies the condition for constructive interference $2\mu t = (n + 1/2)\lambda$ for the smallest $n$. Since violet has the shortest wavelength,it satisfies the condition for constructive interference at a smaller thickness than red. Therefore,violet is the first colour observed as one moves down.

$A$ circular planar wire loop is dipped in a soap solution and after taking it out,held with its plane vertical in air. Assuming the thickness of the film at the top is very small,as sunlight falls on the soap film,the observer receives reflected light. What does the observer see?

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