91Ó°ÊÓ

It is a phenomenon where one light wave superposes over another coherent light wave and this superposition leads to a redistribution of the intensity of light rays at different points.

For constructive interference, the condition is,

$\mathrm{d}=\mathrm{³¾Î»},\mathrm{m}=0,\underline{+1,+2}$ ...(i)

For destructive interference, the condition for path difference is,

$\mathrm{d}=\left(\mathrm{m}+\frac{1}{2}\right)\mathrm{λ},\mathrm{m}=0,\underline{+1,+2}$

...(ii)

Here, $\mathbf{d}$is the path difference between two waves and $\mathbf{λ}$ is the wavelength of light used.

For very thin soap film, the beam gets reflected first at the air-water boundary and the second reflection comes from the water-air interface. Both of these beams that are reflected from different interfaces have the same phase and cause destructive interference. Therefore, the film has a dark appearance.

For thin soap film on glass, the reflection happens at three interfaces- air-water, water-glass and glass-air interface. The third reflection causes the overall interference to be constructive and thus the film appears to be shining.