In Lloyd's mirror experiment (Fig.) a light wave emitted directly be the source `S` (narrow slit) interferes with the wave reflected form a mirror `M`. As s result, an interference fringe pattern is

formed on the screen `Sc`. the source and the mirror are separated by a distance `l = 100 cm`.At a certain position of the source the fringe width on the screen was equal to `Deltax = 0.25 mm`, and after source was moved away from the mirror plane by `Delta h = 0.60 mm`, the fringe width decreased `eta = 1.5` times. Find the wavelength of light.

Explanation

From the general formula
`Deltax = (l lambda)/(d)`
we find that `(Delta x)/(eta) = (l lambda)/(d + 2Delta h)`
since `d` increases tod `+2 Deltah` when the source is moved away from the mirror plane by `Delta h`.
Thus `eta d = d+ 2 Delta h` or `d = 2 Delta h//(eta - 1)`
Finally `lambda = (2Delta hDeltax)/((eta-1)l) = 0.6 mu m`.