Lloyd's Mirror Experiment - Practice Questions & MCQ

Lloyd's mirror experiment

In the Lloyd's mirror experiment, light from a monochromatic slit source reflects from a glass surface at a small angle and appears to come from a virtual source as a result. The reflected light interferes with the direct light from the source, forming interference fringes.

Experimental setup: 

A plane glass plate (acting as a miror) is illuminated at almost grazing incidence by a light from a sliftual image $S_2$ of $S_1$ is formed closed to $S_1$ by reflection and these two act as coherent sources. The expression giving the fringe width is the same as for
the double silt, but the fringe system differs in one important respect.

The path difference $S_2 P-S_1 P$ is a whole number of wavelengths, the fringe at P is dark not bright. This is due to $180^{\circ}$ phase change which occurs when light is reflected from a denser medium. At grazing incidence a fringe is formed at O , where the geometrical path difference between the direct and reflected waves is zero and it follows that it will be dark rather than bright.

Thus, whenever there exists a phase difference of a $\pi$ between the two interfering beams of light, conditions of maximas and minimas are interchanged, i.e.,
$\Delta x=n \lambda$ (for minimum intensity) and
$\Delta x=(2 n-1) \lambda / 2$ (for maximum intensity)