- A beam of light from a slit is incident on the bi prism, which creates two virtual coherent
sources by the principle of refraction. The light from the two coherent sources meet on the
screen/eyepiece and interfere.
- Constructive interference leads to a bright fringe and destructive interference leads to a dark
fringe.
- The condition for constructive interference is
$$\Delta x=nλ \qquad where\;n=1, 2, 3…$$
\(\Delta x\) = path difference,\(\lambda\)= wavelength of light from Sodium-vapour lamp.
- The wavelength is measured by the formula:
$$λ=\frac{βd}{D}$$
Where \(β\) = fringe width, \(D\) = distance from source to screen, \(d\) = distance between two sources.
- The distance between the two virtual sources is found out by displacement method (described
later).