Question

The interfering fringes formed by a thin oil film on water are seen in the yellow light of sodium lamp. We find the fringes
(A) Colored
(B) Black and white
(C) Yellow and black
(D) Colored white yellow

Answer 1

Hint: When light waves undergo interference, the intensity gets redistributed forming bright and dark fringes. These fringes are formed without changing the frequency of the waves. As we know that the frequency of the light wave is the determinant factor for the color, so this means that the color of the light wave will remain constant.

Complete step-by-step solution
During interference, the waves from two coherent sources superpose on each other and the resultant wave has its intensity redistributed which is viewed as bright and dark fringes. During this interference process only the intensity is getting redistributed but the frequency of the light waves remains the same. This is the reason why yellow light from sodium lamps is seen after interference as well.

The light waves under constructive interference form yellow fringes as bright fringes and under destructive interference the black fringes are formed as dark fringes.

Hence, the correct option above is C.

Note: Constructive interference occurs when crest of one wave falls one crest of the other, has phase difference of 0 or 2n \[\pi \] and path difference n $\lambda $ and destructive interference occurs when crest of one wave falls on trough of other, has phase difference of \[\left( {2n + 1} \right)\pi \] and path difference $(2n - 1)\dfrac{\lambda }{2}$ .