Question

Why is the central fringe dark?

Answer 1

Hint: In order to solve this question, we should know about constructive interference and destructive interference, when two waves overlap with each other having same phase, they forms constructive interference and produce bright spots whereas when two waves overlap with out of phase, they produce destructive interference and produce dark spots. Here, we will discuss Newton rings in the reflected system to know the reason for central fringe darkness.

Complete step by step solution:
As we know, Newton rings is an experiment where we use a spherical lens and a flat surface below the lens to produce an interference pattern by reflection of light.
So, when a lens and flat surface is placed then a thin air film having some definite thickness is produced at the centre between the spherical lens and the flat glass plate. So, the geometrical path difference of light between the incident and the reflected light at this centre will be zero. But due to reflection of light the phase of light changes between the incident and reflected light is of ${180^o}$ so, a path difference of $\dfrac{\lambda }{2}$ is produced.
Hence, due to the phase of light between incident ray and reflected ray, and a path difference of $\dfrac{\lambda }{2}$ destructive interference produced at the centre of newton’s ring occurs and destructive interference always produces a dark fringe.

Note: It should be remembered that It was S. Isaac Newton who discovered the phenomenon of Newton rings and this was written in a famous book written by Newton called Opticks. Interference is a phenomenon of superimposing two light waves to produce resultant waves having either lower, higher or same amplitude.