Q1Engineering Physics
Question
Why is it easier to see Newton's rings in reflected light than transmitted light?
Answer
In reflected light, the interference minima have nearly zero intensity resulting in high contrast, while in transmitted light, the interfering waves have vastly different amplitudes leading to poor contrast.
Newton's rings are formed by the interference of light rays reflected from the top and bottom surfaces of the air film between a lens and a glass plate.
In the reflected system, the two interfering rays have almost equal amplitudes. Therefore, during destructive interference, they completely cancel each other out, producing perfectly dark fringes against bright maxima (excellent contrast).
In the transmitted system, the interfering rays consist of one directly transmitted ray (very high amplitude) and one ray that has suffered two internal reflections (very low amplitude). Their interference cannot produce zero intensity at the minima. The fringes appear as slightly less bright rings on a very bright background, making them extremely difficult to see.