Wave Optics Test

Result

Wave Optics Test - 7

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Question 1
1 / -0

If white light is used in the Newton's rings experiment, the colour observed in the reflected light is complementary to that observed in the transmitted light through the same point. This is due to:

A
90^ochange of phase in one of the reflected waves

B
180^ochange of phase in one of the reflection waves

C
145^ochange of phase in one of the reflection waves

D
45^ochange of phase in one of the reflection waves

Solution

The mechanism of formation of Newton's rings is the same as that of the production of colours in thin films. Usually the light beam falls normally on the air film included between the upper lens and the lower glass plate, but we have shown the rays slanting to make things clear.
Newton's rings in the reflected system are formed by the interference of waves 1 and 2 where as Newton's rings in the transmitted system are formed by the interference of waves 3 and 4.
If the phase change of π introduced due to reflection at a denser medium is ignored, the difference between the path lengths of the rays 1 and 2 is the same as the difference between the path lengths of the rays 3 and 4.
In the case of waves 1 and 2 there is an additional path difference of λ/2 because of the phase difference π introduced due to reflection at B. In the case of waves 3 and 4 there is an additional path difference of λ because of the phase difference 2π introduced due to reflections at B and C. If the condition for brightness is satisfied for one colour in the case of the interfering waves in the reflected system, the condition for darkness will be satisfied for the same colour in the case of the interfering waves in the transmitted system. So the colours present in the reflected system will be absent in the transmitted system and vice versa. The basic reason for this is the 180º phase change [Option (2)].