Question

Why are the two-source interference equations not valid for light from an incandescent bulb passing through a screen with two narrow slits onto an observation screen directly behind it?

1. Not monochromatic sources
2. Incoherent sources
3. Observed from a distance similar to or smaller than the separation between the sources

a) 1 only
b) 2 only
c) 3 only
d) 1 and 2 only
e) 1 and 3 only
f) 2 and 3 only
g) All three

Answer 1

The equations for two-source interference are not valid for a non-monochromatic and incoherent light source like an incandescent bulb passing through two slits.The correct answer, d) 1 and 2 only.

Step-by-step explanation:

The two-source interference equations are not valid for light from an incandescent bulb passing through a screen with two narrow slits because the conditions required for interference are not met. Incandescent bulbs are not monochromatic sources; they emit light consisting of multiple wavelengths. The incoherent nature of the light from such a bulb means that the wavefronts emitted are not in phase with one another, which is a key requirement for creating stable interference patterns (constructive and destructive) that can be predicted by these equations. Additionally, interference patterns are typically observed at distances much greater than the separation between slits, ensuring that the wavefronts have space to overlap and interfere properly; in other words, they require observation from a substantial distance, much larger than the separation between the sources.

The correct answer to why the two-source interference equations are not valid under these conditions is d) 1 and 2 only: not monochromatic sources and incoherent sources.