A drop of oil on a pond appears bright at its edges, where its thickness is much less than the wavelengths of visible light .What can you say about the index of refraction of the oil compared to that of water?
Step 1 of 3</p>
The third medium is air in the picture above, the refractive index of the film is bigger than the air. So the ray reflecting from the point A changes its phase
So the path difference of the rays 1 and 2 is
Where the thickness of the film is zero d=0
When d=0 , the path difference is λ/2 so there we have destructive interference
Step 2 of 3</p>
The film is oil and the third medium is water. When the third medium`s refractive index is bigger than the second. The ray reflected from point A doesn’t change its phase.
So the path difference between ray 1 and 2
When the thickness of the fim is minimum d=0
Path difference is zero that means that point is contractive interference. At the minimum thickness of the lens, we have bright fringe.