Unit 14: Waves , Sound , and Physical Optics
Weightage : 15–18%
Most-appropriate topic codes (AP Physics 2):
(b)
The green color is caused by constructive interference of reflected light from the thin oil film.
Some light reflects from the air-oil interface, and some light enters the oil, reflects from the oil-water interface, and then returns to the observer. These two reflected waves overlap.
At the air-oil interface, the reflected wave undergoes a \(180^\circ\) phase shift because it reflects from a lower-index medium to a higher-index medium. At the oil-water interface, there is no \(180^\circ\) phase shift because the oil has a greater index of refraction than water.
The second reflected wave also travels an extra distance through the oil film. Since the wavelength in oil is different from the wavelength in air, \(\lambda_{\text{oil}}=\dfrac{\lambda_{\text{air}}}{n_{\text{oil}}}\), this extra path length affects the phase difference between the two reflected waves.
In the region that looks green, the film thickness causes green light reflected from the two surfaces to return in phase. Therefore, green light undergoes constructive interference and appears brighter than the other colors.
\(\boxed{\text{The oil looks green because reflected green light from the two film surfaces interferes constructively.}}\)
