A thin film of cooking oil ( n = 1.47 ) is spread on a puddle of water ( n = 1.35 ) . What is the minimum thickness D min of the oil that will strongly reflect blue light having a wavelength in air of 456 nm, at normal incidence?

The minimum thickness of the oil is 77.55 nm

Explanation:

Given:

Refractive index of oil

Refractive index of water

Wavelength of light m

From the equation of thin film interference,

The minimum thickness is given by,

Where , thickness

Here we have to find minimum thickness so we use

    m

    nm

Therefore, the minimum thickness of the oil is 77.55 nm

answer and explanation;

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.