Physics

Problem

A transparent film of uniform thickness

350\,\text{nm}

is deposited on a glass substrate. When white light is incident normally from air, strong reflected maxima are observed at

700\,\text{nm}

and

410\,\text{nm}

. Assuming these two wavelengths correspond to successive interference orders, what is the approximate refractive index of the film? a)

1.63

1.20

1.41

2.00

Intuitive explanation

Thin-film interference depends on the optical path difference created when light reflects from both surfaces of the film. The key insight is that successive bright fringes differ by exactly one wavelength of phase difference, so you can use the ratio of two consecutive maxima to find the refractive index without knowing which orders they are. Set up the condition for constructive interference at each wavelength, then use the fact that they're consecutive orders to eliminate the unknown order number and solve for $n$ .

Intuitive explanation

Browse the problem library