Consider a horizontal plane of thin film with a thickness t. This film is located in between air and water (see sketch). Light is directed from the air downward through the film and into the water, perpendicular to the surfaces. The index of refraction of the film n1 = 1.5. The index of refraction of water is n2 = 1.33. The wavelength of the incident wave in the air is λ, and its frequency in air is f = c λ . Air Water t n1 n2 The wavelength λwater of the light in the water is 1. λwater = n2 λ 2. λwater = n1 n2 λ 3. λwater = 1 n2 λ