19. A device called a Fabry-Perot interferometer contains two parallel mirrors as shown below. The multiple reflections occur between the inner mirror surfaces, and the waves transmitted at the top can interfere. Assume that the light waves travel nearly perpendicular to the mirrors, the transfmitted waves interfere constructively, and the spacing between the mirrors is exactly 3.5um. Provide a mathematical solution, to determine possible value(s) for the wavelength in the visible range between 600 nm and 700 nm. [7 marks] These waves interfere. mirrors 20. A thin film of soapy water with an index of refraction of 1.35 has a thickness of 2.50 x 10 'm at one point. The film is surrounded by air. Determine the two longest wavelengths of light that will interfere constructively when reflecting from the soapy film. [5 marks]