Answer Happy

a) Three light sources available in a laboratory are labelled A, B and C and have the following properties: The dominant emission from Source A is derived from an electronic transition from an excited state with lifetime 10-8 seconds. 550 560 570 580 Wavelength (nm) The frequency bandwidth of B is Af = 0.8 MHz. The emission spectrum of source C is displayed in the image above. (i) State what is meant by "the coherence time" of a light source, making reference to Source A. [3] (ii) Find the coherence times of Sources B and C. [5] b) Consider the reflection of light at normal incidence from glass coated with a trans- parent thin film. (i) Explain why the reflectivity of the coated glass is given accurately by the approximation Rafg= | raf+rfg exp(2i3) |², where raf and rfg are the Fresnel reflection amplitudes for the air-film and film-glass boundaries and is the phase thickness of the film. (ii) Describe qualitatively how the presence of a "quarter wave" film with higher refractive index than glass, i.e. nƒ > ng, can enhance the reflectivity of glass. Intensity t 3 - [2] [3]