Answer Happy

A block of material B is placed inside a coating machine and a layer of material A is deposited upon it. During deposition light from the LED considered in part a) is reflected from the sample at normal incidence. Deposition occurs at a constant rate between times t = 0 and t = 1800 seconds. Measured reflectivity results are shown above right in image 2. ni - nt ni + ni The Fresnel amplitude for normal incidence reflection is given by r = n is refractive index and the subscripts indicate the incident and transmission media. where (i) Calculate the refractive index of material B. [4] (ii) For the three turning points in the image, state whether the rays from the air-A boundary and the A-B boundary are in phase or out of phase. You must explain your reasoning. [3] [3] (iii) Find the deposition rate (in nm per second) and the final thickness of the film. (iv) In a separate experiment, the deposition system is set running with a clean B substrate on a Friday afternoon. Despite successful deposition over the week- end, by Monday morning no further reflectivity oscillations are observed. Sug- gest an explanation and hence estimate the reflectivity measured on Monday morning. [4] (v) A new B sample is placed into the deposition system and the LED is replaced by a blue laser. It is found that the reflectivity oscillations die out after only a few minutes of deposition. Suggest an explanation for this observation. [2]