Answer Happy

Reflectivity (%) A film with refractive index nf = 1.88 is deposited onto glass (ng = 1.50) in a vacuum chamber. Dur- ing deposition the sample is illuminated at normal incidence using Source B and its reflectivity R is measured. Results are shown in the image to the right. Deposition occurred at times t > to. Ro t₁ Time (iii) Source B has wavelength 633 nm. What was the thickness of the film at time t₁? [2] (iv) Given that the Fresnel reflection amplitude for normal incidence at medium 2 from medium 1 is: N₁ N₂ 712 = n₁ + n₂ predict the numerical values of Ro and R₁ indicated in the image. [3] (v) Suggest a reason for the initial drop in reflectivity observed at small deposition times in the image. [3] (vi) Describe qualitatively how the experimental reflectivity results would change if Source C (with wavelength 561 nm) or if a white source (with mean wavelength 561 nm and linewidth 200 nm) were used in the above experiment. [4] R₁