A laser beam of wavelength 630nm power output of 10mW and spot size of diameter

Question

A laser beam of wavelength 630nm power output of 10mW and spot size of diameter 3mm (assume the spot is round) is normally incident upon a infinitely thick sapphire block. What is the intensity of the reflected beam in W/m^2? What is the intensity of the transmitted beam just beyond the interface? This beam is to be shone through a saphhire window which is 2mm thick. What is the reflected intensity in this case? What is the transmitted intensity? The window is perfectly transparent, ie you may neglect any intensity loss as the beam moves through the sapphire.

Explanation / Answer

a. Intensity = Power / Area = 10mW / Area Area = pi * diameter^2 /4 = pi * (3/2) ^2 /4 mm^2 Reflected intensity = 10mW / pi * (3/2) ^2 /4 mm^2 = 5.66 mW/mm^2 = 5.66 *10^-3/10^-6 mW/mm^2 = 5660 W/m^2 Transmitted intensity = 0 b. Since, the window is perfectly transparent: Transmitted intensity = Incident intensity = 5660 W/m^2

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