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Main Menu Help Logout Go to the previous resource in the Go to the next resource in the course sequence a Show Metadata Provide my evaluation of this Provide feedback messages or contribute to the course Prepare a printable document discussion about this resource notes and annotations about this resource Set a bookmark for this resource PHY131s17, Homework 9: Coating on a Lens to Minimize Reflection What is the minimum thickness of coating which should be placed on a lens in order to minimize reflection of 401 nm light? The index of refraction of the coating material is 1.39 and the index of the glass is 1.64. Submit Answer Tries 0/20 Post Discussion end Feedback

Explanation / Answer

Difference between two reflected wave must be half the wavelength so that minimum can occur

Hence

Path difference = 2*thickness of coating*refractive index of coating

0.5*401*10^-9=2*d*1.39

gives, d= 72.12*10^-9 m

d=72.12nm

For single slit diffraction,

dsin=m

for first minimum m=1

d*(y/D)=m

D=m/(d*y)

D=(1*578*10^-9)/(0.80*10^-3*2.41*10^-3)

D=0.3m

Width of central maximum = 2D/d = (2*578*10^-9*0.3)/(0.80*10^-3) = 0.43mm

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