Question

A film of magnesium fluoride (n=1.38), 1.25x10^-5 cm thick, is used to coat a camera lens (n=1.55). Are any wavelengths in the visible spectrum intensified in the reflected light?
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Bats use the reflections from ultra-high frequency sound to locate their prey. Estimate the typical frequency of a bat's sonar. Take the speed of sound to be 3.40x10^2 m/s, and a small moth 3.00mm across, to be a typical target.

Answer 1

answers in the back of the book are
6.9 x 10^-5 cm

and

1.13 x 10^5 Hz

Answer 2

Δ=2bn-λ/2 = 2k(λ/2)
2bn =(2k+1)λ/2
λ=4bn/(2k+1)
For k=0,
λ=4bn=4•1.25•10⁻⁷•1.38=6.9•10⁻⁷m=
=6.9•10⁻⁵cm
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λ=3mm=3•10⁻³m
v=340 m/s
f=v/ λ=340/3•10⁻³=1.13 x 10⁵Hz

Answer 3

Thanks so much Elena !

Answer 4

why did you calculatebit as destructive interference? wouldnt we want it to be constructive if we want to intensify the light