Question

1) How much longer would it take for light to travel through 1.0 km of water than 1.0 km of air?
What formula do you use? The answer is 1.1 x 10^-6 s

2) The acetate roll on overhead projector is placed at a distance 1.2f from the lens. (where f is the focal length). At what distance should the screen be placed to properly focus the image?
Why is the answer 6f?

Answers

Elena
1
c /v(water) = n - refractive index (for water n=1.33)
v =c/n = 3•10^8/1.33=2.26•10^8 m/s
t1=s/c=1000/3•10^8=3.3•10^-6 s.
t2=s/v=1000/2.26•10^8=4.4 •10^-6 s.
t2-t1 =(4.4 -3.3) •10^-6 = 1.1•10^-6 s
2
1/f=1/di+1/do
di=f•do/(do-f) =
=f•1.2f/(1.2f-f)=
=1.2f²/0.2f = 6f

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