990_Physics ProblemsTechnical Physics - Chapter 35 P35.42(a...

Chapter 35
331
P35.42 (a)
sin
sin
θ
θ
2
1
2
1
=
v
v
and θ
2
90 0 = ° . at the critical angle
sin .
sin
90 0 1 850
343
°
=
θ
c
m s
m s
so θ
c
= = °
−
sin . .
1
0 185 10 7
a f
.
(b) Sound can be totally reflected if it is traveling in the medium where it travels slower: air .
(c) Sound in air falling on the wall from most directions is 100% reflected , so the wall is a
good mirror.
P35.43 For plastic with index of refraction n ≥ 1 42 . surrounded by air, the critical angle for total internal
reflection is given by
θ
c
n
=
F
H
G
I
K
J
≤
F
H
G
I
K
J
= °
− −
sin sin
.
.
1 1
1 1
1 42
44 8 .
In the gasoline gauge, skylight from above travels down the plastic. The rays close to the vertical are
totally reflected from the sides of the slab and from both facets at the lower end of the plastic, where
it is not immersed in gasoline. This light returns up inside the plastic and makes it look bright.
Where the plastic is immersed in gasoline, with index of refraction about 1.50, total internal
reflection should not happen. The light passes out of the lower end of the plastic with little reflected,