Question of the Day: Which path should a lifeguard (L) take at a beach if she wants to reach the swimmer (S) in the least time? Assume she can run faster in sand than she can swim in water. L sand water S

This is an application of Fermat’s Principle of Least Time

from one medium into another ___________________is the _____________of a wave as it passes_____________________________________. Refraction bending from one medium into another transmitted Ex: It is the ___________________ wave that is __________________ . refracted wavefront ray ____________ diagram: ____________ diagram: medium 1 surface medium 2 reflected Note: The _________________ wave is not shown here.

Huygens Ex. Refraction explained using _______________ wavefronts.  Each point on the surface serves as a _____________ source for new ________________ in the second medium. point "wavelets"

When a light wave refracts, its ____________ does not change. This means its __________________ does not change. But the _______________ does change. color frequency speed In medium 2: In medium 1: v = fl v = f l changes = (same) changes If speed increases, l ___________________ . If speed decreases, l ___________________ . increases decreases This is true for ______________________ . all waves

Top view of wheels on an axle: = pavement = fast medium for wheels grass = slow medium for wheels This wheel slows down while other wheel is still moving fast

Refraction of light causes mirages. sand acts like a mirror

Water wave refraction:

Refraction only occurs when a wave: 1/______________________________, AND 2/ enters the new medium _____________ (at an angle_________________________.) changes speed obliquely other than 00 incident ray reflected ray normal medium 1 boundary medium 2 refracted ray

The _______________ ray is not refracted if EITHER there is: transmitted incident _____________ no speed change (n1 = n2) 1 2 transmitted …OR if: 2. ____________ ____________ q1 = 00 incident 1 not oblique 2 transmitted

The _____________ray will ___________ be in the Important points: The _____________ray will ___________ be in the quadrant that is _______ _______________of the boundary as the incident ray. reflected always incident ray normal on the same side 1 boundary 2 The ______________ ray will ____________be in the quadrant that is __________ of the quadrant of the incident ray. refracted No rays _________ in this quadrant! always opposite

Ex: A ray is shown incident on a boundary, shown by a darker line. Give the quadrant of the reflected and the refracted ray. A/ C/ II I II I refracted reflected III IV III IV refracted reflected B/ D/ II I reflected II I refracted III IV III IV reflected refracted

________________tells exactly how much refraction occurs. Snell’s Law angle of incidence q1 n1 n2 n1sinq1 = n2 sinq2 q2 medium 1 medium 2 angle of refraction Calculators must be in ______________mode, and not in ___________ mode. degrees radians

Page 2: bottom Which medium refracts waves the most? diamond Which medium refracts waves the least? air

Ex 1: Find the angle of refraction, q2 incident 600 air Lucite n1sinq1 = n2 sinq2 q2 1.5 sinq2 1.00 sin600 = 1.00 (0.866) = 1.5 sinq2 0.5774 = sinq2 sin-1(0.5774) = q2 350 = q2

Ex 2: Find the absolute index of refraction of the second medium, n2 300 zircon unknown 460 n1sinq1 = n2 sinq2 1.92 sin300 = n2 sin460 1.92 (0.500) = n2 (0.71934) 1.92 (0.500) = n2 0.71934 1.33 = n2 What medium is the unknown?

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 _________________ angle  bends__________________ 2 decreasing > If v1 ____ v2 (or n1 ____ n2): < 1 Decreasing v:  _________________ angle  bends__________________ 2 decreasing towards normal initial direction If v1 ____ v2 (or n1 ___ n2): < > 1 Increasing v: __________________angle  bends___________________ 2 increasing away from normal initial direction

Ex: In which medium is light faster? 1 2 1 2 H. B. E. 1 2 1 2 I. C. F. 1 2 1 2

apparent sun The atmosphere true sun is below horizon Earth your horizon The sun has already set when you see it setting!

Ex. Pools of water apparent depth actual depth i o farther Objects in pools are _______________ than they appear!