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PHY138 – Waves, Lecture 7
Today’s overview
The Nature of Light
Ray Tracing in Optics
Reflection of Light:
Law of Reflection
Refraction of Light:
Snell’s Law
Reading Assignment
Please read the following from Serway
and Jewett before class on Wednesday:
Chapter 25, Sections 25.7, 25.8 and
Chapter 26, Section 26.1.
A Web-CT quiz is due Wednesday
morning, which tests your basic familiarity
with Chapter 26 assigned reading. It’s the
last Web-CT quiz of this quarter.
The assignment due on Monday,
December 6 at 5:00 PM is now available
online. It includes problems from
Chapter 25 and one from Chapter 26.
It’s the last assignment of the Waves
Test 2 is at 9:00 AM sharp on Friday Dec.
10. One double-sided aid-sheet on
letter-sized paper per student will be
allowed. Test 2 covers material from the
Waves Quarter.
Wave Fronts and Rays
Wave fronts connect points of equal phase
in neighbouring points on an extended
If the wave source is a point, wave fronts
are concentric expanding spheres, all
centred on the source.
A very large or distant source can create
plane wave fronts.
Rays show the propagation direction of
waves, and are always perpendicular to
wave fronts.
The Law of Reflection
1  1
A dentist uses a mirror to
look at the back of a second
molar (A). Next, she wishes
to look at the back of a
lateral incisor (B), which is
90° away. By what angle
should she rotate her mirror?
1. 90°
2. 45°
3. 180°
Specular vs. Diffuse Reflection
Specular Reflection
•The surface is flat at distance scales near
or above the wavelength of light
•It looks “shiny”, like a mirror.
Specular vs. Diffuse Reflection
Diffuse Reflection
•The surface is rough at distance scales
near or above the wavelength of light
•Almost all surfaces reflect in this way.
Index of Refraction
v is the speed of light in a transparent
c is the speed of light in a vacuum
(c=3.00×108 m/s)
n is a new dimensionless constant: n≥1
Snell’s Law of Refraction
n1 sin 1  n2 sin 2