•
•
Exam 1 Review
PHY 1033
Fall 2011 Exam #1 In class (1 hr. 40 min. ending 4:40 pm) Ihas Sept. 20
You are allowed to consult one 8.5”x11” sheet of paper with anything written on it in your own
hand, and to use a calculator, ruler, and pencils or pens. Number your answer to each question
clearly, answering them in numerical order in a “Blue/Green Book” test form. Clearly put your name
on the front of the blue/green book with “Exam #1” and today’s date. Do not violate the UF honor
code. Write on the front of the blue/green book “I have neither given nor received help from
anyone during this exam”, and sign your name. Answers without the required justification or
explanations will be counted wrong, even if they are correct. You may quietly ask me any question
after raising your hand, but my answers will only clarify the exam questions. Good luck!
Science = A Process for Change!
•
•
•
•
•
•
Discovery
Experiment/Observation
Hypothesis
Prediction
Test
Theory
An ongoing cycle – the “scientific method”
Scientific Method: The dynamic interplay between theory and experiment
What is “Physics”
• An attempt to rationalize the observed Universe in
terms of irreducible basic constituents, interacting
via basic forces.
– Reductionism!
What does light do?
Travel in a straight line
Except when it passes from one substance
to another
Angles: Incidence and reflection equal
Measure angles from “Normal” to surface
Light rays and angles are in one plane
Light paths are reversible
From air-to-water: angle of refraction is
less than angle of incidence
Geometric Optics
We are using the Ray Approximation
• Light travels in a straight-line path in a
homogeneous medium until it encounters a
boundary between two different media
Specular
Reflection
• Specular reflection is
reflection from a smooth
surface
• The reflected rays are
parallel to each other
• The ray approximation is used to represent beams
of light
• A ray of light is an imaginary line drawn along the
direction of travel of the light beams
Quick Quiz: What happens if the surface is rough?
A.Angle of incidence equals angle of reflection
B.A single beam reflects at a different angle than incidence
C.The reflection is defuse
D.How rough is rough?
E.All the light goes into the block
Diffuse
Reflection
• Diffuse reflection is
reflection from a rough
surface
• The reflected rays travel
in a variety of directions
• Diffuse reflection makes
the dry road easy to see
at night
Law of Reflection
• The normal is a line
perpendicular to the surface
– It is at the point where the
incident ray strikes the
surface
• The incident ray makes an
angle of θ1 with the normal
• The reflected ray makes an
angle of θ1’ with the normal
The angle of reflection is equal to the
angle of incidence
θ1= θ1’
The Index of Refraction
Refraction of Light
• The incident ray, the
reflected ray, the
refracted ray, and the
normal all lie on the same
plane
• The angle of refraction,
θ2, depends on the
properties of the two
media
Some Indices of Refraction
• When light passes from one medium to another,
it is refracted because the speed of light is
different in the two media
• The index of refraction, n, of a medium can be
defined
n=
speed of light in a vacuum c
=
speed of light in a medium v
Snell’s Law of Refraction
• n1 sin θ1 = n2 sin θ2
– θ1 is the angle of incidence
• 30.0° in this diagram
– θ2 is the angle of refraction
n1
n2
Quick Quiz:
If n1 =1, n2 =1.5, and sin30°=0.5
What is sin θ2 ?
A. 3 B. 1 C. ½ D. 1/3 E. ¼
Total Internal Reflection
• Total internal reflection
can occur when light
attempts to move from
a medium with a high
index of refraction to
one with a lower index
of refraction
– Ray 5 shows internal
reflection
Critical Angle
• A particular angle of
incidence will result in
an angle of refraction
of 90°
– This angle of
incidence is called
the critical angle
n1 sin θ1 = n2 sin θ2
n1 sin θ1 = n2 sin θ2
n1 sin θ1 = n2 * 1
sin θ C =
• The index of refraction in anything except a vacuum depends on the
color of the light
• This dependence of n on color is called dispersion
• Light disperses into colors when refracted
The index of refraction for a
material usually decreases with
from violet to red
Violet light refracts more than
red light when passing from air
into a material
Responsible for rainbows and
all other prism effects
The Rainbow
Refraction in a Prism
• The amount the ray is bent
away from its original
direction is called the angle of
deviation, δ
• Since all the colors have
different angles of deviation,
they will spread out into a
spectrum
– Violet deviates the most
– Red deviates the least
• A ray of light strikes a drop of water in the atmosphere
• It undergoes both reflection and refraction
– First refraction at the front of the drop
• Violet light will deviate the most
• Red light will deviate the least
A ray of light strikes a drop of water
in the atmosphere undergoes both
reflection and refraction
First refraction at the front of the drop
At the back surface the light is reflected
It is refracted again as it returns to the front
surface and moves into the air
n2
for n1 > n2
n1
Dispersion
Following the Reflected and Refracted Rays
• Ray 1 is the incident ray
• Ray 2 is the reflected
ray
• Ray 3 is refracted into
the lucite
• Ray 4 is internally
reflected in the lucite
• Ray 5 is refracted as it
enters the air from the
lucite
What is the critical angle for the
water/air interface?
1. 90°
2. 49°
3. 42°
4. 0°
At the back surface the light is
reflected
It is refracted again as it returns to
the front surface and moves into the
air
The rays leave the drop at various
angles
The angle between the white light and the
violet ray is 40°
The angle between the white light and the
red ray is 42°
Observing the Rainbow
The WAVE THEORY,
THEORY,
advocated by
Christian Huygens
and Robert Hooke,
Hooke,
said that light was a wave.
The PARTICLE (corpuscular) THEORY,
THEORY, advocated
by Isaac Newton and later by Pierre Laplace,
Laplace, said
that light was
made up of a
stream of tiny
particles called
corpuscles.
• If a raindrop high in the sky is observed, the red ray is
seen
• A drop lower in the sky would direct violet light to the
observer
• The other colors of the spectra lie in between the red and
the violet
The whole universe is composed of SHOs
SHO is system with linear restoring force to original
shape/position
Mass/spring
Frequency f =
1 ω
=
T 2π
ωms =
k
m
g
L
λ
X direction (space/not time)
Longitudinal or compression
wave
λ
Chapter 14
Wave motion-Wave velocity c
c
c
t
Types of Waves – Transverse
Each part of spring moves
⊥To wave motion
Like AM radio
Pendulum
ωp =
Oscillators make Waves
All waves carry energy and momentum
Tuning Forks produce single frequency sound
c = λƒ
Always true: Light-transverse
Sound-longitudinal
Any wave
Most waves need a medium-Except light
λ
• As the tuning fork vibrates, a succession
of compressions and rarefactions spread
out from the fork
• A sinusoidal curve can be used to
represent the longitudinal wave
– Crests correspond to compressions and
troughs to rarefactions
Categories of Sound Waves
• Audible waves
– human hearing:
Normally between 20 Hz to 20,000 Hz
• Infrasonic waves
– Frequencies are below the audible rangeEarthquakes or car stereos
• Ultrasonic waves
– Frequencies are above the audible range
Dog whistles
Middle A on the piano is 440 Hz. The speed of
sound in air is about 330 m/s. What is the
wavelength of this sound wave?
1.
2.
3.
4.
4.7 m
1.3 m
0.75 m
0.87
c = λƒ
Always true: Light-transverse
Sound-longitudinal
Any wave