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Module 5 Part 2
Waves and Light
Dr. Paul H. Comitz
pcomitz@live.com

Agenda
 Characteristics of Waves
 The Nature of Light
 The Speed of Light
 Light as a wave
 Light as a Particle: The Photoelectric Effect:
 Lab Photo Electric Effect
 http://phet.colorado.edu/en/simulation/photoelectric

Course Modules
# Module Weeks Reading Quiz
1 Newton's laws 1 Ch 4,5 *
2 Conservation of Energy and
Momentum
2,3 Ch 6,7,8 Quiz 1
3 Thermodynamics 4,5 Ch 12,13,14
4 Electromagnetism 6,7 Ch 17,18 Quiz 2
5 Waves, Sound, and Light 8,9 Ch 16, 20, 21 Quiz 3
6 Modern Physics 10 Ch 23 Final Exam
* it is strongly recommended you read chapters 0 - 3

Module 5
 Reading: Chapters 16,20,21
 Chapter 16 - Wave Motion and Sound
 Chapter 20 - Light
 Chapter 21 – Reflection and Refraction
 Exercise 5, due start of week 10 (4%)
 Quiz 3 – In class tonight
 Labs
 Waves (3.75%)
 Quantum Theory (3.75%) (Photoelectric Effect)
 Final Exam
 In class 8/23

Review: Characteristics of Waves
 Energy may be transferred through wave motion
 Amplitude
 Wavelength
 Frequency
 Period

Characteristics of Waves
Period
 time to complete one vibration
Example: Pendulum makes 2 vibrations in 1
second. Frequency is 2 Hz. Period of
vibration is 1⁄2 second.
Example: AM Radio frequency is 660 kHZ
Period – 1.5 ms
= 1frequency
period
= 1period
frequency

If the frequency of a particular wave is 20 Hz, its period is
A. 50 milliseconds
B. 20 seconds.
C. more than 20 seconds.
D. none of the above
Vibrations and Waves
CHECK YOUR NEIGHBOR

If the frequency of a particular wave is 20 Hz, its period is
A. 50 milliseconds
B. 20 seconds.
C. more than 20 seconds.
D. none of the above
Explanation:
Note when  = 20 Hz, T = 1/ = 1/(20 Hz) = 1/20 second = 50 ms.
Vibrations and Waves
CHECK YOUR ANSWER

The Nature of Light
 Straight Line
Propagation
 Reflection
 Mirror/smooth surface
 Rough surface
 Refraction
 Bending of light as it
passes through boundary
of two media

The Nature of Light
 Wave Theory
 Each point on a wave front
can be regarded as new
sources of small wavelets
that spread out uniformly in
the forward direction at the
same speed
 Particle Theory
 Light is made up of streams
of particles called photons
 Einstein
 Light behaves as both a
particle and a wave

The Nature of Light: Speed of Light
 Initially though to be instantaneous
 1920: Measurement method developed by
Albert Michelson
 Measured
 299,792,458 m/s
 Often abbreviated: 3 x 108 m/s
 In US units: 186,000 mi/s
 Recall
 v = l/T = lf
 l is wavelength
 T is period
 f is frequency

The Nature of Light: Speed of Light
Average speed of light through different
materials
• vacuum—c (300,000,000 m/s)
• atmosphere—slightly less than c (but rounded off to c)
• water—0.75 c
• glass—0.67 c, depending on material
• diamond—0.41 c

The Nature of Light: Refraction
Refraction
 bending of light when it passes from one
medium to another
 caused by change in speed of light

Refraction
Illusions caused by refraction
 Objects submerged in water appear closer to the
surface.

The Nature of Light: Reflection
Law of reflection
 The angle of reflection equals the angle of
incidence.

The Nature of Light: Reflection
Diffuse reflection
 when light strikes a rough or irregular surface
and reflects in many directions
 an undesirable circumstance is the ghost image
that occurs on a TV set when TV signals bounce
off buildings and other obstructions

Light as an Electromagnetic Wave
 Electromagnetic waves are produced by
moving electric charges
 Electric charges have an electric field
 Moving electric charges create a magnetic
field

Electromagnetic Spectrum
Electromagnetic spectrum
 classification of electromagnetic waves according to
frequency
 lowest frequency of light we can see appears red
 highest frequency of light we can see appears violet
 higher frequency of light is ultraviolet—more energetic and
causes sunburns
 beyond are X-ray and gamma ray
 no sharp boundary between regions

The electromagnetic spectrum spans waves ranging from lowest to
highest frequencies. The smallest portion of the electromagnetic
spectrum is that of
A. radio waves.
B. microwaves.
C. visible light.
D. gamma rays.
CHECK YOUR NEIGHBOR

The electromagnetic spectrum spans waves ranging from lowest to
highest frequencies. The smallest portion of the electromagnetic
spectrum is that of
A. radio waves.
B. microwaves.
C. visible light.
D. gamma rays.
CHECK YOUR ANSWER

Which of these is fundamentally different from the others?
A. sound waves
B. light waves
C. radio waves
D. X-rays
CHECK YOUR NEIGHBOR

Which of these is fundamentally different from the others?
A. sound waves
B. light waves
C. radio waves
D. X-rays
Explanation:
All are electromagnetic waves except sound, which is a
mechanical wave.
CHECK YOUR ANSWER

Light as a Particle: The Photoelectric Effect
 Einstein’s view on light
 as a stream of particles, bundles of energy (photons)
 photons interact with matter one at a time
 high-energy photons dislodge electrons from certain
metals
 Einstein was awarded the Nobel Prize in Physics in
1921 for his paper on the Photoelectric effect
 Photon
 A particle representing a quantum of light or other
electromagnetic radiation. A photon carries energy
proportional to the radiation frequency but has zero
mass

The Photoelectric Effect
Quantum Physics
Quantization
 the idea that the natural world is granular rather
than smoothly continuous
Quantum
 any elemental particle that makes up matter or
carries energy

Light As a Particle: The Photoelectric Effect
 A model for how matter radiates
 Light is a stream of photons each with a
particular quantum of energy
 high-energy photons dislodge electrons from
certain metals
 energy in each quantum is proportional to the
frequency of radiation
 E ~ f, or with Planck’s constant h,
 E = hf
 h = 6.62 x 10-34 J s

The Photoelectric Effect
high-energy photons dislodge
electrons from certain metals

Summary
 The Nature of Light
 Straight Line Propagation
 Reflection
 Refraction
 Speed of light c = 3 x 108 m/s
 Light behaves as both a wave and a particle
 Electromagnetic wave
 B and E fields
 Particles
 Photons
 Photoelectric effect
 Ejection of electrons from surfaces illuminated by
UV light

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Waves and Light

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