1. Foundations of Physics
CPO Science
Foundations of Physics
Unit 6, Chapter 18

2. Chapter 18 Wave Properties of Light
Unit 6: Light and Optics
Chapter 18 Wave Properties of Light
18.1 The Electromagnetic Spectrum
18.2 Interference, Diffraction, and Polarization
18.3 Special Relativity

3. Chapter 18 Objectives
Calculate the frequency or wavelength of light when given one of the two.
Describe the relationship between frequency, energy, color, and wavelength.
Identify at least three different waves of the electromagnetic spectrum and an application of each.
Interpret the interference pattern from a diffraction grating.
Use the concept of polarization to explain what happens as light passes through two polarizers.
Describe at least two implications of special relativity with regards to energy, time, mass, or distance.

4. Chapter 18 Vocabulary Terms
x-ray
spectrum
microwave
index of refraction
electromagnetic wave
spectrometer
gamma ray
radio wave
transmission axis
diffraction grating
special relativity
polarization
polarizer rest
energy destructive
interference
ultraviolet
time dilation
infrared
speed of light
constructive interference
visible light
wavelength

5. 18.1 The Electromagnetic Spectrum
Key Question:
What is the electromagnetic spectrum?
*Students read Section 18.1 BEFORE Investigation 18.1

6. 18.1 The Electromagnetic Spectrum
The energy field created by electricity and magnetism can oscillate and it supports waves that move.
These waves are called electromagnetic waves.

7. 18.1 The Electromagnetic Spectrum
Electromagnetic waves have both an electric part and a magnetic part and the two parts exchange energy back and forth.
A 3-D view of an electromagnetic wave shows the electric and magnetic portions.
The wavelength and amplitude of the waves are labeled λ and A, respectively.

8. 18.1 The Electromagnetic Spectrum
The higher the frequency of the light, the higher the energy of the wave.
Since color is related to energy, there is also a direct relation between color, frequency, and wavelength.

9. 18.1 Speed of Light c = f l Wavelength (m) Speed of light
3 x 108 m/sec
c = f l
Frequency (Hz)

10. 18.1 Calculate wavelength
Calculate the wavelength in air of blue-green light that has a frequency of 600 × 1012 Hz.
1) You are asked for the wavelength.
2) You are given the frequency.
3) The speed of light is c = fλ.
4) λ = c ÷ f
= (3 x 108 m/sec) ÷ (600 x 1012 Hz) = 5 x 10-7 m

11. 18.1 Waves of the electromagnetic spectrum
Visible light is a small part of the energy range of electromagnetic waves.
The whole range is called the electromagnetic spectrum and visible light is in the middle of it.

12. 18.1 Waves of the electromagnetic spectrum
Radio waves are on the low-frequency end of the spectrum.
Microwaves range in length from approximately 30 cm (about 12 inches) to about 1 mm.
The infrared (or IR) region of the electromagnetic spectrum lies between microwaves and visible light.

14. 18.1 Waves of the electromagnetic spectrum
Ultraviolet radiation has a range of wavelengths from 400 down to about 10 nm.
X-rays are high-frequency waves that have great penetrating power and are used extensively in medical and manufacturing applications.
Gamma rays are generated in nuclear reactions.

16. 18.2 Interference, Diffraction, and Polarization
Key Question:
What are some ways light behaves like a wave?
*Students read Section AFTER Investigation 18.2

17. 18.2 Interference, Diffraction, and Polarization
In 1807, Thomas Young ( ) did the most convincing experiment demonstrating that light is a wave.
A beam of light fell on a pair of parallel, very thin slits in a piece of metal.
After passing through the slits, the light fell on a screen.
A pattern of alternating bright and dark bands formed is called an interference pattern.

19. 18.2 Diffraction gratings
A diffraction grating is a precise array of tiny engraved lines, each of which allows light through.
The spectrum produced is a mixture of many different wavelengths of light.

20. 18.2 How a Diffraction Grating Works
When you look at a diffracted light you see:
the light straight ahead as if the grating were transparent.
a "central bright spot".
the interference of all other light waves from many different grooves produces a scattered pattern called a spectrum.

21. 18.2 Spectrometer
A spectrometer is a device that measures the wavelength of light.
A diffraction grating can be used to make a spectrometer because the wavelength of the light at the first-order bright spot can be expressed in a mathematical relationship.

22. 18.2 Grating Formula l = d sinq ~ dw L
distance between grating lines (m)
distance between
2 first order bright spots
wavelength
of light (nm)
l = d sinq ~ dw L
distance
between
screen and glasses
d= 13,500 lines/inch = ? lines/m

23. 18.2 Polarization Polarization is another wave property of light.
The fact that light shows polarization tells us that light is a transverse wave.
Most of the light that you see is unpolarized.
That does not mean the light has no polarization.
Unpolarized light is actually an equal mixture of all polarizations.
We call ordinary light unpolarized because no single polarization dominates the mixture.

24. An easy way to think about polarization is to think about shaking a spring back and forth.
If the spring is shaken up and down it makes vertical polarization.
If the spring is shaken back and forth it makes horizontal polarization.
Waves move along the spring in its long direction.
The oscillation of the wave (and its polarization) is transverse or perpendicular to the direction the wave travels.

25. 18.2 Polarization Polarization is a vector.
A wave with polarization at 45 degrees can be represented as the sum of two waves.
Each of the component waves has smaller amplitude.
Most of the light that you see is unpolarized.
That does not mean the light has no polarization.
Unpolarized light is actually an equal mixture of all polarizations.
We call ordinary light unpolarized because no single polarization dominates the mixture.

27. 18.2 Polarization
A polarizer is a material that selectively absorbs light depending on polarization.
A polarizer re-emits a fraction of incident light polarized at an angle to the transmission axis.
Most of the light that you see is unpolarized.
That does not mean the light has no polarization.
Unpolarized light is actually an equal mixture of all polarizations.
We call ordinary light unpolarized because no single polarization dominates the mixture.

28. 18.2 Applications of Polarizers
Polarizing sunglasses are used to reduce the glare of reflected light
The LCD (liquid crystal diode) screen on a laptop computer uses polarized light to make pictures.

29. 18.3 Special Relativity Key Question:
What are some of the implications of special relativity?
*Students read Section AFTER Investigation 18.3

30. 18.3 Special Relativity
The theory of special relativity describes what happens to matter, energy, time, and space at speeds close to the speed of light.

31. 18.3 Special Relativity These effects are observed in physics labs:
Time moves more slowly for an object in motion than it does for objects that are not in motion. This is called time dilation.
As objects move faster, their mass increases.
The definition of the word “simultaneous” changes.
Space itself gets smaller for an observer moving near the speed of light.

32. 18.3 Speed of light paradox
The theory of special relativity comes from thinking about light.
A ball thrown from a moving train approaches you at the speed of the ball relative to the train plus the speed of the train relative to you.
The speed of light appears the same to all observers independent of their relative motion.

33. 18.3 Speed of light paradox
If the person on the train were to shine a flashlight toward you, you would expect the light to approach you faster.
The light should come toward you at 3 × 108 m/sec plus the speed of the train.
But Michelson and Morley found experimentally that the light comes toward you at a speed of 3 × 108 m/sec no matter how fast the train approaches you!

34. 18.3 Consequences of time dilation
In the early 1970s an experiment was performed by synchronizing two precise atomic clocks.
One was put on a plane and flown around the world, the other was left on the ground.
When the flying clock returned home, the clocks were compared.
The clock on the plane measured less time than the clock on the ground. The difference agreed precisely with special relativity.

35. 18.3 Einstein's formula E = mc2
This equation tells us that matter and energy are really two forms of the same thing.
E = mc2
speed of light
3.0 x108 m/sec
Energy (J)
Mass (kg)

36. 18.3 The equivalence of energy and mass
If a particle of matter is as rest, it has a total amount of energy equal to its rest energy.
If work is done to a particle by applying force, the energy of the particle increases.
At speeds that are far from the speed of light, all the work done increases the kinetic energy of the particle.
It would take an infinite amount of work to accelerate a particle to the speed of light, because at the speed of light the mass of a particle also becomes infinite.

37. 18.3 The equivalence of energy and mass
Einstein’s was able to deduce the equivalent of mass and energy by thinking about the momentum of two particles moving near the speed of light.
Since the speed of light must be the same for all observers regardless of their relative motion and energy and momentum must be conserved, as the speed of an object gets near the speed of light, the increase in mass must come from energy.

38. 18.3 Calculate equivalents
A nuclear reactor converts 0.7% of the mass of uranium to energy.
If the reactor used 100 kg of uranium in a year, how much energy is released?
One gallon of gasoline releases 1.3 × 108 joules.
How many gallons of gasoline does it take to release the same energy as the uranium?
1) You are asked for energy in joules and gallons of gas
2) You are given the mass, a percent converted to energy, and J/gallon for gasoline.
3) Einstein’s formula: E = mc2
4) The amount of mass converted to energy is:
m = x 100 kg = 0.7 kg
Energy released is:
E = (0.7 kg)(3 x 108 m/sec)2 = 6.3 x 1016 J
To calculate the equivalent in gasoline divide by the energy per gallon:
N = 6.3 x 1016 J ÷ 1.3 x 108 J/g = 4.8 x 108 gallons
This is 480 million gallons ofgasoline.

39. 18.3 Simultaneity
The two lightning strikes are simultaneous to the observer at rest, but the observer moving with the train sees the lightning strike the front of the train first.

40. Application: Holography