Download Electromagnetic (Light) Waves

Physics R
Date: __________________
Review Questions
1. An ocean wave traveling at 3 m/s has a wavelength of 1.6 meters.
a. What is the frequency of the wave?
b. What is the period of the wave?
Electromagnetic (Light) Waves
Electromagnetic Waves
Until the 20th century, reality was everything humans could touch, smell, see, or hear. Since the initial
publication of the charted electromagnetic spectrum, humans realized what they could touch, smell, see,
and hear is less than one millionth of reality.
Electromagnetic Spectrum
Visible light, infrared light, x-rays, microwaves, gamma rays, etc. are all part of the electromagnetic
spectrum. They all travel with the same _____________ in a vacuum. However, they have different
__________________ (Hz) and ___________________ (m). Find the spectrum in your reference table.
Monochromatic
1
Electromagnetic waves all travel at the same speed in a vacuum
Practice
2. What type of EM wave is the following?
500,000 hertz
_________________________
5.0 x 10-9 meter
_________________________
13
3.0 x 10 hertz
_________________________
14
4.5 x 10 hertz
_________________________
200 meters
_________________________
3. What is the frequency of a 632 nanometer EM wave? What color is it?
4. What is the wavelength of a 5 x 1013 Hz EM wave? What type of wave is it?
Electromagnetic waves are transverse waves.
Radio waves travel from New York City to the antenna on your radio in Hicksville.
How should you orient the antenna on your radio to get reception, horizontally or vertically? Why?
5. The antenna of a device is (approximately) the wavelength of the incoming signal. If you’re
purchasing a home phone and want one with a smaller antenna, which should you buy, a 900 Mhz or a
2.4 Ghz phone? Why?
6. How long should you make an antenna to pick up an AM radio signal of 800 khz?
Doppler effect
Light from a star in distance space is expected to have a frequency of 6.3 x 1014 Hz. Instead the star has
a slightly lower frequency than what would have been expected. Is the star moving towards or away
from Earth? Is this a red shift or blue shift?
2
Speed of an electromagnetic waves (light) in a medium
When light enters a new medium, the speed changes and the wavelength changes but the frequency
remains the same. For all wave phenomena, frequency always remains the same except for the Doppler
Effect.
7. What is the speed of light in:
a. Corn oil
b. Diamond
c. Water
8. Light travels at 1.563 x 108 m/s through a material. What is the composition of the material?
9. Light with a frequency of 5.2 x 1015 Hz is traveling through water.
a. How fast is it traveling?
b. What is the wavelength of the light in water?
10. Light with a frequency of 3.84 x 1014 Hz is traveling through glycerol.
a. How fast is it traveling?
b. What is the wavelength of the light in glycerol?
Energy of an electromagnetic wave (light)
Higher frequency EM waves have higher energy
Ephoton =
(found in the Modern Physics part of the reference table)
A photon is the smallest particle of light – like an atom is the smallest particle of matter.
9. An electromagnetic wave has a frequency of 5.1 x 1014 Hz
a. What color wave is this?
b. What is the energy of the wave?
c. What is the speed of this wave in a vacuum?
3
10. A photon is traveling with a frequency of 8.6 x 1016 Hz
a. What type of EM wave is this?
b. How much energy does this photon have?
c. What is the wavelength of this wave?
11. An electromagnetic wave is traveling with frequency 3 x 107 Hz
a. What type of EM wave is this?
b. How much energy does it have?
c. What is the speed of this wave in a vacuum?
Phase (True for all waves, not just EM waves)
2 points are in phase if they are on the same part of a wave
2 points are 180 degrees out of phase at opposite parts
In Phase
Out of Phase
Polarization
A polarizing filter blocks out all waves except those with
particles moving in a certain direction.
This only works for transverse waves
– longitudinal waves can NOT be polarized.
Reflection
Law of reflection:
12. To the right (using a protractor and ruler)
a. Draw the normal
b. What is the angle of incidence?
c. What is the angle of reflection?
d. Draw the reflected ray.
4
13. To the right (using a protractor and ruler)
An incident ray has an angle of 20 degrees
a. Draw the incident ray (and normal)
b. Draw the reflected ray
Diffraction - Electromagnetic waves diffract just like mechanical waves
Air
Air
There is no change in medium necessary for diffraction
14. A 7x1014 Hz EM wave in air goes through a hole and diffracts
a. What is the speed before it goes through the opening?
b. What is the speed after?
c. What is the wavelength before the opening?
d. What is the wavelength after?
e. Sketch four diffracted wave fronts after the opening
Note: Larger wavelength and smaller opening -> More diffraction
15. Radio waves and light waves go past tall buildings. Which will diffract more? Why?
Interference - Electromagnetic waves interfere just like mechanical waves
If you put a paper to observe light
along B-D-C, what would the pattern
look like?
Mechanical versus Electromagnetic Waves
Mechanical waves
vs
Electromagnetic waves
Can travel in a vacuum
Transverse or longitudinal
Energy is related to
Requires a medium to travel
Tastes good on a pizza
5
Refraction (transmission when light enters a new medium)
When light (EM waves) enter a different medium, there is a change in ____________________
which causes a change in __________________ or ______________________
Recall: as light travels through a different medium, its speed changes according to the index of
refraction
n=
Light changes direction (angle) according to Snell’s Law
16. A ray of light (f=5.09 x 1014 Hz) is incident on the boundary
between air and an unknown material X at an angle of incidence
of 55°, as shown. The absolute index of refraction of
material X is 1.66.
a. Determine the speed of light in material X
b. Calculate the angle of refraction of this light ray
17. A monochromatic light ray (f=5.09 x 1014 Hz) traveling
in air is incident on the surface of a rectangular block of
Lucite (n=1.50).
a. Measure the angle of incidence for the light ray to the
nearest degree
b. Calculate the angle of refraction of the light ray when it
enters the Lucite block.
c. What is the angle of refraction of the light ray as it
emerges from the Lucite block back into the air?
6
18. A ray of monochromatic light having a frequency of 5.09 ×
1014 hertz is incident on an interface of air and corn oil at an
angle of 35° as shown. The ray is transmitted through parallel
layers of corn oil and glycerol and is then reflected from the
surface of a plane mirror, located below and parallel to the
glycerol layer. The ray then emerges from the corn oil back into
the air at point P.
a. Calculate the angle of refraction of the light ray as it enters
the corn oil from air.
b. Explain why the ray does not bend at the corn oil-glycerol interface.
c. On the diagram, use a protractor and straightedge to construct the refracted ray representing the light
emerging at point P into the air.
Dispersion
Why do all refraction problems specify the same frequency (f=5.09 × 1014 hertz)?
Why is the sky blue?
Why is the sun yellow?
Why are sunsets red?
7
19. A wave generator with a constant frequency
produces parallel wave fronts in a tank of water
of two different depths. The diagram below
represents the wave fronts in the deep water. As
the wave fronts travel from the deep water into
the shallow water, the speed of the waves
decreases. On the diagram at right use a
straightedge to draw at least three lines to
represent the wave fronts, with appropriate
spacing, in the shallow water.
20. Orange light has a frequency of 5.0 x 1014 Hz in a vacuum. What is the wavelength of this light?
21. How much time does it take light from the moon to reach Earth?
How much time would it take a radio wave to travel from the moon to Earth?
22. What is the energy in Joules of a photon with wavelength 532 nm?
23. A hydrogen atom emits a photon with an energy of 1.63 x 10-18 J. What type of radiation would this
photon be classified? (i.e. infrared, blue light, x-ray, etc.)
24. Radio waves diffract around buildings more than light waves do because compared to light waves,
radio waves (move faster) (move slower) (have a higher frequency) (have a longer wavelength)
25. A ray of monochromatic light (f = 5.09 x 1014 hertz) in air is incident at an angle of 30° on a
boundary with corn oil. What is the angle of refraction?
26. A monochromatic ray of light (f = 5.09 x 1014 hertz) traveling in air is incident upon medium A at
an angle of 45°. If the angle of refraction is 29°, what is medium A? (water, fused quartz, Lucite, or
flint glass)
27. A ray of light is travelling from water to diamond. If the incident angle is 45°, what will the angle
of refraction be?
8
Review Checklist
Determine the part of the EM Spectrum that a particular wave belongs to. Explain the origin of
electromagnetic waves and contrast this with the production of mechanical waves. Use equation to
determine speed in a medium or index of refraction for a medium.
1. What type of electromagnetic radiation is a wave with a __________?
wavelength of...
Frequency of…
(a) 3.5 x 103 meters
(e) 3.0 x 106 Hz
-8
(b) 4.5 x 10 meters
(f) 2.2 x 1015 Hz
(c) 2.0 x 10-12 meters
(g) 1.5 x 1010 Hz
(d) 6.0 meters
(h) 5.5 x 1014 Hz
2. Determine the speed of a ray of light with a frequency of 5.09 x 1014 hertz as it passes through water
(index of refraction = 1.33).
3. Determine the absolute index of refraction for a medium in which microwaves travel at a speed of 1.8
x 108 meters per second.
Explain the Law of Reflection; and identify ray diagrams in which reflection occurs. Explain Snell’s
Law and identify ray diagrams in which refraction occurs. Use Snell’s Law to determine angle of
incidence; angle of refraction; or index of refraction.
4. A ray of light moving from air into Lucite enters the Lucite at an angle of 35° relative to a line
perpendicular to the Lucite surface. What angle will the light be bent at as it moves through the Lucite?
5. A ray of light is incident on a block of unknown glass at an angle of 45 degrees. The light refracts to
an angle of 25.2 degrees. Determine the material of the unknown glass.
Answers
1. a. radio/long radio
b. x ray
c. gamma ray
d. radio/TV FM
e. radio/AM
f. ultraviolet
g. microwave
h. visible/green
2. 2.26 x 108 m/s
3. 1.67
4. 22.5 degrees
5. Flint glass
9