Physics 102-002
Announcements
• WebAssign –
– Chapter 26 due next Monday
• No class Wednesday
– We’ll cover Ch 28 next week
• Exam 3 Corrections due
Wed, Apr 25
Picture: National Science Foundation's Very
Large Array (VLA) radio telescope image of a
"spectacular and complex structure" in giant
elliptical galaxy M87, the central galaxy of the
Virgo Cluster of Galaxies. This photo shows
two large, bubble-like lobes, more than
200,000 light-years across, that emit radio
waves.
Class Schedule
4/9
Midterm Exam #3
4/11
Chapter 24
Magnetism, (Pg 458-470)
4/16
Chapter 26
Properties of Light
4/18
No class
4/23
Chapter 28
Reflection and Refraction, Part 1 (Pg 530-540)
4/25
Chapter 28
Reflection and Refraction, Part 2 (Pg 540-551)
4/30
Chapter 30
Light Emission (Pg 558-568) Midterm Exam #4
5/2
Review
5/7
Final Exam
Note the changes!!!
Chapter 26
Properties of Light
• Electromagnetic Waves
– Electromagnetic Wave Velocity
– Electromagnetic Spectrum
• Transparent Materials
• Opaque Materials
What is Light?
When you look at something … anything … you are, in fact, seeing light -- light
that somehow left the object far or near and reached your eyes. Light is all our
eyes can really see.
You also encounter light in devices that produce light -- incandescent bulbs,
fluorescent bulbs, lasers, lightning bugs, the sun. Each one uses a different
technique to generate photons (or “pieces” of light)
Modern physicists believe that light can behave as both a particle and a wave.
We will talk about light mostly as waves, because this provides the best
explanation for most of the phenomena our eyes can see.
There are many different ways to produce photons, but all of them use the same mechanism inside an
atom to do it. This mechanism involves the energizing of electrons orbiting an atom's nucleus
Electrons circle the nucleus in fixed orbits. An electron has a natural orbit that it occupies, but if you
energize an atom you can move its electrons to higher orbitals. A photon of light is produced whenever
an electron in a higher-than-normal orbit falls back to its normal orbit. During the fall from high-energy to
normal-energy, the electron emits a photon -- a packet of energy -- with very specific characteristics. The
photon has a frequency, or color, that exactly matches the distance the electron falls.
Electromagnetic Waves
If you shake a charged stick in the air, you create a current … a moving charge.
Recall that a moving charge emits a magnetic field. A changing magnetic field
creates a changing electric field.
If the charge moves back and forth (oscillates), it emits an oscillating magnetic field.
The oscillating magnetic field creates an oscillating electric field, which creates an
oscillating magnetic field, and so on …
Both of these fields continuously regenerate each other and propagate out through
all space.
This is an Electromagnetic Wave, and you can visualize it as shown.
The electric and magnetic fields
oscillate in planes perpendicular
to each other as the wave moves
through space, both moving in the
same direction at the same
speed, and at the same
frequency.
Physics Place figure
Electromagnetic Wave Velocity
Light always moves through a vacuum at 3x108 m/s … ALWAYS!
(it travels a bit slower when moving through other materials, though)
In the 1800s, JC Maxwell was able to use the equations of electrostatics and magnetism to
DERIVE the exact value for the speed of light in vacuum WITHOUT measuring it. His calculated
value has since been experimentally verified many times over.
All electromagnetic radiation moves at this speed!
Maxwell discovered that visible light is electromagnetic radiation in the frequency range between
4.3X1014 Hz and 7x1014 Hz (vibrations per second).
Other types of electromagnetic radiation (radio waves, x-rays, tv signals, radar, etc)
all have the same cause and nature as visible light, but just vibrate at different
frequencies.
Notice that electromagnetic radiation doesn’t need a medium to move through, it
can move through a vacuum! That’s because both electric and magnetic fields are
transmitted through space without needing a medium.
James C. Maxwell
The Electromagnetic Spectrum
The classification of electromagnetic waves according to frequency is the electromagnetic spectrum.
The EM Spectrum by frequency
The EM Spectrum by wavelength
Visible light is less than 1 millionth of
1% of the EM spectrum
The divisions between regions aren’t sharp, they’re a little fuzzy, and they overlap.
Also, some of the regions are further divided. For example, there are Near-, mid-, and far-infrared,
and hard and soft x-rays.
Frequency and wavelength are related. The higher the frequency, the shorter the wavelength.
Further, frequency and wavelength are related to the speed of light.
Physics Place figure
Frequency x wavelength = speed of light
c=3x108 m/s in vacuum
Question 1
Question 1 Answer
Transparent Materials
Recall that light originates from the vibrations of electrons in atoms.
When light strikes a material, some of the electrons in the matter are forced to start vibrating at the
same frequency as the light that struck the matter. These electrons emit light of the same frequency
which strikes adjoining atoms, etc. In this way the light is transmitted through the material.
As the electrons in glass are set
in vibration by incoming light,
they re-emit light at the same
frequency
Light at a given frequency is absorbed at “preferred”
frequencies by atoms in much the same way that sound
of a given frequency can start a matching tuning fork
vibrating.
Because it takes time for atoms to absorb and re-emit light, it takes longer for light to travel through matter
than for it to travel through a vacuum. The speed of light in “dense media” depends on the density of the
medium.
In glass, light travels at 0.67c
Light travels at different speeds through oil and water, which is why an oil film on top of water has a
“rainbow” effect.
Opaque Materials
Sometimes light at the right frequency matches the “resonant” frequency of the atoms in matter. In
cases like this, the light sets the entire atom or molecule in vibration. When this happens, the
electrons don’t undergo a change in energy and light is not re-emitted. Much of the lights energy is
lost in heating up the matter. Light doesn’t get transmitted through the matter, and we say the
substance is “opaque”.
Glass is opaque to infrared and ultraviolet light, but is transparent to visible light.
Clouds are transparent to most ultraviolet light, but not visible light, which is why you can get
sunburned on a cloudy day.
Shadows
Distinct shadow (Umbra) caused by
small nearby light source or a large far
away light source
Fuzzy, blurry shadow (Penumbra)
caused by small nearby light source or
a large far away light source
Question 2
Question 2 Answer