Download Wave and Particle Properties of Light Video Script

Physics 1302 – Wave and Particle Properties of Light
Instructor #1
Light is a wave.
Instructor #2
No, light is a particle.
Instructor #1
It’s a wave, an electromagnetic wave. Scientists like Faraday and Maxwell and Hertz said so, and I
agree.
Instructor #2
Well, Albert Einstein said that light consists of tiny bundles that behave like particles, and I’m with
Einstein.
Instructor #1
It’s a wave.
Instructor #2
It’s a particle.
Instructor #1
You’re wrong. And Mom always liked you best.
VO
Is light a wave or a particle? This question puzzled scientists, much smarter than these two, for
centuries. In today’s program, we’ll look at evidence supporting each view and come to a conclusion
about the nature of light.
Instructor #1
OK. We’re going to approach this argument scientifically. I’ll go first and show you the evidence for
the wave theory of light.
Light behaves like a wave. It reflects, refracts, diffracts, and interferes, just like sound waves and
water waves do. Look at this.
(mirror on screen)
VO
A beam of red laser hits a mirror at an angle. You can see that the light obeys the law of reflection.
Instructor #1
Of course, you observe the reflection of light every time you look in a mirror.
Instructor #2
So light reflects. Big deal. Use that book as a wall, and roll this ball toward it at an angle. It reflects
off the wall, and I believe the ball is a particle.
Instructor #1
OK. So both waves and particles reflect. But I’m not finished. Watch this.
(stick on screen)
VO
Straight sticks appear to be bent in water. The sun often appears to be oval, rather than round when it
appears low in the sky.
Ponds streams appear to be shallower than they really are, and on a hot day, the road often shimmers
as though it was the surface of a lake.
These phenomena result from the refraction of light.
Instructor #1
Did you hear that? Light WAVES bend or refract, just like water waves do.
And another thing: When I roll a ball through an opening like this, does its path bend? No. Waves
diffract, or bend around an obstacle. Particles don’t.
For many years, it was thought that light did not diffract, but scientists finally figured out that the
openings they were using were too large compared to the tiny wavelength of visible light. So they
couldn’t see any diffraction.
(water waves on screen)
VO
Remember that as an opening becomes smaller, curving of water waves becomes greater, so that
straight wave fronts become circular ones. The smaller the size of the opening compared to the
wavelength of the waves passing through it, the greater the diffraction.
Now watch as a razor blade is placed in the path of a beam of red light of a single wavelength. The
light falls on a screen behind the object. The shadow cast by the edges of razor blade is not sharp, as
you might expect. Instead, some of the light has been bent around the edges of the object and into the
shadow, giving it a fuzzy appearance. This bending around an obstacle is due to the diffraction of
light.
Instructor #1
Now, diffraction is pretty convincing evidence that light acts as a wave, but this should be the
clincher. You all know that two objects can’t be in the same place at the same time, or pass right
through each other, but waves can. It’s called interference, and, guess what? Light waves undergo
interference. Watch this.
(slit screen on screen)
VO
If light is allowed to fall onto two narrow slits, which are cut into a thin piece of material, the particle
theory predicts that the light will travel in straight lines through the slits and form the same pattern on
the screen behind. However, the observed result is quite different.
In this experiment, the slit width is only slightly larger than the wavelength of the light, and intense,
monochromatic laser light shines through the slits. Instead of 2 bright bands of light, a spread-out
pattern appears on the screen behind. This curious result can only be explained by the wave model of
light.
Instructor #1
I rest my case. Light has wave properties and does the things that only waves do, like diffracting and
interfering.
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Instructor #2
I must admit that you make a good case, but I have some evidence, too. So go away and let me have
my turn. Go on.
That’s better. Now, my story begins with this man. His name is Heinrich Hertz. Yep, the unit for
frequency is named for him. He took Maxwell’s theories of electromagnetic waves and applied them
to demonstrate how radio waves could be produced and detected. His work led to the invention of
both the wireless telegraph and the radio. So he kind of proved that electromagnetic waves were real.
Then he turned around and accidentally discovered something about light that could not be explained
by the wave theory. It’s called the photoelectric effect. Better get some notes on this.
(green chalkboard on screen)
VO
The word, photoelectric, means “light producing electricity.”
The photoelectric effect occurs when light of certain frequencies shines on a metal and ejects
electrons from the metal surface.
Some uses of the photoelectric effect include solar calculators and solar-powered cars,
electric sensors that turn street lights on when it gets dark outside or that sounds an alarm when a light
beam is blocked by a person at a door,
and light meters on cameras
Instructor #2
As you can see, the photoelectric effect was an important discovery, and scientists like Hertz wanted
to explain how it worked using a concept that we’ve used to explain lots of wave phenomena, from a
singer breaking glass to a microwave oven cooking food. What is it? Tell your teacher.
It’s resonance, of course. And, to be fair, I think I’ll let wave boy tell how wave theory could explain
the photoelectric effect. But please put your pencils down because his explanation might not work.
Instructor #1
Don’t listen to my Particle Pal. This is easy. We all know that electrons can vibrate in a whole range
of frequencies. And electromagnetic waves, like visible light, can match those natural frequencies
and make the electrons start to vibrate. If the light continues to shine on the metal, which holds its
electrons loosely, the electrons’ vibrations will build up until they jump right out of the surface.
Doesn’t that make sense?
Instructor #2
I must admit that your resonance explanation sounds reasonable. But Hertz and others made some
predictions based on the idea of electrons vibrating sympathetically. Then scientists performed
experiments to check them out, and guess what? THEY WERE ALL WRONG. Here’s what really
happened. By the way, kids, we’re talking facts here so you can take notes now.
(table on screen)
VO
The wave theory predicted that any frequency of light would match the natural frequency of electrons
and cause resonance. The brightness of the light should determine whether electrons are ejected or
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not. But scientists found that it was the color or frequency of the light, not its brightness that
determined whether electrons could be ejected or not. Violet and ultraviolet light always produce the
photoelectric effect, but red light never works. Even violet light from a distant star ejects electrons,
but the brightest red light fails every time.
Next, wave theory says that a faint light would be like tiny pushes of a child on a swing. It would
take time to build up the vibrations. But experiments showed that there is never a time delay. The
light either ejects electrons or it doesn’t, but the results are immediate, even for the faintest violet
light.
Finally, wave theory suggested that since brighter light would have more energy, it should increase
the kinetic energy of the ejected electrons. But scientists found that the brightness or intensity of light
does not affect the kinetic energy of the ejected electrons. Brighter violet light ejects more electrons
than faint violet light, but the kinetic energy of the ejected electrons depends on the frequency of the
light, not its brightness.
The wave theory failed to explain the photoelectric effect.
Instructor #2
Well, well. Now we have some facts about light that cannot be explained by the classical wave
theory. So scientists needed another theory to explain the photoelectric effect. In 1905, Albert
Einstein came to the rescue. He used a theory that a German physicist, Max Planck, came up with in
1900. It’s called the quantum theory. Get some notes on the quantum theory, and then we’ll see what
Einstein said.
(green chalkboard on screen)
VO
According to the quantum theory, energy is radiated and absorbed by tiny, indivisible bundles.
Each packet of energy is called a quantum.
A quantum of light energy is called a photon. Albert Einstein invented this term.
The energy of a photon of light depends on the frequency of light.
The equation for calculating the energy of a photon of light is “E equals h f.” “h” is called Planck’s
constant.
(text on screen)
VO
Here’s how Einstein used the idea of little bundles of energy to explain the photoelectric effect. He
said that light is a shower of individual photons, each photon having a certain amount of energy
depending only on its frequency. When the light hits the metal’s electrons, each electron absorbs only
one photon of light.
Now each electron needs a certain amount of energy to jump out of the atom. If the photon can
furnish enough energy, the electron will be ejected. If not, the photoelectric effect will not occur.
And if the photon furnishes more energy than the electron needs to escape, the extra energy will give
the electron more kinetic energy and make it move faster.
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Let’s go back to our experimental evidence and see how well Einstein’s ideas explain what actually
happens.
Since violet and ultraviolet light have high frequency, their photons will have enough energy to eject
electrons. But red photons are very low in energy and will not work.
And electrons are ejected the instant they absorb a photon with the right energy. So there is never a
time delay.
Finally, bright light just consists of many photons that will eject many electrons, but the kinetic
energy given to each electron depends upon the energy of the photon, and that depends on frequency,
not brightness.
Quantum theory explains what wave theory could not.
Instructor #2
In 1905 Einstein received the Nobel Prize in physics for his explanation of the photoelectric effect,
and Plank received the prize in 1918 for his quantum theory. How’s that for impressive evidence for
the particle nature of light?
But, wait, there’s more. Only certain frequencies of light produce photosynthesis when the light is
absorbed by green leaves. And red light is used in dark rooms when film is being developed because
PHOTONS of red light don’t have enough energy to cause a chemical reaction with the film.
And speaking of chemistry, do you remember how the classical wave theory predicted that electrons
would give off energy every time they made a revolution and spiral right into the nucleus? Of course,
that just doesn’t happen. But quantum theory explained how electrons could exist in energy levels
without losing any energy. And it explained how certain colors of light are produced by electrons
making “quantum leaps” between energy levels when atoms are zapped with energy like this.
So I rest my case. Light comes in photons, which are like little particles of light.
Instructor #1
You made some convincing arguments, but don’t forget that my evidence was convincing, too.
(freeze video)
VO
So we’re right back where we started. Is light a particle or a wave? It’s obvious that a compromise in
necessary.
(table on screen)
VO
Let’s take another look at the evidence on both sides. Refraction, diffraction, and interference are
things that only waves do. It looks like light acts like a wave when it’s on the move.
But when the light interacts with matter, like being absorbed by electrons to eject them from a metal
or to produce chemical reactions or to make quantum leaps, it acts like particles.
So the answer is a combination of both theories. It’s called the dual theory of light, or wave-particle
duality.
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When light moves through space, it acts as a wave, and when it interacts with matter, it acts as a
particle.
Instructor
It’s a relief to have a solution. All that arguing with myself was exhausting.
So now that we know more about the nature of light, let’s talk about sources of light. We’ll start with
the sun and the moon. They both look like they give off light, but one does and the other does not.
Better get this in your notes.
(moon on screen)
VO
Objects that are visible because they reflect light are said to be illuminated. The moon reflects light
waves coming from the sun.
This picture of the moon and earth shows that both are illuminated on the sides closest to the sun.
Luminous bodies emit, or give off, light waves. The sun is luminous.
Light bulbs are luminous, too. Let’s look at some examples of how these and other luminous bodies
produce light.
(text on screen)
VO
Let’s get some notes on how light is produced:
Keep in mind that energy is required to make electrons jump to higher energy levels and then fall
back down, giving off photons of light.
On the sun, visible light, as well as many other electromagnetic waves, are produced by the fusion of
hydrogen atoms, a nuclear reaction.
Incandescent materials give off light when they are extremely hot. Examples include candle flames
and the filaments in incandescent light bulbs.
The third type of light production is using electricity to excite atoms of gases in gas discharge tubes.
The most common examples are neon lights.
Next is fluorescence. You’ve all seen fluorescent lights, probably in your classroom. In this method
of light production, ultraviolet waves are produced electronically and shine on the substance coating
the inside of the bulb. The UV waves cause each electron to make a large quantum leap. If the
electron fell back down this far, you would not be able to see it because it would be ultraviolet. But in
fluorescent substances, the electron falls back down in smaller steps, like these. This small fall might
produce low energy red light, this one green, and this one blue. You would see a combination of
these colors.
Chemoluminescence is the result of a chemical reaction that releases light instead of the heat given off
by most reactions. An example is a light stick. You see lots of these around Halloween.
A special kind of Chemoluminescence is bioluminescence, in which the reactions occur in living
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organisms, such as fireflies. You might call them lightning bugs.
Instructor
Let me show you some examples of different types of light production that you just got into your
notes. In gas discharge tubes, only certain quantum leaps are made by the electrons within the atoms.
So only certain colors of light are produced.
But when a solid, like this filament, is zapped with energy, white light is produced. How? Well, the
atoms in a solid are so tightly packed that electrons can leap into energy levels of neighboring atoms.
The different leaps and falls give off a large range of frequencies of light. Put those frequencies
together and we see white.
Now watch this to see examples of black light and bioluminescence.
(painting on screen)
VO
Fluorescent materials can also be used in paints and dyes. They glow when exposed to the UV
radiation coming from a “black light”.
Fireflies and some species of jellyfish and fish give off light as a result of chemical reactions in their
bodies. This effect is called bioluminescence.
Instructor
There are two chemicals in this necklace, separated by a piece of plastic. When I break the divider …
the chemicals react, visible light is produced … like this. OK, turn the lights back on. Thanks.
Next, we’re going to light production one step farther and talk about lasers. I’m sure you’ve all heard
of lasers and know about some of their uses. Well, lasers don’t just produce light. They amplify it.
In fact, the word, laser, is an acronym, a word made from the first letters of several words. And
amplification is in it.
(green chalkboard on screen)
VO
The word, laser, is made from the words, “light amplification by stimulate emission of radiation”.
Although the first laser was invented in 1960, the idea of stimulated emission of radiation was
predicted by Einstein in 1917.
Instructor
Before we describe stimulated emission, let’s see how laser light is different from light from, let’s say,
an incandescent light bulb like this one.
The light produced by a typical light bulb consists of a mixture of photons of different frequencies
going in different directions. This is called incoherent light.
But lasers produce a narrow beam of coherent light. Since this is the most important property of laser
light, I want you to get some notes on it. Then you’ll learn how it is produced by stimulated emission.
(green chalkboard on screen)
VO
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Coherent light is monochromatic, which means of one wavelength.
Coherent light consists of identical waves that are in phase.
The light is amplified due to the constructive interference of the waves.
This constructive interference produces a single, narrow beam of very intense light. Now, watch
carefully to see how coherent light is produced by lasers. Fill in the blanks in your notes as we go
along.
(diagrams of atoms on screen)
VO
First, let’s look at what happens in the type of light production you are familiar with. First, you know
that when electrons are in their lowest energy levels, the atom is in its ground state. Energy is
required to make the electron leap into a higher energy level, which puts the atom into an excited
state. The excited atom will have higher energy levels, so it’s radius will be greater than the atom in
its ground state.
Now, under normal circumstances, the electron will eventually fall back down to its lower energy
level, and a photon of light of a certain frequency will be emitted or radiated. This is called
spontaneous emission of radiation.
But Einstein thought that if an excited atom could be hit with a photon of light identical to the one it is
ready to emit, the electron will fall immediately and give off the photon. In other words, the excited
atom would be stimulated to emit an identical photon, which would be in step with the striking
photon. Einstein called this process stimulated emission.
Now the two photons, in phase, can go on to strike another excited atom, stimulating it into emission,
and so on, and so on. Einstein never tried this, but he predicted that a chain reaction could occur.
Instructor
As many, many scientists can tell you, getting from theory to application is a difficult process. Until
1960, no one had been successful in inventing a device that could produce coherent visible light.
Then in 1960, an American scientist, Theodore Maiman, tried this.
(diagram of ruby crystal on screen)
He took a ruby crystal….
and wrapped a camera flash coil around it to excite the electrons.
Then he attached two special mirrors to the ends of the crystal. One was partially transparent so that
intense light could escape through it.
And that was it. He turned on the exciter, and a beam of intense, coherent, red light came out. Let’s
take a closer look at what happens inside a simple laser.
(diagram of laser on screen)
VO
Instead of a ruby crystal, we’ll use a tube of a gas, like neon or helium. These small red dots
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represent the atoms in their ground states.
When we add the exciter and turn it on, we get lots of atoms in excited states. This flash will continue
to furnish energy, but I’m going to remove it from the picture to simplify things for you.
Now, most of the excited atoms will undergo spontaneous emission in all directions. Only about 10%
of the light will become laser light. But look at this photon. It will hit this excited atom and stimulate
the emission of a photon. So we’ll have two photons, in phase. And they’re in line to strike this
excited atom. So we’ll get 3 photons in phase. They will hit the mirror and be reflected back this
way. Now they can strike other excited atoms, and so on. The photons will flash back and forth
millions of times in a fraction of a second, until the beam is strong enough to escape through the
partial mirror as a laser beam.
(laser beam on screen)
VO
The narrow focus of laser beams allows them to be used as measuring devices.
The highly focused laser light allows it to be used in a number of medical applications, such as eye
surgery. It can be used to remove skin lesions and allows surgical procedures to be done with
minimum blood loss, since laser scalpels cauterize wounds as they cut.
Beams of laser light can be used to scan bar codes, as well as to read the digital information stored on
CD’s and DVD’s. Laser light is also used to produce spectacular laser light shows.
Instructor
Your teacher has the Show What You Know questions for you. I’ll see you next time when we study
colors. Right now, I have to perform laser surgery… or maybe I’ll just produce my own laser show.
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