Light

Our study of the history of understanding of light will take us to visit so many topics that it will take us two days. That's good though, since that will give us a pause to reflect on what we're seeing.

Beyond these two days we'll take one more day to study color and color vision, and another day to study image formation.

Models

I have chosen a timeline of history approach to the study of light because it illustrates the importance of models to the advance of science. Scientific models of light have changed over the years as more and better experiments were done. When an old model no longer explained all of the new experiments it had to be replaced by a newer model. However, the old model still worked for all of the original experiments and so was still somewhat useful. Thus, I still use Maxwell's 1870 model of light as electromagnetic waves even though it was supplanted by Feynman et.al.'s Quantum Electrodynamics, QED, in the 1950's. I only use QED when I am forced to by the failure of Maxwell's equations.

When you ask me a question about light I'll try to answer with the simplest model that gives the correct answer. If you revise your question, I may have to switch my answer to a totally different model.

Teachers and models

One of the important skills of a teacher, is choosing the simplest model that helps a student toward understanding.

Let there be light

We'll start out in the dark with the words of Benjamin Franklin who said;
" About light, I am in the dark."

Ancient History

Hunters knew that if you tried to spear a fish by aiming where the fish appeared to be, you would miss. Anyone could see that the spear bent when it entered the water. Now we say that the spear doesn't bend when it enters the water, but that the light with which we see the spear bends as it leaves the water.

The bending of light as it enters or leaves water is called refraction.

One of the oldest tables of data is a Greek stone tablet into which are carved the angle of incidence and angle of refraction of light going from air to water.

Here is an interesting experiment using refraction. I use this to get students interested in light and ask them , "What does it take to make something invisible?" Answer below. Please do the activity first.

Disappearing glass rods snack

When glass rods, Pyrex, are placed into a liquid with the same index of refraction, Wesson oil, they disappear.

To be invisible, an object must not reflect light, it must not absorb light, and it must not refract light.

To be more quantitative about light refraction, you can use two different activities:

The first uses a bare filament light bulb to generate a beam of light, it is called a:

Light Box

The second uses a laser:

Laser Jell-O snack

Critical angle snack

Math Root on refraction through layers

Galileo

Galileo attempted to measure the speed of light, circa 1600. He used lamps and men on mountain tops. He made several measurements and decided that light traveled so rapidly that he could not measure its speed.

How would you have tried to measure the speed of light with Galileo's tools?

Galileo had two men stand on hilltops, one would uncover a lamp, when the other saw the light he would uncover his lamp and be observed by first man. Galileo used hills separated by different distances and found that the distance between the hills did not change the timing. He was measuring human reaction time and not the travel time for light. He realized that this meant that light was very fast. However, Galileo's experiment will work for measuring the speed of sound.

Science mulch: The speed of light is a foot per nanosecond. 0.3 meters per nanosecond,
a nanosecond is 10-9 s.

Christian Huygens

Huygens (1629-1695) developed an idea by Descarte that light was a wave. He published this in 1690. Wave theory predicted that light would refract because the waves would travel more slowly in glass. Robert Hooke, Newton's rival, supported the wave theory. (I still use Huygens wave model to explain why light travels in straight lines.)(Need image)

Newton

Newton's theory was that light was a particle, published in 1704. This theory could explain why light traveled in a straight line. Light traveling as particles explained shadows and reflections, it even explained refraction &emdash; if the particles sped up in dense media, as if they were being pulled into the media by gravity.

Reflection of light can be understood using Newton's particle model. The particles of light bounce off the mirror just like a ball would. This leads to the law:

The angle of incidence equals the angle of refraction.

You can investigate reflection from one or more mirrors using a small pointer laser in the following exploration:

Scan a Laser

Both the wave theory of Huygens and the particle theory of Newton explained refraction. In the wave theory the waves of light slowed down when they entered water from air. In the particle theory the particles of light sped up. (Actually, in the particle theory only the component of the velocity perpendicular to the surface sped up, the component parallel to the surface remained constant.) The actual experiment to decide between the two theories of refraction could not be carried out until 1849 when Foucault found that the speed of light in water was slower. The particle theory can not explain refraction if light slows down in glass.

Newton knew that if light were a wave it would diffract, or bend, around corners.
Since diffraction was not seen, then light couldn't be a wave.

However, Newton saw and reported on the bending of light around a needle and he also saw Newton's rings both of which we now know are due to diffraction and interference. Although Newton saw the phenomena of diffraction he did not recognize it. He said instead that light was a particle that had "fits."

Here is an activity in which you can see Newton's rings for yourself.

Bridge light snack

Chromatic Aberration

When you come to the Exploratorium visit the exhibit Chromatic Aberration. Here you will see that your eye cannot focus blue light and red light at the same time. You look at a sharp edged dot of light through a magenta filter which passes blue light and red light but no other colors Viewed at a distance the dot has a sharp red center and a fuzzy blue halo, viewed closer it has a sharp blue center and a red halo. This is because the index of refraction which determines the amount of bending of light by your eye is a function of the color (later we'll see that color is related to the wavelength of light.) Human eyes bend blue light more than red light. Most transparent things bend blue light more than red, this is known as dispersion.

Aside: Newton knew about chromatic aberration and said that it could not be removed, later Chester Moore Hall (in 1733) discovered how to make lenses which could focus two wavelengths of light at the same time. These lenses were called "achromats." He did not patent his idea which would have required him to publish how it was done, instead he kept it secret, and so reserved the entire trade in achromatic lenses to himself for far longer than the patent would have. He made his achromatic lenses by combining two lenses with different dispersions. Modern lenses can focus three wavelengths at the same time, these lenses are called apochromats and were invented by Ernst Abbe in the 1860's.

Thomas Young

In 1801, Thomas Young passed light through two slits and observed interference. With one slit, a pattern of light was created, when a second slit was opened by itself the same pattern of light was created. When both slits were opened at the same time, darkness appeared where there had been light with either alone! Thus I say:

Light plus light equals dark!

You can easily project Young's experiment for a class of students using an inexpensive laser pointer.

We'll create two slits and cover one of them, then shine light through the other. A band of light will appear on a distant screen. Next we'll uncover the first slit and cover the other. The light spreads into the same broad band.

However if we uncover both slits and shine light through both of them, dark places appear on the wall where there had been light through either slit alone!

Cover one slit and light appears, open that slit and dark is created.

Light plus light equals dark, you've got to see this to believe it!

The two slit experiment convinced Young that light was a wave. Newton's influence on the Royal Society was so strong however that it took Young years to convince the society that light was a wave.

Two Slit Experiment

Students can then figure out Young's model using waves drawn on paper:

Model of interference

There is much more to understand about interference of light, such as diffraction from large slits and interference by reflection from soap films and oil slicks. We'll return to study interference of light in detail later, during an entire day of exploration dedicated to Interference.

While he was at it, Thomas Young also figured out that in order to perceive all of the colors that humans can see we needed to have three different color receptors. He was right, we have three different types of cone cells which give us our color perception.

Augustin Fresnel

Augustin Fresnel (1788) entered in a competition to elucidate the nature of light. (Three of the judges were Poisson, Biot, and Laplace, all supporters of the particle model) Fresnel could write the equations for the wave model of light but he couldn't solve them! One of the judges however was a great mathematician,Poisson, who solved Fresnel's equations. Poisson pointed out that his solution predicted that there would be a spot of light in the middle of the shadow of a ball. Since there was no such spot of light, Fresnel was wrong.

However the chairman of the judges, Arago, conducted the experiment and found the spot.

Thereafter the spot was named: the Poisson spot.

Later, as head of the lighthouse commission, Fresnel invented the lens that bears his name.

At the Exploratorium you can see the Poisson spot in the exhibit Long Path Diffraction.

Or you can make it yourself with a small pointer laser:

Making a Poisson Spot.

Arago

Arago discovered that light was polarized, this meant light was a transverse wave. This meant that light was not like sound which was a longitudinal wave.

You can easily see that light is polarized by doing experiments with polarizers and plastic.

Polarized light

We'll return another day to explore Polarization in more detail.

Maxwell

If light was a wave, what was it a wave in?

Maxwell found the equations that described light as a wave in electricity and magnetism, an electromagnetic wave.

Tape demo of an electromagnetic wave

For a while, it was thought that electromagnetic waves had to travel through the aether &endash; a thin stiff substance that permeated the universe.

Michelson and Morley did an experiment to measure the speed of light in the aether. However their experiment showed that there was no aether! Einstein made sense of their data with his theory of special relativity.

In Maxwell's wave model the color of light depends on its frequency, the brightness on its amplitude.

Electromagnetic waves are created by accelerating electric charges,

The electromagnetic waves then exert forces on the electric charges causing them to accelerate.

More on the Maxwell model of light.

Hot objects emit light.
Hot objects glow red, they are red hot.
Hotter objects glow white, they are white hot!

Turn on an electric hot plate, or heat a nail in the flame from a torch and notice the color of the glow that results. As the plate and the nail heat up the color goes from deep red to orange to yellow, heat it enough and it would become white. (The nail will melt before becoming white however.)

Planck was able to explain the color of light emitted by hot objects by modeling energy as existing in discrete chunks, quanta.

Albert Einstein

Explained the photoelectric effect by extending the idea proposed by Planck. He suggested that light energy was quantized. Later, the quanta of light proposed by Einstein were named photons. So it looked like light behaved as a chunk of energy, as a particle.

The Exploratorium has an exhibit on the photoelectric effect,
at this exhibit you charge a zinc plate with negative electrons.
Shining visible light on the plate does not change the charge.
Shining ultraviolet light on the plate discharges it.

Photo electric effect

In particular Einstein found that the energy, E, of a photon was proportional to its frequency, f.

E = hf

where the constant of proportionality, h, is Planck's constant.

It is easy to demonstrate the connection between the energy of a photon and its frequency (color) using light emitting diodes which convert the electrical energy change of one electron into one photon.

Light Energy and Color

Irwin Schroedinger

In Schroedinger's model for light it is described as traveling as a wave and interacting like a particle.

Light is a wave in "probability amplitude."

In the quantum mechanical theory for light, color depends on the frequency of the light, the brightness on the number of photons per second.

Quantum mechanics explained the spectrum of light emitted by atoms and molecules.

The Exploratorium used to have a great exhibit called "Solar Spectrum" in which the spectrum of sunlight was spread across a meter. Maybe one day it will return.

Spectra

The spectrum of hydrogen can be derived from the Schroedinger equation for hydrogen, you can explore the spectrum of hydrogen and the energy levels of the electron in hydrogen using the wonderful program "Atom in a Box" available for payment of a small fee through the internet.

DeBroglie modeled particles as waves too, and then Davison and Germer showed that electrons diffracted and thus had a wavelength.

More on the quantum model of light.

Richard Feynman

Quantum mechanics made some errors in predicting the colors of the spectral lines emitted by atoms, to get the right answer, quantum mechanics had to be combined with relativity.

Feynman was one of the scientists who combined quantum mechanics and relativity to produce our current model of light known as Quantum Electrodynamics, or QED.

Feynman said that If you understand the two slit experiment you understand all of quantum mechanics, but that unfortunately, no one understood the two slit experiment.

In this model light travels as a wave, a wave which explores every path in the entire universe on its way from one point to another. The phase of this wave is changed as it interacts with charged particles. So as the wave passes an atom its phase can be slightly delayed. This slight delay gives rise to refraction.

Electro-weak

Recently scientists found a mathematical model that combined electromagnetism with the weak nuclear force. This combined model is know as the electro-weak force.

Inverse Square law snack