Lesson 52: Properties of Light

History of Light

Starting in ancient Greece, people were trying to figure out how light worked, and specifically how it related to their vision.

Before you start thinking that all Greeks were stupid or stubborn, you should know that some of them did have some partially right ideas about light.

Speed of Light

There are several scientists that tried to measure the speed of light.

They each had different degrees of success, but each agreed that light did have a measurable speed.

Galileo

Galileo performed his own experiment to see if he could measure the speed of light.

In the end Galileo realized that the small time difference he measured was probably due to human reaction time.

Figure 1: Because Earth orbits
faster than Jupiter, the distance
separating them can change quite
a bit in only a few months time.

Ole Rømer

About 75 years later a Danish astronomer named Ole Rømer did a pretty good job of measuring the speed of light based on the eclipse of one of Jupiter’s moons.

At certain times, Earth and Jupiter are closer to each other in their orbits… sometimes they are further apart.

Fizeau & Foucault

By the mid 1800’s a French physicist named Armand Fizeau came up with a great way to finally measure the speed of light accurately.

Figure 2: A gear

He shined a narrow, strong beam of light so that it would go in between the teeth of a spinning gear.

He actually got a result that was very accurate!

Albert A. Michelson

Albert A. Michelson used a spinning mirror apparatus that was a better quality version of Foucault's apparatus to bounce around a beam of light between two mountains 35 km apart.

Currently the most precise measurement we have (using basically Michelson's method!) is 2.99792458e8 m/s. This value was measured in 1986.

Example 1: If the sun were to blow up right now, how long would it take before we saw the explosion here on the Earth?

Don’t worry, this isn’t going to happen any time soon. Our sun is a very stable, normal star, at about its midlife point. The nearest date we have to worry about is in about 3 billion years. By that time the nuclear reactions in the sun will have increased to the point where all of the water on the earth will be boiled away. But let’s say something is really weird, and the sun blew up right now… to see it blow up the light from the explosion has to travel from the explosion (at the sun) to our eyes (here on Earth). On average the Earth is 1.4957e11 m from the sun, so…

v = d / t (replace the “v” with a “c” which represents the speed of light in formulas)

c = d / t
t = d / c
t = (1.4957e11m) / (3.00e8 m/s)
t = 500 s = 8.33 minutes!

The sun could blow up and there is no way we could know about it for over eight minutes. And in case you’re thinking that someone near the sun could radio us with the news, remember that radio waves travel at the speed of light also! Light is the fastest speed there is!

Wavelength of Visible Light

We will be focusing on visible light, the stuff that we can actually see.

The visible light we can see is composed of the colours of the rainbow.

Figure 3: The colours of visible light

Maybe you learned ROY G BIV when you were in Junior to remember the order the colours appear in, called the spectrum.

Example 2: You are told that a colour has a wavelength of about 600nm. What colour is it?

You know that red is about 700nm and green is about 500nm. Since we are right in between, we can expect the colour to have an orangish-yellow colour. If you said just orange or just yellow, I’d still say you were right.

Example 3: A violet light is being used in an experiment and we need to know its frequency. How would you calculate it?

Just like any other wave, we can use the formula v = f λ to figure out the frequency. The only change we will make is in the symbol we use for velocity. Since we will always be talking about light in this section, and since the speed of light is a constant 3.00e8 m/s, we will give it the symbol “c” in the formula to remind us of its importance.

c = f λ
f = c / λ
f = (3.00e8m/s) / (350e-9 m)
f = 8.57e14 Hz

Notice how I wrote down the value for the wavelength of violet light. I know that it is 350nm. When I wrote it in the formula I knew that I would have to use standard units, metres. When I converted it to sig digs, I did not try to move the decimal around… I probably would have counted the steps wrong and made the scientific notation wrong. Instead, I look at my data sheet and see that “nano” means 10-9, so I just replaced the “nano” in my number with x10-9.

Sources of Light

If I ask you to give me an example of something that gives off light, almost everyone would probably give me one of two answers: a light bulb and the sun.

A regular light bulb gives off light (mostly at wavelengths of yellowish light) because the filament heats up to the point that it starts to glow.

The sun is giving off light because of a fusion reaction involving hydrogen being changed into helium.

Most objects don’t give off light, they just reflect it.

Ability to Transmit Light

It is also important to realize that different materials will let light travel through them in different ways.