History of Light
Starting in ancient Greece, people were trying to figure out how light worked, and specifically how it related to their vision.
- Go up to an ancient Greek and ask him why he can see something and he’ll probably answer that light leaves his eyes, hits the object, and bounces back up to his eyes.
- In this model, ask him why he couldn’t see in the middle of the night and he would be stumped.
- But with guys like Plato and Pythagoras supporting the theory, not many people disputed it.
- The Greeks thought that regular light from the sun or a candle was somehow different from the way their eyes worked and they had trouble bringing the two ideas together.
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.
- Aristotle was pretty sure that rainbows were somehow created by sunlight reflecting from raindrops, which is basically right.
- Another Greek who you’ve probably never heard of named Empedocles predicted that light traveled at a certain speed, even though it was really fast.
- This went against what most scientists believed for a very long time, before and afterwards
- Most scientists believed the speed of light was infinite, and that it could travel instantaneously (without time passing) over any distance.
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.
- He stood on one hilltop while another person stood on another, both holding covered lanterns.
- Galileo opened his lantern first. As soon as his assistant saw the light from Galileo, the assistant opened his to shine a light back.
- Galileo hoped that by knowing the distance between the two hilltops and the time it took for the light to travel between them he would be able to calculate the speed of light.
In the end Galileo realized that the small time difference he measured was probably due to human reaction time.
- He still believed that the speed of light would someday be measured, just not using his method.
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.
- Rømer had noticed that at some times there was a delay in the time he could see Jupiter eclipse one of its moons.
- The light had to travel the extra distance when Earth was farther away from Jupiter.
- Because Rømer had some rough figures on the distances of the planets in their orbits, he was able to make a rough calculation of the speed of light.
- By today's standards he was quite far off, but it was the first attempt to measure the speed of light that actually came up with an answer.
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.
He shined a narrow, strong beam of light so that it would go in between the teeth of a spinning gear.
- The light would continue on, traveling a long distance (about 8.5km) and hit a mirror and bounce back the way it came.
- If the gear was spinning at just the right speed, one tooth would have had exactly the right amount of time to move out of the way so that a new gap in between the teeth was there, and you would be able to see the beam of light.
He actually got a result that was very accurate!
- A few years later Jean Foucault (another French physicist) refined the method a bit by using mirrors and got an even more accurate measurement.
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.
- Between the 1880’s and 1920’s he made more and more precise measurements of the speed of light. It was his ability to measure the speed of light so well that won him the Nobel prize in 1907.
Currently the most precise measurement we have (using basically Michelson's method!) is 2.99792458e8 m/s. This value was measured in 1986.
- This measurement is such an accepted standard, that we actually use it to define the metre. The distance a beam of light travels in 1 / 2.99792458e8 seconds is one metre.
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.
- Although you might not think of it as light, x-rays, ultraviolet radiation, radio signals, and even microwaves are “light” as well. You’ll have a chance to study these in detail in Physics 30.
The visible light we can see is composed of the colours of the rainbow.
Maybe you learned ROY G BIV when you were in Junior to remember the order the colours appear in, called the spectrum.
- If you didn’t, ROY G BIV stands for Red Orange Yellow Green Blue Indigo Violet.
- Rather than try to remember the wavelengths associated with all the colours (and they’d only be approximations since there are so many shades), it is best to just remember three numbers for the beginning, middle, and end. That way you can always make a guess about the others.
- The longest wavelengths of visible light start at the red end of the spectrum, at about 700nm.
- In the middle we have green at about 500nm.
- On the far end is violet (purple) at about 350nm.
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 HzNotice 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.
- When any object gives off light because it is hot, it is referred to as incandescent.
The sun is giving off light because of a fusion reaction involving hydrogen being changed into helium.
- If a source of light involves chemical reactions, fission or fusion, or is directly from electricity (like lightning), then it is referred to as luminescent.
Most objects don’t give off light, they just reflect it.
- A good example is the moon. It doesn’t make its own light, it just reflects light from the sun.
- Any object that reflects light is referred to as illuminated.
Ability to Transmit Light
It is also important to realize that different materials will let light travel through them in different ways.
- If a material lets light travel through easily and you could clearly see objects on the other side, it is transparent. (e.g. glass)
- If a material lets light through enough that you can see the light on the other side, but can’t see objects clearly, it is translucent. (e.g. frosted glass)
- If a material does not let light travel through it, it is opaque. (e.g. a wall)