Transcript Slide 1 - MisterSyracuse.com

Name ________________________________________ Date _________________ Period ____________
THE SPEED OF LIGHT
The speed of light, boys and girls, is very fast. You might think a car is fast. Even some sort of super
rocket-car. You might even think that the space shuttle is fast. Well, in the grand scheme of fast-ness, you’re
wrong. Dead wrong. Indeed, all these things move at a mere fraction of the ultimate speed possible, light.
Light moves at a remarkable 299,792,458 meters per second. That is, indeed, very fast. Let’s see how
some other items measure up:
• The speed of light is 186,282.397 miles per hour.
• That’s about a meter every three nanoseconds (one billionth of a second).
• The circumference of the earth is 40,975.16 km. That means that light could circle the earth 7.316 times in
one second.
• The earth is 150,000,000 km from the sun. That means that it takes about 500 seconds, or about 8.3
minutes for light to go from the sun to the earth. So sunrise is actually seen about eight minutes after it
happens!
To sum it up, light travels very quickly. Indeed, astronomers use light to measure distances. A light
year is defined as the distance that light travels in one year. This is equal to 9,460,528,400,000,000 (nine
quadrillion, 460 trillion, 528 billion, 400 million) meters! Gosh! Taking this even further, we know that the
nearest star, Proxima Centuri, is 4.2 light years away. That’s 4.2 x 9,460,528,400,000,000. That’s a lot of
meters.
We are concerned with light, however, in that we want to see if we can measure it for ourselves.
Several attempts were made to measure the speed of light using prisms, rotating mirrors, and many other
devices as early back as the 1600s. It was not until the late 1800s, however, that an accurate measurement
was taken.
We know that light travels as a wave, and all waves have a wavelength and a frequency.
• The wavelength is the distance from one crest to the next. It is represented by λ (greek lambda).
• The frequency is how often the wave comes by a given spot. It is represented by an ƒ (the letter f).
The two terms are connected, such that the wavelength multiplied by the velocity is the speed of light:
λƒ=c
So, if we know the frequency of something, and we can measure its wavelength somehow, then we can
calculated what the speed of light is. In air, it should be a constant, so we can use our percent deviation
formula to figure out how far off we were. We can use marshmallows and a microwave to figure this out.
We can get the frequency of the microwave by cheating and looking at the back label. Now we just
need to get the wavelength. We can do this by microwaving a plate of marshmallows on low power, and
seeing where there are “hot spots,” or melting of the mallows. The distance between these spots should be
about half the wavelength of the microwave. Show your calculations for (1) the speed of light and (2) your
percent deviation on the back of this sheet!