Q & A: Speed of Light

Q:
How can light change to another form? How is it possible to measure how fast light goes, if it goes so fast?
- Jaime (age 14)
Brier Terrace Middle, Brier, WA, USA
A:
Jamie,

Let me answer your second question first. It is certainly possible to measure how fast light goes, even though it is moving very fast indeed (about 300 million meters every second, which is about 10 million times faster than the highway speed limit).

The way you can measure the speed of light is the same as the way you can measure the speed of a car: Just see how long it takes to go a known distance. In a car you might paint two lines on the road separated by a mile. You would start a timer when the car was at the first line and then stop the timer when the car reached the second line. If you measured that it took the car one minute to go from one line to the next (one mile) then you know the speed of the car is a mile per minute (which is 60 miles per hour).

To do this with light all you need a fancier timer, as well as a way to produce and detect light very accurately. Light can be produced at a well determined time by very quickly flashing an LED (this is like the car being at the first line), and light can be very accurately detected using a device called a photo-multiplier (this is like the car reaching the second line).

If the distance the light travels between production and detection is a mile, then the time that we will measure using the timer is about 5 microseconds, which is 5 millionths of a second. This seems like a very short time, but actually its not at all hard to measure this with modern "timers".

Now, for your other question, "How can light change to another form?". I'm not sure exactly what you mean, so let me just pick an everyday example of light turning into something else, namely "stored up" energy. Many kinds of plants can turn light into energy using something called "photosynthesis". This is just a fancy word for what the leaves do when special molecules in the leaf absorb light and store this energy chemically. The energy that the light had can now be used to make the plant live and grow.

Mats

(published on 10/22/2007)