This applet shows how light (in the form of a laser) can be used to measure distance (in this case, height.)
To try this
Click and drag your mouse on the figure to raise or lower its height. Then click on the red button to fire the laser and measure the height.
What's Going On?
When you operate this display, a low power LASER is turned on for a short time. The LASER light bounced off of the top of your head and returned to a sensor in the detection unit. The sensor calculated the time it took for the LASER beam to bounce off of your head and return to the unit. The distance the LASER beam traveled from the laser to your head and back to detector is related to the time it took to travel. The equation for this is:
distance = time x speed of light
By dividing the answer by 2, the computer knows how far your head is from the LASER. The computer also knows how high the LASER is from the floor and subtracts your head distance from the LASER height. This gives your height.
This "Time-of-Flight" method is used to calculate the range of objects in many different ways. RADAR sends out a pulsed beam of radio waves and waits for the beam to reflect from a target. By knowing the velocity of the radio wave, the distance to the object can be calculated.
Some cameras use a similar technique incorporating sound waves for automatic focusing. A sound pulse is transmitted from the camera and a microphone waits for the sound pulse to return. By knowing the time it takes sound to move through the air, the distance to the object is calculated and the camera sets the lens for that distance.