card
166

light

OLogy Series
physics
card
166

light

OLogy Series
physics

We see light every day—whether from light bulbs, flames, flashlights, or the Sun (our most important source of light). We see objects because they reflect, or bounce, light into our eyes. Light is a form of energy called electromagnetic radiation. It's made of tiny particles called photons that travel in straight lines. In a vacuum, nothing can travel faster.

Measuring the Speed of Light
In the early 17th century, Galileo tried to measure the speed of light by flashing lanterns from hilltops about one mile apart. This experiment didn't work because light travels too fast for him to measure over one mile. The first scientist to successfully measure light was the Danish astronomer Ole Roemer. While observing Jupiter's moon Io, he noticed that the moon appeared ahead of schedule when the Earth was closer to Jupiter and behind schedule when Earth was farther away. He concluded that when Io was farther away, it was taking more time for its light to reach Earth, making it seem to appear later. Using what he knew about Earth's changing distances from Jupiter, he was able to calculate the speed of light. He estimated 186,000 miles per second -- almost exactly what scientists measure today, using up-to-date instruments and techniques.

A light-year is a measure of:

light

time

distance

Are you right?

### Correct!

A light-year is the distance that light travels in one year, or about six trillion miles. Astronomers use this measurement because the distance between stars and galaxies is so vast. The closest star to our Sun, Proxima Centauri, is about four light-years away.

How long does it take light to travel from the Sun to the Earth?

eight years

eight minutes

It doesn't travel. It's just there.

Are you right?

### Correct!

It takes about eight minutes for rays from the Sun to reach Earth. (To give you an idea how close the Moon is, it only takes about 1.3 seconds for its light to reach Earth!)

The speed of light never changes.

or
?

### Fiction

In a vacuum, light travels at a constant speed of 186,000 miles per second. But some materials (like air, glass, or water) can actually make light bend and slow down.

Many animals can see different types of light than we do.

or
?

### Fact

There are many kinds of light that our eyes cannot detect, like radio waves, microwaves, and X-rays. Bees and butterflies can see ultraviolet waves.

What it is: a form of energy
What it's made of: particles called "photons"
Speed: 186,000 miles per second in a vacuum
Mass: 0
Types of light: gamma rays, X-rays, ultraviolet light, visible light, infrared rays, microwaves, and radio waves
Characteristics: it can be absorbed, reflected, refracted, or passed through an object

Image credits: Eric Hamilton.