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Lenses show that light rays can bend.

Today we understand that light is a wave of electromagnetic energy that slows down when it passes through matter, because of the way light interacts with atoms and molecules. This change in speed causes the light to bend, or refract.

The bending of light that travels through a surface of water, or of glass, was known and used for a long time before it could be described scientifically. Spectacles appeared in Europe in the 13th century, nearly 400 years before Willebrord Snell developed the scientific law of refraction.

The new understanding of refraction allowed devices such as microscopes, and telescopes to be greatly improved. In turn, the new instruments allowed discoveries about the nature of light.

One of the first discoveries made using a telescope was the presence of moons around the planet Jupiter. It was the orbits of these moons that allowed the speed of light to be measured for the first time.

Refraction of light through a glass
magnifyRefraction of light through a glass

Magnifying glass
magnifyMagnifying glass

Replica of Galileo's telescope
magnifyReplica of Galileo's telescope
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