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Junior School: Light

“Books are a uniquely portable magic.”

LIGHT

Let's learn about light | Science News for Students

When we're very young, we have a very simple idea about light: the world is either light or dark and we can change from one to the other just by flicking a switch on the wall. But we soon learn that light is more complex than this.

Light arrives on our planet after a speedy trip from the Sun, 149 million km (93 million miles away). Light travels at 186,000 miles (300,000 km) per second, so the light you're seeing now was still tucked away in the Sun about eight minutes ago. Put it another way, light takes roughly twice as long to get from the Sun to Earth as it does to make a cup of coffee!

REFRACTION

Refraction of laser beams inside crystals

You've probably noticed that water can bend light. You can see this for yourself by putting a straw in a glass of water. Notice how the straw appears to kink at the point where the water meets the air above it. The bending happens not in the water itself but at the junction of the air and the water. You can see the same thing happening in this photo of laser light beams shining between two crystals. As the beams cross the junction, they bend quite noticeably.

Artwork explaining how refraction (the bending of light) happens when light rays slow down

Why does this happen? You may have learned that the speed of light is always the same, but that's only true when light travels in a vacuum. In fact, light travels more slowly in some materials than others. It goes more slowly in water than in air. Or, to put it another way, light slows down when it moves from air to water and it speeds up when it moves from water to air. This is what causes the straw to look bent. Let's look into this a bit more closely.

Imagine a light ray zooming along through the air at an angle to some water. Now imagine that the light ray is actually a line of people swimming along in formation, side-by-side, through the air. The swimmers on one side are going to enter the water more quickly than the swimmers on the other side and, as they do so, they are going to slow down—because people move more slowly in water than in air. That means the whole line is going to start slowing down, beginning with the swimmers at one side and ending with the swimmers on the other side some time later. That's going to cause the entire line to bend at an angle. This is exactly how light behaves when it enters water—and why water makes a straw look bent.

Refraction is amazingly useful. If you wear eyeglasses, you probably know that the lenses they contain are curved-shape pieces of glass or plastic that bend (refract) the light from the things you're looking at. Bending the light makes it seem to come from nearer or further away (depending on the type of lenses you have), which corrects the problem with your sight. To put it another way, your eyeglasses fix your vision by slowing down incoming light so it shifts direction slightly. Binocularstelescopescamerasnight vision goggles, and many other things with lenses work in exactly the same way (collectively we call these things optical equipment).

REFLECTION

Reflection

The most obvious thing about light is that it will reflect off things. The only reason we can see the things around us is that light, either from the Sun or from something like an electric lamp here on Earth, reflects off them into our eyes. Cut off the source of the light or stop it from reaching your eyes and those objects disappear. They don't cease to exist, but you can no longer see them.

James Webb telescope. Six blue mirror segments.

Photo: Now that's what I call a mirror! In fact, it's six segments of the huge mirror from the James Webb Space Telescope. Picture by courtesy of NASA.

Reflection can happen in two quite different ways. If you have a smooth, highly polished surface and you shine a narrow beam of light at it, you get a narrow beam of light reflected back off it. This is called specular reflection and it's what happens if you shine a flashlight or laser into a mirror: you get a well-defined beam of light bouncing back towards you. Most objects aren't smooth and highly polished: they're quite rough. So, when you shine light onto them, it's scattered all over the place. This is called diffuse reflection and it's how we see most objects around us as they scatter the light falling on them.

If you can see your face in something, it's specular reflection; if you can't see your face, it's diffuse reflection. Polish up a teaspoon and you can see your face quite clearly. But if the spoon is dirty, all the bits of dirt and dust are scattering light in all directions and your face disappears.

ABSORPTION

Reflection, Absorption & Transmission - Maggie's Science Connection

A Quick Overview of Light Absorption

In absorption, the frequency of the incoming light wave is at or near the energy levels of the electrons in the matter. The electrons will absorb the energy of the light wave and change their energy state.

There are several options that could happen next, either the electron returns to the ground state emitting the photon of light or the energy is retained by the matter and the light is absorbed. If the photon is immediately re-emitted, the photon is effectively reflected or scattered. If the photon energy is absorbed, the energy from the photon typically manifests itself as heating the matter up.

The absorption of light makes an object dark or opaque to the wavelengths or colors of the incoming wave:

  • Wood is opaque to visible light.
  • Some materials are opaque to some wavelengths of light, but transparent to others. Glass and water are opaque to ultraviolet light, but transparent to visible light.
  • By which wavelengths of light are absorbed by a material, the material composition and properties can be understood.

Light Absorption: Colour

Another manner in which the absorption of light is apparent is by their colour. If a material or matter absorbs light of certain wavelengths or colours of the spectrum, an observer will not see these colours in the reflected light. On the other hand, if certain wavelengths of colours are reflected from the material, an observer will see them and see the material in those colours. For example, the leaves of green plants contain a pigment called chlorophyll, which absorbs the blue and red colors of the spectrum and reflects the green - leaves therefore appear green.