Waves & Different Materials

When light waves meet a material, one of three things can happen. The material can reflect the light, which means it bounces off the surface. It can absorb the light, which means the material soaks it up and takes in its energy. Or it can transmit the light, which means the light passes straight through, like through a clear window. Which of these happens depends on what the material is like: its colour, its texture, and what it is made of. Light can also bend when it moves from one material into another, like from air into water. This bending is called refraction, and it is why a straw looks broken when it sits in a glass of water. Waves can also overlap and add together, which is called interference, and they can spread out around tiny objects or through small openings, which is called diffraction. All of these behaviours help explain how we see the world and how light carries information.

When light hits a surface, some of it bounces off. That bouncing is called reflection, and it follows a simple rule. The angle at which the light arrives always equals the angle at which it leaves. A polished surface, like a mirror or still water, is very smooth, so all the light rays bounce off in the same neat direction. That is why you can see a clear image in a mirror. A rough surface, like a brick wall or a sheet of paper, has tiny bumps all over it. Each light ray still follows the rule, but because the surface points in many slightly different directions, the rays scatter off every which way. That is why rough surfaces do not show reflections of your face. When light meets any material, some is reflected, some is soaked up, and some passes through. The surface itself decides how much of each happens.

Light is only one small part of a much bigger family called the electromagnetic spectrum. This family includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. They are all the same kind of wave, just with different wavelengths and frequencies. Waves with high frequency have short wavelengths and carry more energy. Waves with low frequency have long wavelengths and carry less energy. Different parts of the spectrum are useful for different jobs. Radio waves carry music and phone calls because they travel far and pass through walls. Microwaves heat food and link satellites. Infrared is the warmth you feel from a fire, and remote controls use it to talk to your TV. Visible light is the only part our eyes can detect. Ultraviolet can tan or burn your skin, and X-rays pass through your body so doctors can see your bones. The material a wave meets decides whether it is absorbed, reflected, or let through.

Filtering light means letting only certain colours pass through while blocking the rest. You can try this at home. Coloured cellophane, clear gift wrap, and some see-through plastic objects all work as light filters. Hold a piece of red cellophane up to a lamp. The cellophane lets red light through and absorbs the other colours, so everything you see through it turns red. This is exactly how the colour of any object works. A red apple looks red because its skin reflects red light to your eyes and absorbs the other colours. Try layering two different coloured filters to see which colours can make it through both. Shine a torch through the filters onto white paper to see the colour clearly. You can also test sunglasses, coloured drink bottles, or sweet wrappers. Each material has its own way of treating light, based on what it is made of and how it is coloured.

Electromagnetic waves come in a huge range, sorted by their wavelength and frequency. At one end are radio waves, with wavelengths as long as a building. At the other end are gamma rays, with wavelengths smaller than an atom. In between sit microwaves, infrared, the rainbow of visible light, ultraviolet, and X-rays. Each kind is made differently and behaves differently when it meets matter. Radio waves are made by vibrating electric currents in antennas, and they bounce off the upper atmosphere, which lets radio stations reach far away. Infrared is given off by anything warm, so night-vision cameras can spot people and animals in the dark. Visible light comes from very hot objects like the Sun and from light bulbs. Materials respond to each part of the spectrum in their own way. Glass transmits visible light but blocks much ultraviolet. Your skin absorbs ultraviolet, which is why you can sunburn. Metal reflects radio waves, which is why satellite dishes are made of it.