# Light

Light is a type of radiation that moves through space. Animals such as humans have eyes sensitive to light to see and understand the world around them.

Although light seems special, it is really just another kind of electromagnetic energy, like microwaves and radio waves. Light normally travels in straight-line rays, and reflects and refracts (bends) in very precise ways as it speeds through the world.

Most of the light we see with our eyes is very weak because it has already reflected off things. Not all light is so weak, however: light beams made by lasers are super concentrated and can be powerful enough to slice through metal.

### Reflection

We see things because light bounces off them into our eyes. If a surface is smooth, like a mirror, the light rays all bounce off at the same angle to make a single beam. This is called specular reflection. If the surface is rough, the rays bounce off randomly in different directions. This is called diffuse reflection.

The law of reflection

A light ray shooting at a mirror bounces off again at exactly the same angle. In more scientific terms, we say the angle of incidence is equal to the angle of reflection.

### Refraction

Light rays travel slower in more dense (thicker) substances such as water and glass than in air. The change in speed causes light to bend (refract) as it passes from air to glass and back. Lenses use refraction to magnify things - as the lens bends the light, the rays seem to be coming from a point closer to us, making objects appear bigger.

Changing direction

Light rays slow down and bend inward as they pass from air to glass, and speed up and bend outward as they go from glass to air.

Real and apparent depth

Refraction makes fish appear nearer to the surface. Because our brains assume light rays travel in a straight line, rather than bend, we see the fish higher up than they really are.

### Interference

When two or more light rays, water waves, or sound waves meet, they combine to make interference. In some places, the waves add together (interfere constructively) and in others, they subtract or cancel out (interfere destructively). The result is a new wave that’s bigger in some places and smaller in others. This explains why an ocean wave can be followed by another wave that may be much bigger or smaller.

Constructive interference

When two waves of the same length and height (amplitude) overlap exactly in step (in phase), they add together. The new wave they make has the same wavelength, but twice the height. If two light waves added together like this, it would make light twice as bright.

Destructive interference

When two identical waves add together, but are moving out of sync (out of phase), they cancel out altogether. The wave they make has zero amplitude. If two light waves added together like this, they would make darkness.

Colorful soap bubbles

Interference makes soap bubbles swim with color. The soapy film varies in thickness over the bubble. As light rays shoot into a bubble and reflect off its inner and outer surfaces, they add or subtract to make waves of different colors.

### Diffraction

Light waves spread out when they pass through tiny gaps or holes. The smaller the gap, the more spreading (diffraction) that occurs.

Diffraction through a narrow gap

For diffraction to work, the gap has to be about the same size as the wavelength of the waves. Sound waves also diffract, which is why we can hear through open doorways and around corners into other rooms.

### Lasers

Lasers make very powerful beams of light. Unlike in normal lamps, the light waves from lasers are in sync and add together (constructive interference). This is why laser light is strong enough to travel incredible distances and why the most powerful lasers (carbon dioxide lasers) can weld metal or slice right through it.

Inside a laser

Lasers make light when a power source flashes energy through a central tube. The atoms become excited, give out flashes of light (photons), and excite other atoms so they flash too. The light bounces back and forth between two mirrors until it emerges from one end as a concentrated beam.