What a ray represents in a particle model or a wave model.

Regardless of whether you think light is a wave or a particle (or both, as quantum mechanics would say), most properties of light can be described using a ray model. A ray is simply a bit of light which is all travelling in a certain direction. It is represented by an arrow. Light rays behave in a fairly simple way.

Sources of Light Rays

Light rays are emitted from any number of different sources of course: stars, light bulbs, ... even you and your chair are a source of (infrared) light rays. Let's consider the simplest possible source of light, a point source. (Larger sources can be thought of as collections of point sources.) Light rays are emitted radially by a point source in all directions (unless something blocks them).

A detector (such as our eye) can determine the distance of a point source by measuring how much the rays they receive diverge. When the detector is close to the light source, it receives rays which are travelling in many different directions. As the detector moves farther back, however, most of those rays end up missing the detector, and only rays which are aimed more or less towards the detector will make it through. Far from the source, the rays are practically parallel to each other.

Parallax Demo

This is the reason that the moon seems to follow you as you drive along in a car, while the positions of nearby objects seems to shift wildly as you move: a phenomenon called parallax. Astronomers can use parallax to determine the distance to a nearby star, by watching how its position shifts in comparison to more distant stars.

Ray Physics

Light rays follow their own version of Newton's First Law. A ray of light will continue to move in a straight line at constant speed forever, until it hits something. Rays of light do not interact with each other at all: if I shine one flashlight beam so that it crosses another, the first one's beam is not affected at all. (You may have heard about light interference, but that's a bit of a misnomer. We'll discuss that later. )

A ray only stops moving in a straight line when it enters, or attempts to enter, a new medium: e.g. when a light ray passes from air into glass, or when it hits the surface of a table. At this point, some or all of these things may happen:

We will discuss light reflection and transmission in the next two sections , but first it will be useful to establish some vocabulary.