Rays

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:

• Some of the light may be absorbed by the medium and lost. Dark glasses do this, for example. Most materials will absorb different colors of light at different rates, which is what gives them their color. Leaves, for example, absorb most visible light except for green, which is why they appear green to us. We won't discuss light absorption in any great detail.
• Some of the light is reflected back into the original medium.
• If the new medium is transparent or translucent, some of the light may pass through, or transmitted. Transmitted rays of light usually change direction, a process called refraction.
We will discuss light reflection and transmission in the next two sections , but first it will be useful to establish some vocabulary.
• The interface is the dividing line between the materials. The division between medium isn't always a sharp line: for instance, as light travels from air into water it passes through a layer of water vapor at the surface, which is a gradual transition. Reflection and refraction will still occur in these cases, but the physics is more complicated.
• The initial ray of light is called the incident ray.
• If we draw a line perpendicular to the interface, through the place the incident ray intersects the interface, that is called the normal to the surface. ("Normal" is a mathematical term meaning "perpendicular".)
• Once the incident ray hits the interface, it may break apart into a reflected ray and a transmitted ray, as shown.
• To quantify the directions of the three rays, we define the incident angle θi, the reflection angle θr, and the transmission angle θt. These are always measured from the normal, never from the interface. For example, an incident ray which hits the interface dead-on, right along the normal, has an incident angle of 0°.