Reflection

What happens to light when it strikes a surface?

First, what is the difference between a ray of light and a beam of light?  A ray shows the path of the light, but the ray is defined to have no thickness.  The ray only has length and direction.  The ray is something we draw to explain the direction that light is traveling, but you will never see an individual ray.  A beam of light can actually be seen. Examples include a laser beam or when light passes through a small opening and travels as a beam on the other side.

Figure 1, Laser Beams (shown above) have width. 

Figure 3, Multiple rays describe the path of a very wide beam of light. 

The answer is, of course, yes and you have seen diffuse reflection (the other type) before.  When a beam of light strikes a ‘rough’ surface, the light will reflect off at many angles and be seen by everyone in the room.  When the laser beam strikes the board below, all people standing in front of the board can see the green dot on the board where the beam hit.  For this to happen, the green light must reflect off the board and enter each person’s eyes.  This means that light is reflecting off the board at many angles.

Is there another type of reflection?

Figure 2, Rays describe the paths of different parts of the light .

When a ray of light strikes a surface, what happens?  The ray will reflect off the surface, but in what direction?

To keep track of the reflection, we will draw a dashed line called a normal (see Figure 4b).  A normal is drawn at a 90o angle to the surface and passes through the surface at the point where the ray strikes the surface.  The ray coming from the source is called the incident ray.

We will then measure the angle between the normal line and the incident ray.  Next, we measure the angle between the normal line and the reflected ray.  How do these angles compare when light reflects off a prism?

Do the reflected angle and incident angle below appear to be equal?  The reflected angle and incident angle will be equal for surfaces like a glass prism.

reflected ray

incident ray

incident

angle

reflected

angle

50o

50o

Watch the movie below to see how changing the incident angle affects the reflected angle.  Do the two angles always appear to be equal?

normal

The type of reflection shown in the video is called specular reflection and occurs on very smooth surfaces.  Specular reflection is when a beam of parallel light rays strike a surface and the reflected beam leaves the surface at an equal angle to the normal as the angle the incident ray makes with the normal.  This is what happens on the surface of a mirror.  The way beams are reflected in specular reflection is similar to the way a ball bounces off a surface. Click here to see the video.

Why can’t you see the laser beam as it travels through the air?  (click here to find out)

Below, you can see a magnification of a somewhat rough surface.  Adjacent parts of the surface are not even.  As a beam made of parallel light rays strike the surface, they hit at many different angles, due to the uneven surface.  Each incident angle leads to an equal reflected angle.  Since there are many different incident angles, there are many different reflected angles.  This sends light in many different directions and most people in the room can see the reflected light.  This is similar to how the whiteboard at the front of the classroom reflects light, but different than how a mirror does.

What causes diffuse reflection?

Rough Surface

What causes diffuse reflection?

Figure 4b, The incident beam is hard to see because as the laser travels through the air, very little of the light is reflected (to your eye).  But as the laser travels through the glass, a significant portion of the laser light is reflected in many directions (called scattering).  Your eye sees this reflected light.  The reflected beam hits the tabletop, after reflection, making the beam easy to see.

Figure 4a,  An incident beam and reflected beam are shown above. Why is the incident beam hard to see?

Copyright  © 2009-2012, by Marcus Milling

edited by Julia Pian

_________________Click HERE to see the reflection video. ________________