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 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 a ray.  A beam of light can actually be seen, such as a laser beam, or when light passes through a small opening and travels as a beam on the other side.

Laser Beams, these beams have width  (figure 1)

Multiple rays describing the path of a very wide beam of light 

(figure 3)

The answer is of course yes, and you have seen diffuse reflection before.  When a beam of light strikes a ‘rough’ surface the light will reflect off at many angles and be seen by every one 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?

Rays describing the paths of different parts of light  (figure 2)

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 4).  The 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 the light reflects off the prism?

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

reflected ray

incident ray

incident

angle

reflected

angle

50o

50o

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

normal

This type of reflection is called specular reflection and occurs on very smooth surfaces.  When a beam of parallel light rays strike a surface the reflected beam leaves the surface at an equal angle to the normal.  This is what happens on the surface of a mirror.  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-up 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.  But 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 the whiteboard at the front of the classroom, but different than a mirror.

What causes diffuse reflection?

Rough Surface

What causes diffuse reflection?

Figure 4b, The incident ray is hard to see because as the laser travels through the air very little of it is reflected (to your eye).  But as the laser travels through the glass, much of the laser light is reflected in many directions (called scattering).  So your eye sees this reflected light.  The reflected beam hits the table-top after reflection and this makes it easy to see.

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

copyright Marcus Milling  2009

_________________Click HERE to see the reflection video _________________