Mirrors and Images

When you see an image of an object in a mirror, what path do the rays take from the object to your eyes?

When light rays leave the wrestler (shown below), the rays strike the mirror and reflect.  How do these light rays form an image of the wrestler when they enter your eye?  Do clear images form by specular reflection or diffuse reflection?  What kind of surfaces help to form clear images?  When reading below remember that the object is the actual wrestler and the image is what you see when the reflected light rays reach your eye.

Figure 1, As the light rays leave the wrestler, what path do they take to the mirror and then to your eye?

The picture below is another view of the situation above.  Remember that light reflects off the mirror at an equal angle to the normal, as compared to the incident angle.  As light leaves the top of the wrestler’s head, where along the mirror must light reflect so that it enters your eye?  Is this the only point on the mirror where light from the top of the wrestler’s head reflects off the mirror?  Where must light reflect off the mirror (at an equal angle) so that a person on the other side of the basketball can see the wrestler?

Play the movie here.  From how many different spots can you see the image of the wrestler?  What does this mean about the number of light rays leaving a spot on the wrestler and in how many places do these light rays strike the mirror?

Does the image of the wrestler always seem to be in the same spot relative to the actually wrestler (the object)?  When the image of the wrestler is viewed from many different positions are the reflected light rays always coming from the same spot on the mirror?  Let’s draw some diagrams below to explore these many questions. 

How do light rays from all parts of a book reflect off the mirror to from a clear image?

From Fred’s perspective, each reflected ray seems to be coming from somewhere straight back along the path of the reflected ray.  If we dash each of these paths back, we can locate the position of the image.  From Fred’s perspective, the image of the book seems to be located behind the mirror.  The image seems to be located an equal distance behind the mirror compared to the distance the book is in front of the mirror.

Figure 2, In what direction do light rays leave the top of the wrestler’s head?  At how many points do these rays hit the surface of the mirror?

Figure 3

In Figure 3  below, one can see a single incident ray leaving the book and the reflected ray that results.  Notice that the reflected angle and the incident angle are equal, as you observed in your experiments.  Where does Fred see the image of the book?

In Figure 4 below, two more incident rays are shown leaving the book, and reflecting off the mirror into Fred’s eye.  Notice all the reflected angles are equal angles compared to their incident angles.

Other rays that leave these three points on the book reflect off the mirror at different points and do not make it into Fred’s eye.  This means Fred never sees those other reflected rays.

Where does Fred see the image of the book?

reflected

angle

incident

angle

Figure 4

Figure 5

Finally, if we follow an incident ray from the book, as it reflects into the eye of a second observer, we can locate the image that the second observer sees.  If we dash the reflected ray seen by Gwen, back along the path from which Gwen sees it coming, we locate the image.  This ‘dash back’ line intersects the same point on the image that Fred sees.  This means that Gwen sees the image in the same location as Fred.

Note that the incident angle and reflected angle are equal for the light ray that Gwen sees.

Important! To accurately locate the position of the image, you must follow the path of two light rays that leave the object from the same point and enter the eye of two different observers.

Figure 6

Copyright  © 2009-2012, by Marcus Milling

edited by Julia Pian

This light ray is not seen by Fred.