The first Law of Reflection

Water waves are reflected away from the barrier at the same angle as they approach (this is easier to see in the video on reflection). The ray of red light shows up the angles much better. Angles are measured to the “normal” . The normal is a line constructed at 90 degrees to the reflector at the point where the light or waves meet the surface.

Normal, incident ray and reflected ray

Normal, incident ray and reflected ray

 

The First Law of Reflection is that the angle of incidence “i” equals the angle of reflection “r”.

First Law of reflection

Think about how your image looks when you stare into a mirror. You look the wrong way around in that you seem to be switched left to right.  An image formed by reflection in a plane (flat) mirror is reversed left to right (laterally inverted to use the fancy term). The image looks as if it is behind the mirror. It seems to be as far behind the mirror as the object is in front. Because the light cannot go through the mirror the appearance that a real picture is there is an illusion, the light cannot really be coming from there. This image in the mirror is called a "virtual image".

Note that the reflection of the ruler in a glass fronted mirror is not sharp. This is because there are two overlapping images, one reflecting from the silvering and one from the front of the glass.

 

The Second Law of Reflection

This law is hard to demonstrate in video and hard to draw.  It states that the incident ray, the reflected ray and the normal all lie in the same plane. That means that if we took a flat sheet of card we could always place it so that these two rays and the normal could be placed or drawn upon it.

The video at the end of this page is an easy and quick way to run through ideas on reflection. Other notes and video lessons on the physics of light include:    

 Refraction of light      

Total internal reflection and optical fibres - notes and video lesson         

Drawing ray diagrams for lenses - notes and video lessons