Finding the mirror image
Where is the mirror image? Some of your students probably believe it is on the mirror's surface. This experiment only costs seconds, but makes it evident that the image we see in the mirror lies behind, and not on the reflective surface.
The virtual image we see in a mirror actually lies behind the mirror.
Look into a mirror you hold about 10 cm from your face. Then hold a paper with some writing or drawing just above the mirror.
1. If you move the paper forward and backward, at what distance can you see both your mirror image and the paper in focus?
2. Can you explain why this is the case?
While observing each other, your students will quickly note that they hold the paper at roughly twice the distance of the mirror.
Different students might take different approaches to show why this is the case. Some might explain that light needs to travel from the face to the mirror and back, while the light from the paper only goes one way. Others might use a geometrical argumentation and some might illustrate the formation of a virtual image with a ray optics diagram. Help your students to see what they can learn from each of the approaches they come up with.