Learning Aims:
  • Understanding that the image in a plane mirror is not located at the mirror surface

Three thick whiteboard markers, Plane mirror, Retort stand, Paper, Pencil, Cocktail sticks, Shiny metal tin
Suggestions for use:

The students should initially investigate the phenomenon of parallax by placing one of the markers vertically on their desk and lining up the other two behind it. They should describe what they observe when they look along this line of markers, with one eye closed, and move their head sideways -

Does the nearest marker appear to move a greater or lesser distance than the farthest marker?

Where would the markers need to be placed so they appear to move together?

The students should then be asked to determine how they might use this method of parallax to determine where the image in a plane mirror is formed. They can hold the mirror vertically and use the paper to mark the locations of the object pen, the mirror plane, and the image itself. The students should plot the path the light takes to the eye, and the path to the image.

Finally, the students should set their tin on the sheet of paper and attempt to angle a number of cocktail sticks on the paper so that they appear to lie parallel in the tin. They should then attempt to explain how the image is formed in this convex mirror based on their understanding of ray optics.

Possible questions:
  • Does the angle of incidence equal the angle of reflection for a curved mirror?
  • How does the curve affect the direction of light reflected from different points?
  • Where is the image formed in the convex mirror? What if it was concave?