Josh Pendreigh, of Mosgiel West School, asks :-

Morgan Allington, of Balclutha Primary School, asked:-

Why is it that when I look into my dinner spoon my reflection is upside down but when I look at the back it is the right way up?

John Campbell, who teaches optics at the University of Canterbury, responded.

A very astute observation Josh. To explain this I will need to start with a simpler example. Look into a flat mirror. Notice where the reflection of your eyes is. The line between your eyes and their reflection is a very special line, we say it is at right angles to the surface of the mirror. Put another way, it is the line along which incident light rays travel back along the same line after reflection from the mirror.

This is the same as throwing a ball head-on to a wall. If the wall is rough it can bounce back in any direction. We call that a diffuse reflection, the sort we get from light reflected from a painted surface. If the wall is smooth the ball will bounce back in the direction it came from. But throw the ball at any other angle to the wall and it will bounce in another direction which makes the same angle but on the opposite side of the line which is at right angles to the wall. This is what we call a mirror reflection.

Light from the Sun or a light bulb is diffusely reflected off the top of our head and scattered in all directions. We see an image of the top of our head because some of these light rays hit the mirror and are reflected back to our eye(s). Because this requires a mirror reflection the right angle line for this image passes half way between our eyes and the top of our head. Hence the image of the top of our head appears above our eyes, ie the right way up.

For a curved surface the line between our eye and its image also passes through the centre of curvature of the mirror. When looking at the back of a spoon, the image of the top of our head is, like that for a flat mirror, reflected from above the line between our eye and its image so the image is the correct way up. But when looking at the front of a spoon the image of the top of our head is reflected from below the line to the image of our eye so our head appears up-side down.