Reflection and how mirrors work
Physics · Key Stage 2 · The School
The rule of reflection
Light bounces off a surface at the same angle it arrived at. The angle in equals the angle out. You measure both from a line at right angles to the surface. Light travels in straight lines, so that one rule tells you exactly where a reflected ray will go. It is how a periscope works. Two tilted mirrors let you see over a wall.
What a mirror image is like
A mirror image is the same size as the real thing. It sits as far behind the glass as the object is in front. But LEFT AND RIGHT ARE SWAPPED. That is why writing held up to a mirror is hard to read. It is also why an ambulance has AMBULANCE printed backwards on the front. A driver ahead reads it the right way round in the mirror.
Why paper is not a mirror
A mirror is very smooth. Rays that arrive side by side leave side by side, so the image survives. Paper looks smooth but it is bumpy close up. Its rays scatter in all directions. You still see the paper, but no image forms. So the difference between a mirror and a wall is not what they are made of. It is how smooth they are.
Stand 1 m from a mirror and ask a friend to guess how far away your image looks. It looks 2 m away — 1 m to the glass and 1 m behind it. Test it: hold a book at arm's length and focus a phone camera on the book, then on your reflection. The camera has to refocus, because the image is genuinely further away than the glass. Step back another metre and the image retreats two.