Reflection and plane mirrors

Mirrors change the direction of light

Hold a shiny steel plate or a flat mirror in sunlight and turn it: a patch of light jumps onto a wall that the Sun does not reach. Tilt the mirror and the patch moves. Light still travels in straight lines, but on meeting the shiny surface it sets off in a new direction. We call this turning of light by a mirror the reflection of light. A mirror with a flat surface, rather than a curved one, is a plane mirror.

A beam meets a mirror

To watch reflection closely, cover all the gaps of a comb with black paper except one, so that it makes a single thin slit. Lay a white sheet on a table, stand the comb upright on it and shine a torch through the slit: a thin beam of light runs across the paper. Now place a plane mirror in its path. The beam strikes the mirror and comes back along a new line. That is reflection at the mirror.

The image in a plane mirror

Put a pen in front of a plane mirror and a second pen seems to stand behind the glass. That apparent pen is the image; the real pen is the object. Move the pen around and you find that the image is always the same size as the pen, and always erect, the right way up. Try catching the image on a sheet of paper behind or in front of the mirror: it never appears there, because no light actually reaches the place behind the mirror. Next, stand at different distances from a mirror. Step closer and your image comes closer; step back and it moves away. The image is always as far behind the mirror as you are in front of it.

Left and right swap

Raise your left arm in front of a mirror and the image raises the arm on its right side; touch your right ear and the image touches its left ear. This left-right swap is called lateral inversion. It is why AMBULANCE is painted reversed on the front of ambulances: in the rear-view mirror of the vehicle ahead, the word reads correctly. Hold a paper with your name up to a mirror and the letters appear reversed, though letters such as A, H, O and T, which are the same on both sides, look unchanged. Long before glass mirrors, people polished stone or metal to see themselves; in Kerala, the metal mirrors called Aranmula Kannadi are still made today.

Periscopes and kaleidoscopes

Two plane mirrors fixed at 45° in a Z-shaped tube make a periscope. Light from an object enters the top, reflects off the upper mirror down the tube and off the lower mirror into your eye, so you see objects you could not see directly. Submarines, tanks and soldiers in bunkers use periscopes, and you can use one to see over taller friends. Three mirror strips joined in a triangle make a kaleidoscope: pieces of coloured bangles at one end are reflected again and again, so a new symmetric pattern appears every time you turn the tube.

Why it matters

Mirrors are everywhere: in bathrooms, vehicles, shops and scientific instruments. Compare the mirror image with the upside-down picture in the pinhole camera, and revise how shadows are formed. Answers to the textbook questions are in the NCERT solutions for Chapter 11; see all of Chapter 11 notes.