Reflection of light

During the shadow activity, one student noticed something odd. A shiny object, like a polished steel plate, made a shadow on the screen — but also a bright spot of light on the wall on the opposite side. Why?

Activity 11.5: take a shiny flat steel plate, or a plane mirror (a mirror that is flat, not curved), outside into the sunlight. Turn it until it throws a spot of sunlight onto a wall the Sun is not shining on. You may have done this for fun already.

Now tilt the plate or mirror different ways. The spot moves. Light still travels in straight lines, but it changes direction where it hits the shiny surface.

So a mirror or shiny surface changes the direction of the light falling on it. This change of direction of light by a mirror is called reflection of light.

Activity 11.6 shows the bend clearly. You need a plane mirror on a stand, a torch, a comb, white paper and a strip of black paper. Cover all the gaps in the comb with black paper except one, to make a thin slit.

Lay the white paper on a table, hold the comb upright on it, and shine the torch through the slit until a thin beam of light runs along the paper. The slit is there to make the light into a narrow beam you can follow.

Put the mirror in the beam's path, keeping the comb still. The beam changes direction at the mirror and goes off in a new straight line. That is reflection, happening at the mirror's surface.