P Lucas of Palmerston North asks :-

Why do we see a reflection off the clear windscreen of a car?

John Campbell, a physicist at the University of Canterbury, responded.

Whenever a wave passes from one medium to another, in which its speed is different, part of the wave is reflected at the interface. Light passing from air into glass, or glass into air, is just one example of this.

Of our transparent media, light in air changes speed the least in passing into water; it slows down to three quarters of the speed in air. The biggest change for visible light is on entering diamond; for which it slows down to half the speed in air. For materials such as glass the light slows down to about two thirds that in air.

Because of the large change in speed, diamonds sparkle the most and water the least. For glass, about 4 percent of light incident at right angles to the window is reflected. The amount of reflection depends on the angle of incidence of the light. At grazing incidence almost all of the light is reflected and surfaces such as glass or polished lino appear to be mirrors.

Light is partially polarised on reflection. This is why fishermen and women use polarising sunglasses to supress the amount of reflected light they see off the water, and can thus see more clearly the fish below the water surface. You can check if your sunglasses are polarizers, or merely tinted, by looking at a reflection off a window or polished bench top and rotating your sunglasses. Tinted ones wont change the reflected image.

The surface of windscreens are toughened, to prevent sharp edges should it break in a collision, using a heat treatment which requires the surface to be cooled very quickly. When wearing polarised glasses a car windscreen may appear blotchy because the glass surface now polarises light. This blotchyness is sometimes seen weakly without polarising sunglasses. This is because the light from the sky, ie sunlight scattered by the atmosphere, is weakly polarised.