Cutting out glare
Look out of the window and imagine that it is a very hot, sunny day. The Sun has climbed in the sky all morning and is now at its highest point. This means that the place where you are is receiving the greatest number of light rays it can. The light strikes everything in your view. Some of it is absorbed by rough surfaces such as walls made from bricks and concrete. More is absorbed by the trunks of trees and makes them appear dark. All these surfaces reflect some light but because they are rough the light they scatter travels in all directions so only small amounts reach your eye. However, when light shines on a very smooth surface such as glass, shiny metal or the polished paintwork of cars, something different happens.

When light strikes a smooth surface only a small amount of it is absorbed. This means that a large amount of light is reflected. On a rough surface one light ray may strike a bump while the one next to it strikes a hollow and both are reflected in different directions. On a smooth surface there are no bumps and hollows to scatter the light, so all the light rays that strike the surface leave together. If the light is not too bright all the rays create reflections of nearby objects in the shiny surface. If the light is very bright, as on a sunny day, the light from a smooth surface is so strong that when it enters your eye it dazzles you and you cannot see. We call this bright light glare.

Glare is unpleasant and it can be dangerous. After a shower of rain a wet road surface can reflect so much sunlight that it causes glare to the drivers travelling along. The drivers can be blinded by the glare and cannot see where they are going. If one car stops suddenly the driver of the car behind may not see it, because of the glare, until it is too late.

Light is a form of energy. So one way to cut down on the energy entering your eyes is to wear sunglasses made of light energy absorbing materials.

Sunglasses do not necessarily have lenses which correct for defects of vision such as long or short sight. They do contain materials which absorb some of the light that passes through them. That is why they look black. However, they not only absorb light coming from the shiny surface which causes glare, but they also absorb light from any surface in the field of view. This makes everything look darker, not just the surfaces that cause glare. There are some sunglasses which do not behave in this way. They can reduce the glare without making everything else look dark. These are called polarising sunglasses.

The energy in a light ray vibrates like the ripples in a puddle after you have tapped the edge. It travels in a form of wave. The wave behaves as if it has some properties of electricity and some properties of a magnet. It is called an electromagnetic wave. In a light ray the light waves vibrate up and down or from side to side. In polarising sunglasses there is a material that acts as if it has slits in it like a comb. These slits are very tiny and are arranged vertically in each lens of the pair of sunglasses. When light strikes the front of the lens, those waves which are vibrating up and down pass through the slits but those that are vibrating from side to side are blocked.

Imagine looking out of the window while a cloud passes across the Sun. It is still bright but there is no glare from the smooth surfaces in your view. The surfaces of all the objects are sending you light waves, which vibrate both up and down and from side to side. Now imagine putting on a pair of polarising sunglasses. The slits in the lenses stop light waves that vibrate from side to side from entering your eyes and the view looks slightly dimmer. Now imagine that the cloud moves away from in front of the Sun and the smooth surface sends you stronger rays of light that cause glare. When they reach your polarising sunglasses, all the light waves which are vibrating from side to side are blocked and the amount of light that passes through is greatly reduced. At the same time light waves which vibrate up and down from other objects still pass through. The light from them has not been cut down as it would be in ordinary sunglasses so you can see the objects more clearly but without the glare.

Next time you put on a pair of sunglasses look into water on which light is shining. If the glare is reduced but the water appears dark, the sunglasses have lenses, which absorb all forms of light energy. If the glare is reduced but you can see deeper into the water, you are wearing sunglasses made from polarising material.

Does the Sun really move up and down in the sky?
No. The Sun stays where it is in space but it does spin round once in just under twenty-five days. The Sun appears to move across the sky because of the way the Earth turns and the way it tilts either towards or away from the Sun. In the northern hemisphere, when the Earth is tilted towards the Sun, the Sun rises further north on the eastern horizon at dawn, climbs high in the sky at noon and sets further north on the western horizon at dusk. This occurs in the Summer time. When the Earth is tilted away from the Sun, the Sun rises further south on the eastern horizon at dawn, climbs to a lower point in the sky at noon and sets further south on the western horizon at dusk. This occurs in winter. In the Southern hemisphere the Sun rises and sets further south in Summer and rises and sets further north in winter.

Why does a place receive more light rays when the Sun is overhead than when it is lower in the sky?
Imagine that you had a torch and shone it vertically down onto a table. The full power of the light beam with its countless light rays is striking the surface and making a very bright circle of light. Now turn the torch a little to the side so that it shines more across the surface of the table. The circle of light now becomes a larger oval but because only the same number of light rays are shining onto a larger piece of the surface the oval appears dimmer than before. The rays from the Sun behave in a similar way. They form the most concentrated beams of light when the Sun is high in the sky. It is these beams that can cause the most glare.

Why is there still daylight when it is cloudy?
The Sun is still shining in space when it is cloudy on Earth. Some of its light is reflected back into space but some passes through the clouds because they are translucent. They scatter the light in all directions so that there are no strong beams to create glare.

Why can there be glare on a wet road?
A dry road has bumps and hollows, which scatter light in all directions so there is no glare. When the road is wet, the water forms a film over the road surface, which has a smooth surface. This surface reflects strong beams of light to cause glare.

Sometimes on a hot day you can see pools of water on a road that disappear when you reach them. Are they caused by glare?
No. They are mirages. They are caused by the light rays changing their direction. On a hot day the air above a road becomes hot. Light rays coming down from the sky strike the hot air and their paths are turned round so that they shine upwards into the eyes of anyone walking or driving along the road. The light rays are from the blue sky but appear to be coming from out of the road. The light from the blue sky appears as a pool of water on the road.

Can you get glare from light sources?
Yes. Glare does not occur just from smooth surfaces but also from very powerful light sources. The Sun causes glare but you must never look directly at it as the light is so strong it could damage your eyes. You should not even look at the Sun with sunglasses. Common light sources, which cause glare, are the headlights on cars and trucks.