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MOVIES and TV serials seem to dictate fashion, even in sunglasses.

Sunglasses filter out the unpleasant, high-intensity light. If the prevailing light is of high intensity, our eyes try to adapt to it in two ways.

1. By shrinking the pupil. (Pupil is the opening in the centre of our eyes.) The smaller the pupil, the lesser the light that is let in.
   
2. By reducing the sensitivity of the retina. Retina is where images form in our eyes — it’s like the film in a camera.

In fact, there are lots of similarities between a camera and our eyes. If the light is too much, the photographer reduces the size of the aperture, just like our eye reduces the size of the pupil. The smaller the aperture, the lesser the light that is let in.

If the light is too intense, we feel uncomfortable. That’s because our eyes cannot adjust. If we lower the intensity of light, it will be easier on our eyes. This is where sunglasses or dark glasses come handy.

High-intensity light could harm us, too. When our eyes are exposed to intense light, especially ultraviolet rays, the inside parts of the eye are gradually damaged. The lens inside the eye becomes clouded, and in course of time, it turns opaque and stops functioning as a lens.

This condition is called cataract. You could become blind if cataract worsens. Prolonged exposure to high-intensity light is the common cause of cataract. And, cataract is the most common cause of blindness in India and other tropical countries.

Apart from causing cataract, high- intensity light can harm the eyes in other ways too. Infra-red rays present in light could damage the central part of the retina and permanently damage the eye.

Some people develop white rings in their eyes due to exposure to high-intensity light. Though these rings do not cause any defect in vision, they can be very unsightly.

This could probably be avoided with the use of sunglasses.

Apart from the damage it does to the eye, light could damage our skin —and to some extent our immune system, as well. Exposure to sunlight could partly be the cause of certain types of skin cancer. This, of course, cannot be reduced or prevented by the use of sunglasses.

Sunglasses should not be worn to look hep and stylish. So then, how do we choose our sunglasses?

Is it necessary to buy the most expensive glasses? Does price have anything to do with its protective quality? Does the brand of glasses you use matter?

Most sunglasses are made of dark-coloured glass, which are fitted on to a plastic or metal frame. Some are inexpensive: but then, there are all those expensive snob brands that brandish four-figure prices. Sunglasses made from a variety of plastics, called acrylic, are available too.

The colour of the glass indicates the colour of light that will pass through.

For example, a yellow glass filters out blue and green and allows only yellow light to pass through. A green glass filters out blue and red and allows only green light to pass through. A neutral grey glass filters out all hues of light uniformly and lets in a less intense light without changing the colour of the light that gets in.

Most sunglasses filter out ultraviolet rays too. Some amount of ultraviolet rays will pass through, whatever be the colour of the glass. Acrylic ‘glasses’ actually do better job of cutting out ultraviolet rays than real glasses.

Blue light also can damage the eye, but it could be easily filtered out. If you use blue glasses, you actually let in more blue light, and this causes more damage. Any glass of any colour, except blue, will do the trick.

Pink and yellow filter out blue and ultraviolet rays better than any other colour and are probably the best colours to use.

But, pink and yellow do not seem to be very popular.

If you use sunglasses that are very dark, the pupil becomes larger to let in more light. When the pupil becomes large, it is dilated. A dilated pupil lets in more ultra-violet rays. So it’s safer to use sunglasses that aren’t very dark.

Some sunglasses have an anti-reflection coating on them: It helps cut down glare and ultraviolet rays. If your glasses do not have this anti-reflection coating, do not despair: you are absolutely safe with plain sunglasses.

Minor defects in the glass do not cause permanent damage to the eye. It may cause some temporary strain. (According to British Medical Journal, nothing will go wrong even if you wore someone else’s power glasses.)

Expensive, branded glasses do not necessarily offer greater protection. They, of course, have snob value.

Acrylic ‘glasses’ are better at cutting out ultraviolet rays, but they are generally more expensive and, worse still, they get easily scratched. (By the way, plastic ‘glasses’ are not necessarily acrylic.)

Make sure your sunglasses cover your eyes completely. If your sunglasses let in light through the sides and onto your dilated pupils, they defeat their very purpose.

Small sunglasses, which look stylish, are useless.

Choose a frame that sits comfortably on your face: the frame should not cause any irritation.

And, you should wear sunglasses only when you are in the sun. Do not use dark glasses at night, especially while driving. They could strain your eyes — and even land you in accidents.

Dr Ranjit Grover MS