Callum Craik, of Balclutha Primary School, asks :-

Why does looking at the sun make you go blind?

Gordon Sanderson, an ophthalmologist at the Dunedin School of Medicine, responded.

The sun is a powerful source of light, but we can only see some of it. It is what we can’t see such as infrared and ultra-violet light that causes damage to our eyes.

Firstly the infrared, is what makes us feel warm when we are in the sun, it is also a form of light that can be used for cooking. So if it ever reaches our retina it can cook that too. If you focus a magnifying glass on a piece of paper on a sunny day you will already know how you can burn paper. Your eye contains magnifying lenses called the cornea and the crystalline lens; they focus the sun’s rays onto our retina. If we stare at the sun for too long it may burn the photoreceptors and permanently destroy them.

Since it is the photoreceptors that enable our brain to form images of the world, any damage is a serious problem. You can experience a similar but temporary form of blindness, by staring at a much weaker source of light such as a light bulb. When you look away you will notice a black spot in your vision, especially if you blink your eyes a lot. This is called an after-image and is caused by bleaching of the photoreceptors. Fortunately after a few minutes our vision returns to normal. If you had stared at the sun it would not.

The second form of damage occurs with ultra-violet light. This is also an invisible form of light. It is this form of light that causes sunburn. Initially our skin reacts to protect itself from ultra-violet by turning red then brown, but our eyes cannot do this. So a lot of exposure to ultraviolet light such as we experience when skiing, can cause sunburn of the eye. Our eyes are very sensitive so this is very painful, but it wears off after a few days. It is often called snow-blindness.

Fortunately we can protect our eyes by wearing filters. Normally these would be in the form of sunglasses or skiing goggles, they absorb dangerous light. If we wanted to look at an eclipse of the sun we would use very strong filters that absorbed all of the infrared light.