Brooke Walsh, of Balclutha Primary School, asks :-

How does light make us see in the dark?

Logan Mitchell, an ophthalmologist at the University of Otago's Dunedin School of Medicine, responded.

To answer this question we need to think of our eyes. Right at the back of the eye is a film (just like the film in a camera) called the retina. The retina has millions of cells called photoreceptors, which send out an electrical signal when light hits them. What we ‘see’ is determined by the signals these photoreceptors send to the brain. So light lets us see because that’s what our photoreceptors respond to! The reason we can’t see in the dark is that there’s not enough light to stimulate our photoreceptors to send out an electrical signal.

There are very few places where there is absolutely no light. One of those places is right at the bottom of the sea. There are amazing creatures there that don't have eyes, because they don't need them - there's no light!

What happens when we need to see in the dark, like outside on a dark night? Our pupils get bigger - letting more light in! Have you noticed that at night you can only see shades of grey/blue at night? That is because we have got four types of photoreceptor in our retinas, and they are sensitive to different colours, but only one type sees really well when it's very dark and it is sensitive to blue!

Some animals’ eyes have extra features that let them see better in the dark, like cats, who have a special reflective layer (tapetum) on their retina to keep bouncing the light around their eye so that no light is wasted. This is why a cat's eye shines back at you sometimes at night!

But what is 'light'? Light is actually just one type of radiation, just like X-rays, ultra-violet and radio waves, all basically the same thing. But we call light ‘light’ because it happens to be the type that our photoreceptors are sensitive to. Did you know bees can see ultra-violet light!