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What happens when light hits the cornea?


Your eyes are the windows to the world surrounding you. Your eyes show you who or what is around you, and what's going on. It starts with light. For you to see the objects in front of you, they have to be illuminated -- by the sun or by another source of light. Light rays reflect from the surface of objects and reach your eyes.

But what is it in your eyes which allows you to perceive the light and interpret the signals? How does sight work? What happens when Leon sees that flower? The front surface of the eye is a clear and transparent membrane, which protects the eye from dust and contact. If anything touches the eye surface, the membrane will trigger a reflex that closes the eye in a flash.

Also, tears are released that wash away dust and particles. This part of the eye is called the cornea. This is the way a human eye looks in a cross section. As you can see, it is almost spherical in shape, and is sometimes called an Eyeball. What happens when the light reaches the cornea?

Have a look. It is refracted, and directed towards -- a small opening. That opening varies in size depending on how much light reaches it. If the light is strong, the opening will contract. If the light is weak, the opening will expand, to allow in more light.

This opening is called the pupil. Around the pupil, there's a membrane with a muscle controlling the size of the opening. This membrane is called the iris. The iris is the tissue that gives the eye its colour. When the light has passed the opening, the pupil, it will reach the lens of the eye, that is positioned right behind the iris.

The lens functions just like a magnifying glass. It refracts the light, and focuses it. A muscle surrounding the lens changes its shape. If the flower that we're looking at is far away, the lens needs a different shape than if it's close. If the vision is blurry, the reason sometimes is that the muscle around the lens is not able to adjust the shape of the lens properly.

A pair of glasses can help, by refracting the light and compensating for the inability of the muscle. So, the light is refracted by the cornea, and by the lens. It radiates deeper in, and reaches a membrane at the back of the eye. This is the retina. An image of the flower is projected onto the retina.

Due to the shape of the lens, it's turned upside-down. The retina contains millions of light-sensitive cells. There are two kinds. Some respond to the brightness of the light, and are called rods. The others recognise colours, and are called cones.

The cones need more light than the rods to react. If the light is soft or dimmed, it's harder for us to see colours, but thanks to the rods we can still see something. The rods and the cones convert the image on the retina into tiny electrical signals. The nerve system transmits these signals to the brain, that receives and interprets them. The image was upside down on the retina, remember?

The brain adjusts the image, and we perceive it the right way up. So, the light never reaches the brain. It is converted from light to nerve signals. Now you know what happens in your eyes when you spot a flower. But wouldn't one eye be enough?

Why do we have two? Yes, we can see the flower with one eye, but having two eyes next to each other enables us to estimate distances. This is why moving around in the world - or driving for instance - is more difficult if we don't have both eyes working well. Close both eyes for a while, and then open just one eye. Your distance judgement won't work as well as with two eyes.

Try it. --