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24 September 2014
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Nervous system - Sight

Retina: Part of your eye where light rays are turned into images

Rods: Photoreceptors detecting shades of grey when it's dark

Cones: Photoreceptors detecting colours

3D vision: The result of your brain merging the two images captured by your eyes

In your eyes

Your eyes are your organs of sight. In order to see, however, you also need your optic nerves to transmit nerve impulse to your brain, which then adjusts and interprets the images of the world surrounding you.

Your eyes are hollow fluid-filled spheres. Their walls are made up of three different layers, or coats:

  • Outside is your white sclera, which has a transparent window at the front of your eye called the cornea
  • In the middle is a coat containing your coloured iris
  • Inside, at the back of your eye, is your retina, where light rays are turned into images

Light-sensitive retina

Your retina contains millions of receptor cells. They respond to light entering your eye, which is why they are called photoreceptors.

Your photoreceptors contain visual pigments. When light strikes these pigments, they briefly lose their colour. This bleaching process triggers nerve impulses, which are transmitted to your brain via your optic nerve. Your optic nerve runs out of the back of your eye. It originates in an area of your retina called the optic disc, where your have no photoreceptors. This part of your retina is commonly known as your 'blind spot'.

You have two types of photoreceptor cells: rods and cones

Grey vision and movement

Your rods detect colours in shades of grey and they can sense shape and movement. You have about 120 million rods. They are densest at the edge of your retina and don't need a lot of light to work. So you use them in dim light.

Colour vision

Your cones see colours. You have three different types of cone, each of which contains a pigment that responds to a different wavelength of light - green, red or blue. If your brain receives impulses from more than one type of cone at the same time, you see mixed colours. You have around 7 million cones. They are densest at the centre of your retina and need bright light to function. This is why, when it's dark, you can only detect shades of grey.

Secrets of your lens

Before light rays touch your retina, they travel through a disc-shaped lens. Your lens bends light rays to focus them exactly on your retina. Muscles in your eye can change the shape of your lens, thereby adjusting its light-bending power. This allows you to focus on different objects.

By focussing images on your retina, your lens turns them upside down. So, for you to see properly, your brain has to turn them the right way up again. Your brain also needs to merge the two slightly different images captured by each of your eyes into one. By doing so, your brain creates a 3D picture.

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