The Eye

We use sense organs to detect an external stimulus such as a sound, a smell or a ray of light. Each sense organ contains unique receptors to detect different stimuli.

For example, the skin is a large sense organ containing many different receptors which can detect touch, pressure, temperature and even pain.

The human eye is a specialised organ containing receptors that detect light and colour. So the eye allows us to see whether something is coming towards us by detecting the light from objects.

Unlike some other animals, humans have both eyes on the front of our faces, which are capable of facing the same direction. This is called binocular vision and it allows us to perceive the depth and relationship between objects.

Structure of the Eye

Cornea The transparent layer at the front of the eye, covering the iris, that allows light into the eye.

It is responsible for the initial focusing of the eye.
IrisControls the amount of light that can enter the eye. It does this by altering the diameter of the pupil.

This is why sometimes your pupil looks larger or smaller than usual, especially when in a very bright room or pitch-black room.

The iris is also the coloured part of the eye.
Ciliary musclesThese muscles change the shape of the lens by relaxing and contracting while pulling on the suspensory ligaments.

Changing the shape of the lens is required for additional focusing.
Suspensory ligamentsAttaches the lens to the ciliary muscles, so it holds the lens in place.
LensChanges shape to focus light onto the retina.
ChoroidContains the blood vessels of the eye.

This pigmented layer of tissue also prevents the reflection of light rays inside the eyeball.
Blind spotThe area of the retina that has no light receptors and where the optic nerve leaves the eye.
RetinaA layer covered in light-sensitive receptors which are connected to the optic nerve.
Optic nerveCarries impulses between the retina and the brain.
FoveaA small pit at the centre of the retina, which provides detailed and coloured vision because it is packed with cones (a type of receptor cell that detects colour).
PupilThe hole through which light passes into the eye.
ScleraThe tough, white layer of the eye that helps to protect it from injury

Receptors in the retina

Receptor cells in the retina convert the light that enters the eye into electrical signals, which are sent to the brain and interpreted as an image. These electrical signals are sent to the brain through the optic nerve.

The retina contains two types of receptor cells:

  • Rods
  • Cones


These receptors work best in low-intensity light. They respond to differences in light intensity, not wavelength. Also, they are more sensitive in low light conditions and cannot distinguish between different colours (they detect black and white only). Rod cells are found all over the retina.


These receptors work best in bright high-intensity light (they detect colour). Cones respond to different wavelengths, so they respond to different colours. They work well in bright light, that’s why we only see objects/images in colour when the light is bright enough.

  • Cones are mostly clustered around the fovea