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Sight

The human eye is a sense organ adapted to allow vision by reacting to light. The eye contains structures that allow it to perceive light, movement and colour differences.

Eye structure

  • Conjunctiva – a thin, clear mucous membrane covering the eyeball front and inner surface of the upper and lower eyelids. It makes eye movement and blinking easier.
  • Sclera – the outer layer of the eyeball. It is made of tough, interwoven fibres and protects the eyeball. Muscles attach to the sclera allowing eye movement.
  • Cornea – a clear, round window at the eyeball front. It allows light into the eye, protects the eye and bends light rays so images can focus on the retina.
  • Iris – the coloured part of the eye with the pupil at the centre. Different coloured irises result from different amounts of the pigment melanin. The iris is a muscle that can adjust the pupil size so light can enter, forming a retinal image.
  • Pupil – a dark space in the middle of the iris. It enlarges in low light, allowing light in, and it constricts in bright light, reducing the light entering.
  • Lens – found behind the iris inside a clear capsule. It bends light, forming an image on the retina. The cornea bends 70% of incoming light and the lens 30%. Fibres hold the lens in place. Looking closely at an object changes fibre tension, causing the lens shape to change to accommodate the object’s distance.
  • Optic nerve – nerve fibres connecting the retina and brain. While retinal cells change sight into electrical impulses, the optic nerve sends them to the brain’s visual cortex. The image is upside down but the brain turns it up the right way.
  • Retina – a lining inside the back of the eyeball that contains light-sensitive receptors – rods and cones. It receives light rays from the lens, forming an image. These are changed into electrical signals and sent to the brain.

How colour vision works

Rods and cones are involved in colour vision. In normal colour vision, light enters the eye hitting the retina. Rods are more numerous than cones but are not colour sensitive. They are responsible for low light and peripheral vision and sense motion.

Cones are found in the centre of the retina (macula area), concentrated at the fovea (the area of best vision). In bright light (daylight), they give clear vision detecting colour and fine details.

There are 3 cone types – red, blue and green – and each has a different range of light sensitivity. Colour perception comes from the stimulation of cones in different combinations.

Colour blindness

The inability to distinguish some colours is called colour blindness and happens when cones are missing or non-functioning. People with normal colour vision will see colours differently to those with colour blindness, for example, a green cucumber may seem to be the same colour as a carrot to those with defective colour vision.

Nature of Science

Scientists often use labelled diagrams to communicate detailed information. Scientists in a particular area will have a common understanding of these diagrams and the scientific language used.

Useful links

View an animation showing how human sight occurs.
www.bbc.co.uk/science/humanbody/body/factfiles/sight/sight_animation.shtmlexternal link

Information on a cow’s eye dissections, eye lenses and eye structure.
www.exploratorium.edu/learning_studio/cow_eye/how.htmlexternal link

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