Science & Technology in Action

9th Edition

The Miracle of The Eye


This lesson deals with the function and operational components of the human eye. It describes the structure of the eye and outlines how images are formed. The function of rod and cone cells, spectral range and the formation of colour vision are described.

Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

Colour vision
The normal human eye has three different kinds of cone cells, commonly labelled blue, green and red; they are also called S-cones, M-cones and L-cones (referring to short, medium and long wavelengths of visible light that they detect). The peak sensitivities of the S, M and L cones are 420 nanometres (violet), 560 nm (yellow-green), and 630 nm (orange-red) respectively, but there is considerable overlap in the range of wavelengths that the different types of cones detect, especially between the M-cones and L-cones. The relative response of the different photoreceptors enables us to distinguish different colours.

The incidence of colour blindness varies with race. Among Caucasians about 8% of men and about 0.5% of women have some form of colour blindness, the most common kind being red-green colour blindness. Colour blindness results when one or more of the cone types is defective or absent.

The peak sensitivity of rod cells is around 500 nm (blue-green). They are more sensitive to low light levels than the cone cells and enable people to see in dim moonlight or even starlight, but with reduced detail.

Visual acuity
The area of the retina is a little over 1000 mm2 (i.e. 10 cm2). The central part of the retina is called the macula, occupying about 20 mm2, in the centre of which there is a depression called the fovea, about 2 mm2 in area. The fovea, which occupies less than 1% of the retina, contains densely packed cone cells (about 160,000 cone cells per square millimetre) and few if any rod cells. This is one reason why the fovea has the greatest visual acuity; the other reason is that about half of the optic nerve fibres transmit signals from the fovea.

In the rest of the retina (99%) the packing density of cells (mostly rods) is only about 40,000 per square millimetre. Peripheral vision is therefore not as sharp as the central gaze.

To read the full lesson, download the pdf above.

Quiz questions

  1. The lens provides most of the focusing in the eye. false
  2. The front of the eye is called the cornea. true
  3. The human eye normally has three types of colour receptors (cone cells). true
  4. An inverted image is formed on the retina. true
  5. The curvature of the lens in the eye is controlled by the iris. false
  6. The hole in the centre of the iris is called the pupil. true
  7. The size of the pupil changes with the brightness of the object or scene. true
  8. The normal human eye has about 120,000,000 photoreceptors. true
  9. A neuron goes from every rod cell to the brain. false
  10. A convex lens can enable a person with hyperopia bring nearby objects into focus. true
  11. Age-related macular degeneration reduces peripheral vision. true
  12. Diabetic eye disease is the leading cause of blindness among the working age population. true

Glossary of terms

bipolar cell
cilliary muscles,