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