Site author Richard Steane
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How the eye works

The path of light

On its way through the midline of the eye, light rays pass through the following:



Aqueous humour

(pupil)- i.e a gap in the middle of the iris


vitreous humour

(blood vessels, nerves):on most of the retina these structures interfere with the passage of light, but these are not present on the fovea (yellow spot)

retina - rods

- cones

* see below

Refraction - bending of the rays of light (due to changes in refractive index)

occurs at

- the cornea (actually more refraction occurs here, but it is only by a "fixed amount")

- the lens (a variable amount or refraction- see later).

Image formation

The image which forms on the retina is said to be




The light sensitive cells in the retina are called rods and cones. Most of these cells are cones, clustered in the centre of the fovea where there are no obstructing blood vessels and nerves. Rods are more widely distributed around the edge of the retina.

Rods can detect dim light and are responsible for monochrome vision.

(Different types of) Cones detect different colours of light and give colour vision, and require better illumination. Due to their packing, they give a more detailed image.

In addition some cells may detect different patterns of light falling on the retina.

Depending on the illumination of the different cells, nervous impulses pass along the optic nerve to the brain where this information is processed in the visual cortex.

Although it is said that the eye receives an inverted (upside down) image, it is not strictly true that the brain "turns it the right way up", as it is part of the normal image processing.

Adjustments to focussing (accommodation)

In order for us to see near objects the lens must be thicker (fatter); to see far objects the lens must be thinner.

This is achieved because the lens is normally quite elastic, and it can change shape according to the tension it is under.

Normally the (hydrostatic) pressure inside the vitreous humour causes tension to be passed - via the suspensory ligaments - to the lens so that it is pulled thin. Thus parallel rays of light from distant objects are brought to a focus.

For close-up vision, the light rays must be refracted more. In this case the ring of muscle inside the ciliary body is contracted. This removes the tension mentioned above, and allows the lens to bulge to create a fatter, more refractive shape.

n..b. (1) This only occurs when the lens is elastic; in middle age the lens becomes less flexible and so reading glasses are increasingly required!

n.b. (2) The ciliary muscle does not squash the lens fatter; if it did then reading glasses would not be needed!

n.b.(3) This is one of the few examples of a muscle which does not have another, antagonistic, muscle which performs the opposite action.

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