Physics Made Easy

The Eye

The eye, like all other sensory organs, is a biological transducer. In the case of the eye, this means that it converts light energy to electrical impulses (and amplifies the effect).


Accommodation: the process in which the eye adapts to view close or distant objects as required.

  1. Distant object


  1. Near object


Pupil reflex: controls the amount of light entering the eye depending on environmental light intensity. The muscles in the iris are arranged as below:


When light intensity is low, the radial muscle contracts and the circular muscle relaxes. This leads to an increase in pupil size, so the eye can gather as much light as possible. When light intensity is high, the radial muscle relaxes and the circular muscle contracts, leading to a decrease in pupil size.

Structure of light sensitive cells (aka rods and cones- see below)

Rod cell:


Cone cells have the same overall structure; however, the outer segment is cone shaped and contains different pigments.

The retina contains rod and cone cells with bipolar neurones in front of them. The neurones synapse to the end bulbs of the light sensitive cells. The retina is said to be inverted because the sensors are actually behind the neurones.

Rod cells are light sensitive cells which give black/white vision. Several rods will synapse to one optic neurone, allowing summation of symbols. This means that rods are sensitive even in low light intensities, as the combined stimulation of a number of rods is enough to generate an AP in one neurone. However, this sharing of neurones means that rods give poor visual acuity.

Rods are found evenly distributed over the whole of the retina, except in the fovea.

Biochemistry of rod vision:

  1. Rhodopsin (aka visual purple) absorbs light and is broken down into retinene (requires vitamin A for its formation) and opsin (a protein). This chemical change generates an AP.
  2. Rhodopsin is reformed by enzymes in dark conditions from retinene and opsin. Mitochondria provide energy.

Cone cells are light sensitive cells which give colour vision. There are three types, which are stimulated by different wavelengths of light (blue, green and red). Each cone synapses with its own individual neurone- this gives high visual acuity but means that there is no summation effect. Therefore, cones only function in high light intensities.

Cones are mostly concentrated in the fovea.

Biochemistry of cone vision:

  1. Cone rhodopsin (aka iodopsin) comes in three different forms (to perceive red, green and blue). Bright light breaks cone rhodopsin down into retinene and photopsin (a protein). This chemical change generates an AP.
  2. The three different types of cone rhodopsin are formed by combining different proportions of retinene and photopsin. This reformation takes place slowly but continuously.

Trichromatic colour vision: stimulating different combinations of cones enables us to perceive colours other than pure red, green and blue, e.g.

Red+Green –> Orange/Yellow

Green+Blue –> Cyan

Red+Blue –> Magenta

Red+Blue+Green –> White

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