1. Photoreceptors

2. Photoreceptor cells
The retina is a thin sheet of cells that contains photoreceptor cells. Photoreceptor cells are those containing light-sensitive pigments. They convert light images into electrochemical signals that the brain can interpret.
The photoreceptors contain photopigments, which are coloured proteins that absorb light and undergo structural changes that lead to the production of action potentials and the start of a nerve impulse.
There are two types of photoreceptors in the retina:
Rods - most numerous at the periphery of the retina and detect shape, movement and light and dark changes.
Cones - sensitive to three different colours - green, red and blue

3. Role of Rhodopsin in rods
Rhodopsin is a photosensitive pigment. It consists of two molecules joined together; they are retinal (a derivative of vitamin A) and opsin. When light falls on rhodopsin a series of chemical reactions break the molecule rhodopsin into retinal and opsin.
This generates electrical impulse that is transmitted to the bipolar cells, the ganglion cells and then through the optic nerve where the signal is interpreted by the brain. The molecules then reform as rhodopsin again and the process is repeated.

4. Three types of cones
Humans have three types of cone cells which mean they can detect the full visible spectrum. These are:
Red cones (contain erythrolabe) type L - these respond to long wavelengths of light (red at 564 nm)
Green cones (contain chlorolabe) Type M - these respond to the middle wavelengths of light (green at 533nm)
Blue cones (contain cyanolabe) Type S - these respond to short wavelengths of light (blue and violet at about 435 nm)

5. Types of eye structure
Simple eyes (called ocelli) are found in worms, mollusks and crustaceans. Eyes that are located in a hollow are called a cup eye. These eyes do not detect colour and only give information on the direction of light source
Compound eyes made up of a large number of separate light receptors called ommatidia. Insects have compound eyes and they have three colour vision including the ultraviolet range spectrum. Each ommatidium has its own cornea and a lens made up of a crystalline cone. Compound eyes can have high flicker speeds for detecting movement, can detect ultraviolet light and the polarization of light
Single lens eye is a more complex camera type of eye found in mammals, all vertebrates and cephalopods. These eyes can focus and form an image. There are three different types of receptors found in the eye: colour vision, visual acuity and night vision. Having two eyes depth perception can be achieved

6. Rods and cones
The retina consists of a thin sheet of photoreceptor cells. These are light-sensitive cells which are activated by light energy to produce an impulse which travels along the neurons that link them to the brain.
In the retina there are two types of photoreceptor cells: rods and cones. Both of these cells are modified neurones. They are not distributed around the retina uniformly.
Rods are long rod-shaped cells, which are sensitive to low levels of light but are unable to discriminate between colours. The image formed by the brain using information form rod cells lacks detail. Rods are linked in groups to single neurones. Rods are found mainly around the periphery of the retina and there are none at the fovea. They are more suitable for night vision. When the pupil is dilated more rods will be exposed. Rods also detect movement very well.

7. Rods and cones
Cones are conical cells which contain a pigment which is only sensitive to high intensities of light but exist in three different forms so that these cells can distinguish between colours. They have extensive nerve connections with the brain and produce a more detailed image. The number of cones increases towards the centre of the back of the retina. At the centre of the retina is a small area, known as the fovea, which has densely packed cones only. The fovea corresponds to the region of maximum visual acuity.
Cones are more suitable for day vision. In bright light, when the pupil is contracted, it will be mainly the cones that are activated. As cones require light of high intensity to stimulate them, it follows that we cannot see colours in poor light.
Visual acuity is dependent on the number of cone cells per unit area. The more there are the greater the number of impulses which will pass to the brain and the more detailed the image.