Light and The Eye

Retina

the light-sensitive inner surface of the eye, containing photoreceptors including rods and cones plus layers of neurons that begin the processing of visual information

Rod Cells

a type of photoreceptor specialized for low levels of light intensity, such as those found at night

Cone Cells

a type of photoreceptor that operate best in bright light; enable high-acuity, color vision

Lens

the transparent structure behind the pupil that changes shape to help focus images on the retina

Fovea

the central focal point in the retina, containing the highest density of cone cells in the retina

Bipolar cell

a bipolar neuron located in the middle layer of the retina, conveying information from the photoreceptors to the ganglion cells

Ganglion cells

In the retina, the specialized neurons that connect to the bipolar cells; the bundled axons of these cells form the optic nerve.

Visible light

electromagnetic waves ranging from 400-700 nms

Isomerisation

Process by which retinal changes its shape within the disc membrane in the outer segment of photoreceptors, in response to the absorption of light.

Retinal

The light-sensitive part of the visual pigment molecule. It is attached to the protein molecule opsin to form the visual pigment.

Opsin

A class of protein that, together with retinal, constitutes the photopigments

Spatial Summation

The sum of multiple synapses firing at different locations at one time to create a net effect.

Neural Convergence

When several presynaptic neurons synapse with a single postsynaptic neuron. Increases light sensitivity and decreases spatial resolution of rod cells.

Sclera

Hard, white, strucutral part of the eye

Cornea

The transparent layer forming the front of the eye.

Pupil

The adjustable opening in the centre of the eye through which light enters

Iris

A ring of muscle tissue that forms the colored portion of the eye around the pupil and controls the size of the pupil opening

Myopia

Nearsightedness; difficulty seeing distant objects when focal point of light rays falls in front of the retina

Hyperopia

Farsightedness; difficulty seeing close objects when focal point of light rays falls behind the retina

Presbyobia

Farsightedness; caused by weakening of ciliary muscles and hardening of lens due to aging

Near point

The distance at which the lens can no longer accommodate enough to bring close objects into focus.

Focal point

The point at which rays parallel to the optical axis reflect and meet

Ciliary Muscles

Muscles which relax or contract and alter the shape of the lens

Cone-based Neurotransmission

On average 6 photoreceptors transmitting to a single ganglion cell. Often one-to-one transmission with individual ganglion cells, particularly in the fovea, resulting in high visual acuity but low sensitivity to low levels of light.

Rod-based Neurotransmission

On average 120 photoreceptors transmitting signals to a single ganglion cell. Results in low visual acuity but high sensitivity to low levels of light.

Accommodation

The process by which the eye's lens changes shape to focus near or far objects on the retina

Blind spot

The point at which the optic nerve leaves the eye. No photoreceptors cells are located there, creating a gap in our vision.

Dark Adaptation Curve

The function relating sensitivity to light to time in the dark, beginning when the lights are extinguished

Light Adaptation

The process whereby the eyes become less sensitive to light in high illumination

Rod Monochromats

A person who has a retina in which the only functioning receptors are rods.

Light Sensitivity

The ability to detect an object in dim light

Ultraviolet Light (UV)

Part of the electromagnetic spectrum that consists of wavelengths that are shorter than those of visible light and longer than those of x-rays

Gamma Rays

Electromagnetic waves with the shortest wavelengths and highest frequencies. Penetrating electromagnetic radiation of a kind arising from the radioactive decay of atomic nuclei.

Radio Waves

Electromagnetic waves with the longest wavelengths and lowest frequencies