The Human Eye

In many ways, the human eye is similar to a camera. Light enters through an opening, is focused through a lens, passes through a light-tight (dark) space, and forms a smaller, inverted, real image on a light-sensitive surface at the back.

Parts of the Human Eye Cornea (clear covering) Iris (coloured disc) Pupil Cornea Iris Lens Ciliary Muscle Vitreous Humour Fovea Retina Optic Nerve Sclera

Path of Light into the Eye Cornea – Transparent, outer layer of eye. – Refracts light towards pupil. Pupil – Black hole in the iris. – Allows light to pass through to the lens. Iris – Circular band of muscle and pigment. – Controls the size of the centre hole (contracts or dilates the pupil).

Focusing the Light Lens – Transparent, flexible, crystalline convex lens behind pupil. – Refracts and focuses (converges) light. Ciliary Muscle – Muscle fibres from sclera to lens. – Controls shape of lens to allow lens to focus.

Support Structure of Eye Sclera – White, dense outer layer of eye ball. – For eye muscle attachment, holding eye shape, and to keep inside of eye dark. Vitreous humour – Transparent fluid gel that makes up most of the eye ball. – Holds shape of eye; allows light to pass from lens to retina.

Image Formation in the Eye Retina – Inner lining of light-sensitive cells that covers back of eye; surrounded by network of nerves. – Detects light (of the real image), and transmits this as a signal to the optic nerve. – Photoreceptive cells in the retina are referred to either as “rods” or “cones”. – Rod cells are more sensitive to light (therefore details). – Cone cells detect either red, green, or blue wavelengths of light. (therefore colour vision).

Image Formation in the Eye Fovea – Area of retina which contains the highest concentration of photoreceptive cells. – Responsible for sharpest image formation. (hey, HD!) Optic Nerve – Bundle of all the nerves from the retina, located at the back of the eye ball. – Transmits all the optic signals to the brain for image processing.

Accommodation The distance between the lens and the image (d i ) is constant in the eye. However, the eye can focus on objects far away or very close to the eye. How is this possible?

Accommodation Accommodation: the ability of the eye to change the focus between objects at different distances by altering the curvature of the lens. What changes the shape of the lens in our eyes?

Accommodation When the ciliary muscles contract, the lens is stretched so that less refraction takes place. This allows us to see objects that are far. When the ciliary muscles relax, the lens thickens so that more refraction takes place. This allows us to see objects that are near. Note: This is UNLIKE a camera, where the lens is moved forward or backward to focus.

Accommodation Not Perfect Either by heredity or age, accommodation of the human eye can be flawed. In general, people can either suffer from Myopia (near-sightedness) or Hyperopia (far-sightedness). People can also suffer from Astigmatism or develop Presbyopia.