The Eye

Cross-section of an eye

muscle

optic nerve

aqueous humour

lens

pupil

iris

cornea

ciliary muscle

vitreous humour

blind spot

retinasclera

• Good eyesight requires precise focusing of light rays onto the retina. The entire eye is a focusing system that involves the cornea, the lens, and the retina.

Parts of the eye• Cornea:

– Outer Layer of the Eye– Made of living cells that are completely clear– The light rays arrive at your eye & are refracted– Directs the light into pupil, so an image can be produced

– When the muscles contract, the lens becomes more spherical & thicker to focus on up-close objects

– When the muscles relax, the lens becomes less spherical & thinner to focus on far-away objects.

• Lens– Your eye includes a convex lens– The lens can adjust its focal length depending on the distance

of the object being examined– The lens is attached to muscles inside the eye that can

change shape

• Retina– Acts as a projector screen for the light entering your eye

– The light has already been focused by the lens & cornea

– The image formed is inverted, but your brain interprets the image as being right side up

Changing the shape of the lens

• The eye has a convex lens, which collects light and directs it to a focal point.

• The convex lens is able to change its focal length.

• Muscles supporting the lens contract or relax allowing the lens to change shape.

• When the muscles contract the lens becomes thicker and more spherical. Greater refraction of light (a) FOCUS ON NEAR OBJECTS

• When the muscles relax, the lens is pulled into a flatter and

thinner shape. Less refraction of light (b) FOCUS ON DISTANT OBJECTS

• Most eye problems fall into one or more categories: far-sightedness, near-sightedness, and astigmatism.

• Far-sightedness: people can see distant objects, but they cannot see nearby objects.

• The eye cannot make the lens thick enough to refract diverging light rays from nearby objects correctly on the retina.

Use a convex lens to correct far-sightedness.

• Near-sightedness: people can see nearby objects clearly but cannot see distant objects clearly.

• The light that enters the eye from the object is refracted too much. This happens because the eye cannot make the lens thin enough.

Use a concave lens to correct near-sightedness.

• Astigmatism: is a common condition in which the eye is unable to form a clear image because of an irregular shape of the cornea or lens.

• Astigmatism

Take care of your eyes!