External Structures of an Eye:

• Tear gland:

  • Secretes tears which lubricate the eye, nourish the cornea and keeps it free from dust.

• Eyelid:

  • Squinting is the partial closure of eyelids. This prevents excess light from entering the eye and damaging the light-sensitive tissues

  • Blinking spreads tears over the cornea and conjunctiva and wipes dust particles off the cornea

• Eyelashes:

  • Shield eyes from dust particles.

• Iris:

  • Contain radial and circular muscles that control the size of pupil

  • Pigment of iris gives the colour of eyes

• Pupil:

  • Allows light to enter the eye

• Sclera:

  • Tough white outer layer of connective tissue

  • Continuous with cornea

• Conjunctiva:

  • Thin, transparent mucous membrane that covers the sclera

  • Secretes mucous to lubricate the eye

• Cornea:

  • Transparent refractive layer covering the iris and pupil.

  • continuous with the sclera.

Internal Structure of Eye:

• Choroid:

  • Middle layer of the eyeball, between the sclera and retina.

  • Contains blood vessels that supply oxygen and nutrients, and remove metabolic waste products.

  • Pigmented black to prevent an internal reflection of light.

• Retina:

  • Innermost layer of the eyeball which contains photoreceptors, which are connected to the optic nerve.

• Lens:

  • Transparent biconvex structure that refracts light onto the retina.

  • The lens can change its curvature to focus light onto retina

• Fovea:

  • Also called yellow spot, a small depression in the retina where images are usually focused

  • The fovea contains the greatest concentration of cones, but no rods.

• Ciliary body:

  • Contains ciliary muscles which control the curvature of the lens.

  • It is also responsible for producing aqueous humour.

• Suspensory ligament:

  • Connects the ciliary body to the lens

• Aqueous chamber:

  • The space between the lens and the cornea.

  • Transparent aqueous humour keeps the front of the eyeball firm and helps to refract light into the pupil.

• Vitreous chamber:

  • The space behind lens

  • Transparent vitreous humour keeps the eyeball firm and helps to refract light onto the retina.

• Optic nerve:

  • Transmits signal from the retina to the brain.

  • There are no photoreceptors in the area of the retina where the optic nerve leaves. This area is called the blind spot

Photoreceptors:

Photoreceptors in the retina consist of rods and cones. The photoreceptors are connected to the nerve endings from the optic nerve.

CONES:

• Cones enable us to see colours in bright light.

• There are three types of cones, red, blue, and green (RBG) that allow us to see a wide variety of colours by containing different pigment which absorbs light of different wavelengths.

• Cones do not work well in dim light.

RODS:

• Rods enable us to see in dim light, but only in black and white.

• Rods are sensitive to light of low intensity as they contain pigment called visual purple. When the eye is exposed to bright light, all the visual purple is bleached.

• Visual purple must be reformed for a person to see in the dark. Therefore, it takes awhile for one to see in dark after being in a bright environment as time is taken for visual purple to reform.

• Formation of visual purple requires vitamin A.

How we see

• When light falls on an object, light rays are reflected from the object

• Light rays are refracted through the cornea and the aqueous humour onto the lens

• The lens causes further refraction and the rays are brought to a focus on the retina.

• The image on the retina stimulates the photoreceptors, either the rods or the cones, depending on the intensity of the light.

• Nerve impulses are produced and sent to the brain via optic nerve. The brain interprets the impulses and the person sees the object.

Accommodation:

A reflex action where the lens is adjusted so that clear images of objects at different distances are formed on the retina.

Focusing on a distant object:

• Light rays reflected off distant objects are nearly parallel and enter the eyes

• Ciliary muscles relax, causing suspensory ligaments to tighten

• Suspensory ligaments become taut, pulling on the edge of the lens.

• Lens becomes thinner and less convex, increasing its focal length, causing less refraction of the rays of light

• Light rays from the distant object are sharply focused on the retina.

Focusing on a close object:

• Light rays reflected off close objects are diverging and enter the eyes

• Ciliary muscles contract, causing suspensory ligaments to become relax

• Suspensory ligaments slacken, relaxing their pull on the lens.

• The lens becomes thicker and more convex, decreasing its focal length, causing more refraction of the rays of light,

• Light rays from the near object are sharply focused on the retina.

Pupil Reflex:

• Reflex action (involuntary) where the pupil changes size in response to changes in light intensity.

• In low light intensity, pupils dilate allow more light to enter the eye for better vision.

• In high light intensity, pupils contract to restrict light to enter to prevent excessive light from damaging the retina.

• In dim light:

  • Radial muscles of the iris contract

  • Circular muscles of the iris relax

  • The pupil enlarges or dilates, increasing the amount of light entering the eye.

• In bright light:

  • Circular muscles of the iris contract

  • Radial muscles of the iris relax

  • The pupil becomes smaller or constricts, reducing the amount of light entering the eye.