Structure and Function of the Eye

Photoreceptors: Rods and Cones

• Have you ever noticed that you cannot see colours well in the dark? This is due to the specific light-detecting cells at the back of your eye.
• The retina contains millions of specialised cells called photoreceptors that convert light energy into electrical nerve impulses.
• Rod cells are highly sensitive photoreceptors used for vision in dim light. They provide monochromatic (black-and-white) vision and have low visual acuity (sharpness).
• Cone cells require bright light to function (low sensitivity) but provide high visual acuity and colour vision. There are three types of cones: red, green, and blue.
• Cones are highly concentrated in the fovea (the point of sharpest vision on the retina), whereas the fovea contains no rods.

The Pupil Reflex: Control of Light Intensity

• When you step out of a dark cinema into bright sunlight, your eyes physically react to protect themselves from damage.
• The iris is a coloured ring of tissue containing two sets of antagonistic muscles: circular muscles (ring-like) and radial muscles (spoke-like).
• These muscles control the size of the pupil, the hole in the centre of the iris that regulates light entry.
• In bright light, the circular muscles contract and the radial muscles relax. This causes the pupil to constrict (become smaller) to protect the retina from damage.
• In dim light, the radial muscles contract and the circular muscles relax. This causes the pupil to dilate (become wider) to maximise light entry.

Light Condition	Circular Muscles	Radial Muscles	Pupil Size
Bright	Contract	Relax	Constricts
Dim	Relax	Contract	Dilates

Accommodation: Focusing on Near and Distant Objects

• The lenses in your eyes change shape dynamically, unlike the rigid glass lenses in a camera.
• Accommodation is the process of changing the shape of the elastic lens to "fine-tune" the focus of light onto the retina.
• It is controlled by a ring of ciliary muscles surrounding the lens, which are connected to the lens by fibrous strands called suspensory ligaments.
• Focusing on a near object: The ciliary muscles contract (moving inwards towards the lens), which causes the suspensory ligaments to slacken. The lens becomes thicker and more curved, refracting light more strongly.
• Focusing on a distant object: The ciliary muscles relax (moving away from the lens), which pulls the suspensory ligaments tight. The lens is pulled thin and less curved, refracting light only slightly.

Vision Defects and Correction

• Why do so many people need glasses to see clearly? Often, the shape of the eyeball or the cornea prevents light from focusing exactly on the retina.
• Myopia (short-sightedness) occurs when light focuses in front of the retina. This happens if the eyeball is too long or the cornea is too curved (too much refractive power).
• Hyperopia (long-sightedness) occurs when light focuses behind the retina. This happens if the eyeball is too short or the cornea is too flat (too little refractive power).
• Myopia is corrected using a concave lens, which diverges (spreads out) light rays before they enter the eye, moving the focal point further back.
• Hyperopia is corrected using a convex lens, which converges (brings together) light rays, pre-focusing the light so it lands further forward on the retina.
• Other treatments include contact lenses (thin lenses sitting directly on the cornea), laser surgery (permanently reshaping the cornea: slimming it down to reduce refractive power for myopia, or reshaping to increase curvature for hyperopia), or artificial lens replacement (replacing the natural lens with plastic, often used to treat a cataract).
• Note that soft contact lenses carry a higher risk of eye infections than hard lenses if hygiene is poor.
• The magnification of an object on the retina can be calculated using:

$$\text{Magnification} = \frac{\text{Size of Image (on retina)}}{\text{Size of Real Object}}$$