Eyes
- Our sense organs are part of our nervous system.
- Receptor cells in sense organs detect stimuli.
- Nerves transmit this information to our central nervous system.
- Parts in front of the retina are transparent so light can pass through them. The muscles in the iris can change the size of the pupil, but the iris itself is not transparent.
- Radial and circular muscles work together to regulate how much light can enter the eye.
Bright Light:
Circular muscles contract, and the pupil becomes smaller.
Dim Light:
Radial muscles contract, and the pupil becomes larger.
Rods and Cones:
- The retina contains two types of light receptors that transform light into a nerve impulse that the brain receives.
- Rods detect light intensity.
- Cones detect light wavelength (colour).
- Rods are x1000 more sensitive to light than cones, but cones give much clearer vision.
Colour Blindness:
- A genetic disorder stemming from an error in a single gene results in malfunctioning cone cells.
- Red-green colour blindness is the most prevalent type.
- Sometimes the fault is not actually caused by faulty cone cells, but rather by the pathway from the retina to the brain.
Vision
Monocular and Binocular Vision:
- Humans have binocular vision. This means we see objects with both of our eyes.
- Because our eyes sit side by side, each eye captures a slightly different view. The brain superimposes (compares) and processes the signals from the two eyes to create a single, depth-filled picture. We get a 3D picture and can work out how far something is.
- Most birds and lizards have monoculars. They view objects with one eye on each side of their head.
| Advantages | Disadvantages | |
|---|---|---|
| Monocular vision | The expansive field of view allows you to see what's behind you. | Have poor depth perception and cannot judge distances well. |
| Binocular vision | Can judge distances well | Smaller field of view |
Focusing and Accommodation:
- The light needs to focus on the retina for us to be able to see it clearly.
- The cornea does most of the refraction, and the lens makes fine adjustments.
Looking at a Distant Object:
- When the lens is thick, it refracts light more than when it's thin.
- It changes shape using ciliary muscles and suspensory ligaments.
Distant Object:
- When the ciliary muscles relax, the suspensory ligaments become taut, resulting in a thin lens.
Front view:
Close Object:
- When the ciliary muscles contract, the suspensory ligaments are loose, and the lens is thicker (more curved).
Front view:
Correcting Vision
Contact Lenses:
- Work like glasses, correcting refractive errors.
- We place a thin lens on the surface of the eye.
Soft Lenses:
- A silicone-based gel forms the product's structure.
- The cornea is capable of transmitting oxygen freely.
- More comfortable than rigid lenses.
Hard Lenses:
- More rigid and durable.
- Also gas permeable.
Surgery:
- Laser surgery changes the shape of the cornea.
- The eye usually recovers in a few days.
- With serious refractive error/age-related problems with the lens, an avascular lens is inserted.
- Some modern lenses are multifocal; the patient will regain the ability to see objects at different distances.
