10.1.1 Physics of vision

Structure of the Eye:

• The sclera is the tough outer layer, protecting the eye.
• Inside the sclera, the choroid contains dark pigments that absorb stray light, reducing reflections and preventing blurred images.
• The transparent cornea at the front of the sclera is responsible for initial focusing of light.

Focusing Mechanism:

• The lens is controlled by ciliary muscles around the edge of the cornea. These muscles adjust the shape of the lens to focus on objects at different distances—a process called accommodation.
• For distant objects ($5$ m to infinity), ciliary muscles relax, causing the lens to flatten slightly, which is optimal for distant vision.
• For closer objects, the ciliary muscles contract, making the lens more rounded, which shortens its focal length to bring nearby objects into focus.

The Iris and Light Control:

• The iris is a ring of muscle that controls pupil size, regulating the amount of light entering the eye:
  • In dim light, the iris dilates the pupil to let in more light.
  • In bright light, the iris contracts to reduce the amount of light, improving focus and reducing glare.

The Retina and Photoreceptors:

• The retina is the layer at the back of the eye where light is focused. It contains photodetectors—cells sensitive to light—primarily rods and cones.
• Rods are more sensitive to low light levels and provide grayscale vision. They contain rhodopsin, a pigment that allows low-light vision but needs time to regenerate in the dark (dark adaptation).
• Cones respond to higher light levels and detect primary colours (red, green, and blue). They provide detailed, colour vision, especially in bright light.

Fovea and Visual Acuity:

• The fovea is a small depression on the retina directly behind the lens, densely packed with cones. It offers the highest visual acuity, allowing us to see fine details.
• Moving away from the fovea, cone density decreases and rod density increases, making peripheral vision less detailed but more sensitive to low light.

Optical Power and Refracting System:

• The eye's refracting system (cornea and lens) bends light to focus it on the retina. The optical power of the eye is the combined refractive ability of these parts.
• The near point (closest distance for clear focus) is about $25$ cm for a typical eye, while the far point is at infinity.
• As a converging lens system, the eye can focus light from distant and near objects by adjusting the lens shape.

Accommodation and Lens Curvature:

• When viewing objects at different distances:
  • For distant objects, minimal lens curvature is needed.
  • For nearby objects, the lens becomes more curved to decrease focal length and increase optical power, achieved by ciliary muscle contraction.