Draw a labelled diagram of cross-section of human eye, and explain the mechanisms of image formation, ability to see in dark and discriminate colour. Add a note on the main defects of human eye and how they can be corrected.

1. Labeled Diagram of the Human Eye

Key Components:

• Cornea: Transparent outer layer.
• Iris: Controls pupil size.
• Pupil: Opening for light entry.
• Lens: Focuses light onto the retina.
• Retina: Contains photoreceptor cells (rods and cones).
• Fovea: Central pit of the retina for sharp vision.
• Optic Nerve: Transmits signals to the brain.
• Vitreous Humor: Gel-like substance filling the eye.
• Aqueous Humor: Fluid between cornea and lens.

2. Mechanisms of Image Formation and Adaptation

Image Formation: Light rays enter the eye and are refracted (bent) by the cornea and lens. The lens adjusts its shape (accommodation) to focus the light precisely onto the retina. This creates a real, inverted, and diminished image on the retina. Photoreceptor cells (rods and cones) convert light energy into electrical signals, which are then transmitted to the brain via the optic nerve for interpretation.

Ability to See in Dark (Dark Adaptation): When transitioning from bright light to darkness, vision is initially poor. This is because the cones are saturated and the rods are not fully activated. Dark adaptation involves several processes:

• Pupil Dilation: The iris widens the pupil to allow more light to enter.
• Rhodopsin Regeneration: Rods contain rhodopsin, a light-sensitive pigment. In darkness, rhodopsin regenerates, increasing the rods' sensitivity to light. This process takes about 30-45 minutes for full adaptation.
• Switch from Cone to Rod Vision: Initially, cone vision dominates, but as rhodopsin regenerates, rod vision becomes more prominent, enabling vision in low light.

3. Color Discrimination

Color vision is mediated by cone cells, which are concentrated in the fovea. There are three types of cones, each sensitive to different wavelengths of light:

• S-cones (Short wavelength): Detect blue light.
• M-cones (Medium wavelength): Detect green light.
• L-cones (Long wavelength): Detect red light.

The brain interprets color based on the relative activation of these three cone types. For example, yellow light stimulates both L and M cones, resulting in the perception of yellow. Color blindness occurs when one or more cone types are deficient or absent.

4. Defects of the Human Eye and Their Correction

Common Defects:

• Myopia (Nearsightedness): Difficulty seeing distant objects clearly. Corrected with concave lenses.
• Hyperopia (Farsightedness): Difficulty seeing near objects clearly. Corrected with convex lenses.
• Astigmatism: Blurred vision due to an irregularly shaped cornea. Corrected with cylindrical lenses.
• Presbyopia: Age-related loss of accommodation, making it difficult to focus on near objects. Corrected with bifocal or progressive lenses.
• Cataracts: Clouding of the lens, leading to blurred vision. Corrected with surgical removal of the clouded lens and replacement with an artificial lens.
• Glaucoma: Damage to the optic nerve, often due to increased intraocular pressure. Managed with eye drops, laser treatment, or surgery.

Corrective Measures: The choice of corrective measure depends on the specific defect and its severity. Refractive surgeries like LASIK (Laser-Assisted In Situ Keratomileusis) can permanently correct myopia, hyperopia, and astigmatism by reshaping the cornea.