Human Eye
Physics · मानव नेत्र
📋Quick Overview
The human eye works like a camera. Light enters through the cornea, passes through the pupil, is focused by the lens onto the retina, and the optic nerve sends signals to the brain. The eye can adjust its focal length to see near and far objects — this ability is called 'power of accommodation'. Exams love questions on eye defects and their corrections!
📖Parts of the Human Eye
| Part | Function |
|---|---|
| Cornea | Transparent front covering — bends (refracts) most of the light entering the eye |
| Iris | Colored part (brown/blue) — controls size of pupil (amount of light entering) |
| Pupil | Black hole in center — opens wide in dark, becomes small in bright light |
| Lens | Flexible, convex lens — changes shape to focus on near/far objects (accommodation) |
| Retina | Screen at the back — contains rod cells (dim light/black-white) and cone cells (bright light/color) |
| Optic Nerve | Carries image signals from retina to brain |
| Ciliary Muscles | Change the shape (curvature) of lens for focusing |
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Near point = closest distance at which eye can see clearly = 25 cm (for normal eye). Far point = farthest distance = infinity (∞). Image on retina is REAL, INVERTED, and DIMINISHED — brain flips it!
📖Vision Defects & Corrections
| Defect | Problem | Image Forms | Corrected By | Lens Used |
|---|---|---|---|---|
| Myopia (Near-sightedness) | Can see NEAR clearly, FAR objects blurry | Before retina (too early) | Concave (diverging) lens | Minus (-) power |
| Hypermetropia (Far-sightedness) | Can see FAR clearly, NEAR objects blurry | Behind retina (too late) | Convex (converging) lens | Plus (+) power |
| Astigmatism | Blurry vision at ALL distances (uneven cornea curvature) | Distorted image | Cylindrical lens | Toric lens |
| Presbyopia | Old-age defect — lens becomes stiff, can't focus on near objects | Near objects blurry | Bifocal lens (both convex + concave) | Progressive lens |
📝Color Blindness & Other Facts
- •Color blindness: Cannot distinguish certain colors (usually red-green). Genetic defect, more common in males. Discovered by John Dalton.
- •Persistence of vision: Image stays on retina for 1/16th of a second. This is why movies (24 fps) look smooth.
- •Night blindness: Cannot see in dim light — caused by deficiency of Vitamin A
- •Cataract: Lens becomes cloudy/opaque → treated by surgery (lens replacement)
- •Power of lens (in Diopters) = 1 / focal length (in meters). D = 1/f