👁️10.1 The Human Eye
| Part | Function |
|---|---|
| Cornea | Acts like the outer protective glass of a camera. First surface that bends light entering the eye. |
| Aqueous Humour | Liquid between cornea and lens. Maintains pressure and helps focus light. |
| Pupil | Adjustable hole that controls how much light enters (like a camera aperture). |
| Iris | Colored muscle controlling pupil size based on brightness. |
| Lens | Transparent, convex, and flexible. Focuses light onto retina just like a camera lens. |
| Ciliary Muscles | These muscles pull or relax the lens to adjust focus depending on object distance. |
| Vitreous Humour | Jelly-like substance filling the large rear part of the eye, keeps the shape intact. |
| Retina | The light-sensitive screen at the back. Forms the image using rods (black & white) and cones (color). |
| Optic Nerve | Sends the image to the brain in the form of electrical signals. Brain then interprets it. |
🔄 10.1.1 Power of Accommodation
- The eye lens can change shape to focus on near or far objects.
- This is done by the ciliary muscles:
- For distant objects: Muscles relax, lens becomes thin.
- For near objects: Muscles contract, lens becomes thicker.
🔹 Near Point (Minimum distance for clear vision): 25 cm
🔹 Far Point (Maximum distance): Infinity
📌 Accommodation = Eye’s ability to adjust the lens for focusing.
⚠️ Eye Problems & Corrections
1. Myopia (Near-sightedness)
- Can see near, but not far
- Eye is too long or lens is too powerful
- Image forms in front of retina
✅ Corrected with concave lens (diverges rays)
2. Hypermetropia (Far-sightedness)
- Can see far, but not near
- Eye is too short or lens is too weak
- Image forms behind retina
✅ Corrected with convex lens (converges rays)
3. Presbyopia
- Happens with age, due to weak ciliary muscles
- Can’t see near (sometimes even far)
✅ Bifocal lenses (top = distance vision, bottom = reading)
4. Cataract
- Lens becomes cloudy, light can’t pass properly
✅ Corrected by surgery, artificial lens replacement
🧠 Trick to remember:
My Near = Myopia → Near is clear
Hyper Far = Hypermetropia → Far is clear
🌈 10.3 Dispersion of Light
🧪 Newton’s Prism Experiment:
- White light through prism → splits into 7 colours (VIBGYOR)
- Violet bends most, red bends least
📌 Dispersion = Splitting of white light
📌 Recombination = Merging back into white light using a second prism
🧠 Memory Tip:
VIBGYOR = Violet, Indigo, Blue, Green, Yellow, Orange, Red
Very Intelligent Boys Grow Young Over Rainbows
🌦️ 10.4 Rainbow Formation
Rainbows form due to:
- Refraction (light enters rain drop and bends)
- Dispersion (white light splits inside the drop)
- Internal Reflection (light reflects inside the drop)
- Refraction again (light exits and bends again)
👉 Rainbow is visible when:
- The Sun is behind you
- Rain is in front
🌀 Rainbow always forms opposite the Sun’s direction.
🌌 10.5 Atmospheric Refraction
⭐ Why Do Stars Twinkle?
- Light from stars bends due to air layers with varying temperatures
- This bending changes continuously → Star looks like it’s twinkling
🎯 Also makes the star appear slightly higher than its actual position.
☀️ Sunrise & Sunset
- Sun appears 2 minutes early at sunrise and 2 minutes late at sunset
- Because of atmospheric refraction, we see the Sun even when it’s below the horizon
📌 Total Extra Daylight = 4 minutes
💡 10.6 Scattering of Light
🌌 Tyndall Effect:
- Scattering of light by tiny particles in a medium
- Makes light beams visible (like torchlight in fog)
🔵 Why is the Sky Blue?
- Blue light (short wavelength) is scattered more by air molecules
👉 That’s why we see blue all around
🔴 Why are Sunsets Red?
- During sunrise/sunset, light travels a longer path through the atmosphere
- Blue gets scattered away, and only red/orange light reaches your eyes
🧠 Memory Tricks & Keywords
| Term | Meaning | Keyword |
|---|---|---|
| Accommodation | Focus adjustment | Lens Flexibility |
| Myopia | Near-sighted | Concave lens |
| Hypermetropia | Far-sighted | Convex lens |
| Dispersion | Splitting of light | Prism |
| Refraction | Bending of light | Atmosphere |
| Scattering | Random redirection | Sky colour |
| Tyndall Effect | Visible scattering | Dust beam |
| Recombination | Merge back to white | Second prism |
🧡 Real-Life Applications
- Rear-view mirror → Convex lens: wider field of view
- Sunglasses → Polarisation: reduce glare
- Contact lenses → Artificial lenses: correct vision
- Optical fibers → Use total internal reflection to transmit light signals in telecom