📑 Topics on this page (Jump to Topic):
📌 1. Nature of Light & Reflection
- Light is a transverse electromagnetic wave; speed in vacuum 3 × 10⁸ m/s. Sunlight reaches the Earth in about 8 min 20 s (≈500 s).
- Visible light ≈ 400–700 nm (violet shortest, red longest wavelength). Light has dual nature — wave and particle (photon).
- Laws of reflection: angle of incidence = angle of reflection; incident ray, reflected ray and normal lie in one plane.
- Plane mirror: image is virtual, erect, same size, as far behind as the object is in front, and laterally inverted (AMBULANCE is written in reverse). To see a full image the mirror must be at least half the person's height. Two mirrors at angle θ give 360/θ − 1 images.
| Mirror | Image | Uses |
|---|---|---|
| Concave | Real-inverted (mostly); virtual-enlarged when very close | Shaving/make-up mirror, dentist, reflectors of torch and headlights, solar cooker |
| Convex | Always virtual, erect, diminished; wide field of view | Rear-view mirrors of vehicles, road bends, street lamps |
📌 2. Refraction, Lenses & Total Internal Reflection
- Refraction: bending of light on passing from one medium to another (due to change in speed). Refractive index n = c/v; water ≈ 1.33, glass ≈ 1.5, diamond ≈ 2.42.
- Examples: a coin/pool looks shallower, a stick in water looks bent, stars twinkle, the Sun is seen ~2 min before sunrise and ~2 min after sunset.
- Convex lens = converging, concave lens = diverging. Power P = 1/f (m), unit dioptre; convex positive, concave negative.
- Total internal reflection (TIR): from denser to rarer medium, angle of incidence > critical angle. Examples: sparkle of diamond, optical fibres (endoscopy, internet), mirage (illusion of water in deserts and on hot roads).
📌 3. Dispersion, Scattering & Colours
- Dispersion: a prism splits white light into seven colours — VIBGYOR. Violet bends most, red least. A rainbow = refraction + internal reflection + dispersion in raindrops; it appears opposite the Sun.
- Scattering (Rayleigh): shorter wavelengths (blue) scatter more → blue sky; Sun looks red at sunrise/sunset; sky looks black from space/Moon. Danger signals are red — least scattered, seen far away.
- Primary colours: red, green, blue (RGB); red + green = yellow; all three = white. An object looks the colour it reflects — a red rose looks black in green light.
- C.V. Raman — Raman effect on scattering of light (1928), Nobel 1930; 28 February = National Science Day.
📌 4. Human Eye & Optical Instruments
| Defect | What is seen | Cause | Correction |
|---|---|---|---|
| Myopia (short-sight) | Near clear, far blurred | Image forms in front of retina | Concave lens |
| Hypermetropia (long-sight) | Far clear, near blurred | Image forms behind retina | Convex lens |
| Presbyopia | Near blurred with age | Loss of accommodation | Convex / bifocal lens |
| Astigmatism | Horizontal and vertical lines not clear together | Uneven curvature of cornea | Cylindrical lens |
- The eye lens is convex; the image on the retina is real and inverted. Least distance of distinct vision 25 cm. Persistence of vision ≈ 1/16 s — the basis of cinema.
- Cataract — the lens turns cloudy; replaced surgically with an artificial lens. Night blindness — vitamin A deficiency.
- Microscopes and telescopes use convex lenses; a periscope uses two plane mirrors/prisms (submarines). Both camera and eye form real inverted images.
⚠️ Exam Traps:
- A diamond sparkles due to total internal reflection, not ordinary reflection.
- Stars twinkle but planets do not (planets are extended sources).
- A mirage results from refraction and total internal reflection.
📌 5. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Which mirror is used as a rear-view mirror in vehicles? Practice
Correct answer: (D)
A convex mirror gives an erect, small image and a wide field of view.
A convex mirror gives an erect, small image and a wide field of view.
Q2. Myopia is corrected using a: Practice
Correct answer: (D)
A concave (diverging) lens shifts the image back onto the retina.
A concave (diverging) lens shifts the image back onto the retina.
Q3. The sky appears blue due to: Practice
Correct answer: (B)
Blue (short wavelength) scatters the most.
Blue (short wavelength) scatters the most.
Q4. The sparkle of a diamond is mainly due to: Practice
Correct answer: (A)
Diamond has a very high refractive index (2.42) and small critical angle (~24°).
Diamond has a very high refractive index (2.42) and small critical angle (~24°).
Q5. Light from the Sun takes about how long to reach the Earth? Practice
Correct answer: (D)
1.5 × 10¹¹ m ÷ 3 × 10⁸ m/s ≈ 500 s.
1.5 × 10¹¹ m ÷ 3 × 10⁸ m/s ≈ 500 s.
Q6. A rainbow is formed due to: Practice
Correct answer: (A)
Raindrops act like tiny prisms.
Raindrops act like tiny prisms.
Q7. Why are danger signals red? Practice
Correct answer: (C)
Long wavelengths scatter less and are visible far, even in fog.
Long wavelengths scatter less and are visible far, even in fog.
Q8. Optical fibres work on the principle of: Practice
Correct answer: (A)
Light travels by repeated total internal reflection at the fibre walls.
Light travels by repeated total internal reflection at the fibre walls.
Q9. A shaving mirror is a: Practice
Correct answer: (A)
Held close, a concave mirror gives an erect, enlarged image.
Held close, a concave mirror gives an erect, enlarged image.
Q10. A pond looks shallower than it really is because of: Practice
Correct answer: (B)
Rays from water to air bend away from the normal.
Rays from water to air bend away from the normal.
Q11. Which colour bends the most through a prism? Practice
Correct answer: (B)
Violet has the shortest wavelength and highest refractive index.
Violet has the shortest wavelength and highest refractive index.
Q12. A lens of power +2 D has a focal length of: Practice
Correct answer: (D)
f = 1/P = 1/2 m = 50 cm; positive = convex.
f = 1/P = 1/2 m = 50 cm; positive = convex.
Q13. A mirage in a desert is due to: Practice
Correct answer: (C)
Air near the hot ground is rarer; rays bend and are totally reflected.
Air near the hot ground is rarer; rays bend and are totally reflected.
Q14. A red rose seen in pure green light will look: Practice
Correct answer: (C)
The rose absorbs green and there is no red light to reflect.
The rose absorbs green and there is no red light to reflect.
Q15. Two plane mirrors are at 60°. The number of images formed is: Practice
Correct answer: (D)
n = 360/60 − 1 = 5.
n = 360/60 − 1 = 5.
Q16. The image formed on the retina of the human eye is: Practice
Correct answer: (B)
The eye lens is convex; the brain interprets the image as upright.
The eye lens is convex; the brain interprets the image as upright.
Q17. The Raman effect is related to: Practice
Correct answer: (C)
Discovered by C.V. Raman in 1928; Nobel Prize 1930.
Discovered by C.V. Raman in 1928; Nobel Prize 1930.
Q18. The instrument used in submarines to see above water is the: Practice
Correct answer: (B)
Two plane mirrors/prisms at 45° are used.
Two plane mirrors/prisms at 45° are used.
Q19. Stars twinkle because of: Practice
Correct answer: (C)
Air layers of changing density keep shifting the light path.
Air layers of changing density keep shifting the light path.
Q20. Astigmatism is corrected using a: Practice
Correct answer: (A)
A cylindrical lens compensates for uneven corneal curvature.
A cylindrical lens compensates for uneven corneal curvature.