⚡ PHYSICS · CLASS 10

Light — Reflection & Refraction

Mirror formula, lens formula and Snell's law — animated ray diagrams

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🔦 Reflection of Light

When light hits a surface and bounces back, that's reflection.

  • Angle of incidence (i) — angle between incident ray and normal
  • Angle of reflection (r) — angle between reflected ray and normal
  • Law 1: Incident ray, reflected ray and normal are in the same plane
  • Law 2: Angle of incidence = Angle of reflection (∠i = ∠r)
Mirror Formula 1/v + 1/u = 1/f    Magnification: m = −v/u
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🪞 Animated Ray Diagram — Concave Mirror

Watch the incident ray hit the mirror and reflect. The image forms at the intersection of reflected rays!

For concave mirror: f > 0  |  For convex mirror: f < 0
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💧 Refraction of Light

When light travels from one medium to another, it bends. This is refraction.

🔵 Snell's Law

The ratio of sin of angles equals the ratio of refractive indices.

n₁ sin θ₁ = n₂ sin θ₂

🔷 Refractive Index

n = speed of light in vacuum / speed in medium

n = c/v

🌊 Bending Rule

Light bends towards normal when going into denser medium (water, glass).

n_glass ≈ 1.5

🌈 Dispersion

White light splits into colours (VIBGYOR) when passing through a prism.

n_violet > n_red
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🔦 Animated Refraction at a Surface

Watch light bend as it crosses from air into glass — it slows down and bends towards the normal!

Lens Formula: 1/v − 1/u = 1/f    Power: P = 1/f (dioptre)
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✏️ Solved Example

Q: An object is placed 20 cm from a concave mirror of focal length 15 cm. Find the image distance and magnification.

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Given

u = −20 cm (object in front of mirror), f = −15 cm (concave mirror)

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Apply mirror formula

1/v = 1/f − 1/u = 1/(−15) − 1/(−20) = −1/15 + 1/20

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Solve for v

1/v = (−4 + 3)/60 = −1/60 → v = −60 cm (real image, same side)

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Magnification

m = −v/u = −(−60)/(−20) = −3 (inverted, 3× enlarged)

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🧠 Quick Quiz

Q: A convex lens has focal length 10 cm. What is its power?

Optics mastered! All 8 lessons complete! 🎉✨

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