Light – Reflection and Refraction

Physics Formulas & Ray Optics

🪞 Spherical Mirror Formulas

Applies to both Concave and Convex mirrors using the New Cartesian Sign Convention.

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Mirror Formula: $$\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$$

Relation between f and R: $$f = \frac{R}{2}$$

Magnification (m): $$m = \frac{h’}{h} = -\frac{v}{u}$$

🔦 Laws of Refraction

Bending of light when it travels from one medium to another.

Snell’s Law: $$\frac{\sin i}{\sin r} = \text{constant} (n_{21})$$

Refractive Index (Absolute): $$n = \frac{c}{v}$$

($c = 3 \times 10^8 \text{ m/s}$ in vacuum)

[Image of refraction of light through a rectangular glass slab]

🔍 Spherical Lens Formulas

Applies to Convex (Converging) and Concave (Diverging) lenses.

Lens Formula: $$\frac{1}{v} – \frac{1}{u} = \frac{1}{f}$$

Magnification (m): $$m = \frac{h’}{h} = \frac{v}{u}$$

[Image of ray diagrams for convex lens]

🔋 Power of Lens

The degree of convergence or divergence of light rays achieved by a lens.

Formula: $$P = \frac{1}{f \text{ (in meters)}}$$

Unit: Dioptre (D)

Combination of Lenses: $$P_{total} = P_1 + P_2 + P_3 + …$$

📝 Quick Sign Convention Guide

1. u (Object distance): Always negative ($-$).

2. f (Concave Mirror/Lens): Always negative ($-$).

3. f (Convex Mirror/Lens): Always positive ($+$).

4. m (Real Image): Negative ($-$).

5. m (Virtual Image): Positive ($+$).