ICSE Class 10 Physics: Light — Refraction & Lenses Notes 2026

Refraction of Light

• Bending of light at the interface of two media due to change in speed
• Snell's law: n₁ sin θ₁ = n₂ sin θ₂; refractive index n = c/v
• Conditions for no bending: normal incidence (θ=0)

Refractive Index and its Significance

• Optical density vs. physical density; higher n = slower light
• Relative refractive index: ₁n₂ = n₂/n₁
• Critical angle and total internal reflection: sin C = 1/n

Refraction Through a Glass Slab

• Emergent ray parallel to incident ray; lateral displacement d = t·sin(i−r)/cos r
• Applications: optical fibres, diamond sparkle, mirages
• Why objects in water appear shallower: real depth vs. apparent depth

Lenses — Types and Terminology

• Convex (converging) lens: thicker at centre; bends light inward
• Concave (diverging) lens: thicker at edges; bends light outward
• Principal axis, optical centre O, principal focus F, focal length f

Image Formation by Lenses — Ray Diagrams

• Three standard rays: parallel to axis → through F; through O → straight; through F → parallel
• Convex lens image positions for object at infinity, 2F, between F and 2F, at F, inside F
• Concave lens always forms virtual, erect, diminished image on same side as object

Lens Formula and Magnification

• Lens formula: 1/v − 1/u = 1/f; sign convention (Cartesian)
• Magnification m = v/u = h'/h; positive m means erect image
• Power of lens P = 1/f (in metres); unit dioptre (D); convex lens P>0

Applications of Lenses

• Human eye: convex lens focuses on retina; near point 25 cm
• Myopia corrected by concave lens; hypermetropia by convex lens
• Microscope and telescope: ray diagrams for ICSE Level