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Physics · Class 10 · Chapter 14: Refraction of Light and Optical Instruments

8 min

Key points

Refraction is the bending of light when it passes from one medium to another of different optical density
Laws of Refraction: (1) The incident ray, refracted ray and normal all lie in the same plane (2) Snell's Law: sin i / sin r = constant
Refractive index is the ratio of speed of light in vacuum to speed of light in that medium (μ = c/v)
Critical angle is the angle of incidence in denser medium for which angle of refraction in rarer medium is 90°
Total internal reflection occurs when light travels from denser to rarer medium at angle greater than critical angle
Simple microscope (magnifying glass) uses a single convex lens to form virtual, erect and magnified image
Compound microscope uses two convex lenses - objective lens forms real inverted image, eyepiece acts as simple microscope
Telescope is used to see distant objects - objective lens forms real diminished image at focus of eyepiece
Power of lens P = 1/f where f is focal length in meters, unit is dioptre (D)

Definitions

Refraction: Refraction is the bending of light when it passes from one medium to another of different optical density
Refractive Index: The refractive index of a material is the ratio of speed of light in vacuum to the speed of light in that material
Critical Angle: The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°
Total Internal Reflection: Total internal reflection is the phenomenon in which light is completely reflected back into the denser medium when it strikes the boundary at an angle greater than the critical angle
Power of Lens: The power of a lens is the reciprocal of its focal length measured in meters

Worked example

Define refraction of light. State Snell's law and write its mathematical form. Calculate the refractive index of glass if light travels from air to glass at incident angle 60° and refracted angle 35°. (6 marks)

Refraction is the bending of light when it passes from one transparent medium to another having different optical density. Snell's law states that the ratio of sine of angle of incidence to sine of angle of refraction is constant for given pair of media. Mathematical form: n₁sin θ₁ = n₂sin θ₂. Given: θ₁ = 60°, θ₂ = 35°, n₁ = 1 (air). Using formula: 1 × sin 60° = n₂ × sin 35°, therefore n₂ = sin 60°/sin 35° = 0.866/0.574 = 1.51

Common mistakes

Students confuse angle of incidence with angle of refraction in Snell's law calculations
Always convert focal length to meters before calculating power of lens
Students forget that for total internal reflection, light must travel from denser to rarer medium
In ray diagrams, students often draw refracted ray bending wrong direction
Students mix up the functions of objective and eyepiece in compound microscope

Quick recap

Refraction is bending of light between different media following Snell's law (n₁sin θ₁ = n₂sin θ₂). Total internal reflection occurs from denser to rarer medium when incident angle exceeds critical angle. Simple microscope uses single convex lens for magnification (M = D/f). Compound microscope uses objective and eyepiece lenses for higher magnification.

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