Reflection and Refraction of Light
NIOS · Class 12 · Physics
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Reflection of Light – Laws and Spherical Mirrors
- Law 1: The incident ray, reflected ray, and the normal at the point of incidence are all in the same plane.
- Law 2: Angle of incidence (∠i) = Angle of reflection (∠r). This holds for both plane and spherical mirrors.
- A spherical mirror is a part of a hollow sphere. If the inner (concave) surface is polished, it is a concave mirror. If the outer (convex) surface is polished, it is a convex mirror.
Refraction of Light – Snell's Law and Refractive Index
- Refraction is the bending of light when it travels from one medium to another due to a change in its speed.
- When light travels from a rarer medium (like air) to a denser medium (like glass), it bends TOWARDS the normal. When it travels from denser to rarer, it bends AWAY from the normal.
- Snell's Law (Law of Refraction): sin i / sin r = μ₁₂, a constant for a given pair of media and a given colour of light.
Total Internal Reflection (TIR) and Its Applications
- Total Internal Reflection (TIR) occurs when light travelling from a denser medium to a rarer medium is incident at an angle GREATER than the critical angle.
- Two essential conditions for TIR: (1) Light must travel from an optically denser medium to a rarer medium. (2) The angle of incidence must be greater than the critical angle (iₐ).
- Critical angle (iₐ): The angle of incidence in the denser medium for which the angle of refraction in the rarer medium is exactly 90°.
Refraction at a Spherical Surface
- When light passes from one medium (refractive index μ₁) to another (refractive index μ₂) through a curved (spherical) interface, the image position can be found using a derived formula.
- Sign convention is the same as for mirrors: distances measured from the pole P in the direction of incident light are positive; opposite direction is negative.
- The derivation uses the fact that for small apertures (paraxial approximation), sin i ≈ i and sin r ≈ r.
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