Overview
This lecture covers the topic of refraction of light at plane surfaces, focusing on its definition, behavior through different media (including glass slabs), relevant laws, refractive index concepts, and includes solved numerical examples.
Refraction of Light: Introduction
- Refraction is the bending of light as it passes from one transparent medium to another.
- It occurs due to a change in the speed of light in different media.
- Refraction is different from reflection, where the light bounces back in the same medium.
Behavior of Light During Refraction
- When light travels from a rarer (less dense) medium to a denser medium, it bends towards the normal.
- When light travels from a denser to a rarer medium, it bends away from the normal.
- At normal incidence (light entering perpendicular), there is no deviation; light passes straight.
Angles and Deviation in Refraction
- Angle of incidence (i): between incident ray and normal.
- Angle of refraction (r): between refracted ray and normal.
- Angle of deviation (δ): δ = i – r (rarer to denser), δ = r – i (denser to rarer).
Laws of Refraction (Snell's Laws)
- First Law: Incident ray, refracted ray, and normal all lie in the same plane.
- Second Law: The ratio of sine of angle of incidence to sine of angle of refraction is a constant (refractive index, μ).
Refractive Index
- Absolute refractive index: Ratio of speed of light in vacuum to speed in the medium (μ = c/v).
- Relative refractive index: Ratio of speed of light in two different media.
- Refractive index is always greater than 1 and has no units.
Effects of Refraction on Light Properties
- Speed of light decreases in denser media and increases in rarer media.
- Frequency of light remains constant; wavelength changes with medium.
- Relation: v = fλ, so λ ∝ v for constant frequency.
Factors Affecting Refractive Index
- Nature of medium (density and velocity of light).
- Physical conditions like temperature (increased temperature decreases refractive index).
- Wavelength/color of light (refractive index is highest for violet, lowest for red).
Principle of Reversibility of Light
- Light follows the same path if its direction is reversed.
Refraction Through a Rectangular Glass Slab
- Light bends towards normal (air to glass), then away (glass to air).
- Emergent ray is parallel to incident ray; the perpendicular distance between them is lateral displacement.
Lateral Displacement
- Lateral displacement increases with thickness of the block, angle of incidence, and refractive index.
- It is maximum for light with the shortest wavelength (violet).
Sample Numerical Problems
- Refractive index calculations using μ = c/v.
- Determining speed, frequency, and wavelength changes as light passes through different media.
- Application of Snell's law and wavelength relation in different scenarios.
Key Terms & Definitions
- Refraction — bending of light when passing between transparent media due to speed change.
- Normal — line perpendicular to the boundary surface at the point of incidence.
- Angle of incidence (i) — angle between incident ray and normal.
- Angle of refraction (r) — angle between refracted ray and normal.
- Refractive index (μ) — ratio of speed of light in vacuum to that in a medium.
- Lateral displacement — perpendicular distance between the incident ray (extended) and emergent ray.
Action Items / Next Steps
- Review textbook exercises on refraction and refractive index problems.
- Practice drawing ray diagrams for refraction at plane surfaces and through glass slabs.
- Prepare for the next lesson on advanced refraction phenomena.