Overview
This lecture explains the concept of refraction of light, covering its causes, rules, laws (Snell’s Law), and effects such as lateral displacement and situations involving glass slabs.
Introduction to Refraction
- Refraction occurs when light passes from one transparent medium to another, causing it to bend.
- Everyday examples: a pencil appears bent in water, swimming pools look shallower, and arrows appear reversed through a glass of water.
- Eyeglasses work based on the principle of refraction.
Rectilinear Propagation & Reflection Review
- Light travels in a straight line (rectilinear propagation) only when moving through a single transparent medium.
- Reflection is the bouncing back of light from a shiny, opaque surface like a mirror.
- Law of reflection: angle of incidence (i) = angle of reflection (r).
Refraction of Light: Key Concepts
- Refraction is the bending of light when it crosses the boundary between two transparent media.
- The bending occurs due to a change in the speed of light between different media.
- Light speeds: 3 × 10⁸ m/s in air, 2 × 10⁸ m/s in glass (example values).
Direction of Bending
- From rarer (less optically dense, e.g., air) to denser medium (e.g., glass), light bends towards the normal.
- From denser to rarer medium, light bends away from the normal.
Laws of Refraction (Snell’s Law)
- First Law: Incident ray, refracted ray, and the normal at the interface all lie in the same plane (the paper/screen).
- Second Law (Snell’s Law): For a given pair of media, sin(i) / sin(r) = constant (the refractive index).
- This refractive index depends on the pair of media involved.
Refractive Index
- Defined for light traveling from medium 1 to medium 2 as n₂₁ = v₁/v₂ = sin(i)/sin(r), where v is the speed of light in each medium.
- Example: For air to glass, the refractive index is 1.5.
Refraction Through a Glass Slab
- When light enters a glass slab (air → glass → air), it bends towards the normal and then away, emerging parallel to the incident ray.
- Angle of incidence = angle of emergence.
- The emergent ray is shifted sideways (lateral displacement), not bent.
Special Cases
- At normal incidence (light hits perpendicular to the surface), no bending occurs but speed changes.
- There is partial reflection at the boundary, but it is usually minor compared to refraction.
Key Terms & Definitions
- Refraction — Bending of light as it passes from one medium to another due to a change in speed.
- Rectilinear propagation — Light travels in a straight line in a single medium.
- Reflection — Bouncing back of light from a surface.
- Angle of incidence (i) — Angle between the incident ray and the normal.
- Angle of refraction (r) — Angle between the refracted ray and the normal.
- Refractive index (n) — Ratio of speed of light in two media; measures how much refraction occurs.
- Emergent ray — The ray that emerges from the second boundary of a slab.
- Lateral displacement — Sideways shift of the emergent ray relative to the incident ray.
- Normal incidence — Light striking the surface at 90°, causing no bending.
Action Items / Next Steps
- Answer and comment on: When light travels from glass to water, does it bend towards or away from the normal?
- Review and practice drawing refraction diagrams and applying Snell’s Law.