Refraction of LightCambridge IGCSE Physics: Revision notes

Refraction is the bending of light as it passes from one transparent medium to another. To describe refraction accurately, we use three essential terms:

• Normal: An imaginary line drawn perpendicular (at 90°) to the surface where light enters or leaves a medium.
• Angle of incidence (i): The angle between the incident ray and the normal.
• Angle of refraction (r): The angle between the refracted ray and the normal.

Both angles are always measured from the normal, not from the surface itself. Light bends towards the normal when entering a denser medium (e.g. air to glass) and away from the normal when entering a less dense medium (e.g. glass to air).

Refraction can be observed using transparent blocks of different shapes. A typical experiment uses a rectangular or semi-circular glass block:

1. Set up a light ray hitting the flat surface of the block at an angle (not perpendicular).
2. Trace the incident ray, the refracted ray inside the block, and the emergent ray leaving the block.
3. Mark the point where the ray enters the block and measure the angle of incidence and angle of refraction using the normal.
4. Use a semi-circular block to observe refraction when light enters at the curved surface (perpendicular incidence) and then exits at the flat surface at an angle.
5. Repeat with different angles of incidence and record all angles and observations.

Key observations:

• Light bends towards the normal when entering glass from air (denser medium).
• Light bends away from the normal when leaving glass into air (less dense medium).
• The incident ray, refracted ray, and normal all lie in the same plane.
• For a rectangular block, the emergent ray is parallel to the incident ray but laterally displaced.

Refractive index (n) is a measure of how much a material slows down light compared to its speed in a vacuum (or air).

Definition: Refractive index is the ratio of the speed of light in one medium to the speed of light in another medium:

n = c / v

Where:

• c = speed of light in vacuum (3 × 10⁸ m/s)
• v = speed of light in the medium

Alternatively, using Snell's Law:

n = sin i / sin r

Where:

• i = angle of incidence
• r = angle of refraction

Key facts about refractive index:

• Glass typically has n ≈ 1.5; water has n ≈ 1.33; air has n ≈ 1.0
• A higher refractive index means light travels more slowly in that medium and bends more.
• Refractive index is always greater than 1 for any material denser than air.
• For a given pair of media, n is the same regardless of the angle of incidence (within the visible spectrum).

Critical angle (c) is the angle of incidence at which the refracted ray grazes along the surface of the boundary (refraction angle = 90°). For any angle of incidence greater than the critical angle, total internal reflection occurs.

Total internal reflection is the phenomenon where light is completely reflected back into the first medium with no light escaping into the second medium. This can only occur when light travels from a denser medium (larger n) to a less dense medium (smaller n).

Calculating critical angle:

n = 1 / sin c

Rearranged: sin c = 1 / n

Where:

• c = critical angle (in degrees)
• n = refractive index of the denser medium

Key conditions for total internal reflection:

• Light must be travelling from the optically denser medium to the optically less dense medium.
• The angle of incidence must be equal to or greater than the critical angle.
• When the angle equals the critical angle exactly, the refracted ray emerges along the surface (r = 90°).
• When the angle exceeds the critical angle, all light is reflected; none is refracted.

Everyday examples:

• Diamonds sparkle because their high refractive index (n ≈ 2.42) creates a very small critical angle, so light reflects internally multiple times.
• Looking at water from underneath: if you look at a shallow angle upwards from underwater, you may see a mirror-like reflection instead of the world above.
• The silvered back of a mirror uses total internal reflection principles.

Optical fibres are thin strands of glass or plastic that transmit light signals over long distances. They exploit total internal reflection to keep light trapped within the fibre.

Structure and principle:

• An optical fibre consists of a core (high refractive index) surrounded by a cladding (lower refractive index).
• Light enters at one end of the fibre at a shallow angle relative to the fibre's length.
• The light repeatedly undergoes total internal reflection at the core-cladding boundary.
• This keeps the light trapped and travelling along the fibre with minimal loss.

Advantages in telecommunications:

• High bandwidth: Can carry vast amounts of data as pulses of light.
• Low signal loss: Total internal reflection means very little light is absorbed or scattered.
• Long-distance transmission: Signals can travel many kilometres without needing amplification.
• Immune to electromagnetic interference: Unlike copper cables, optical fibres are not affected by radio waves or electrical noise.
• Secure: Difficult to tap into without breaking the fibre, making them safer for sensitive communications.

How data is transmitted:

• Digital information (1s and 0s) is encoded as pulses of light (on/off).
• A laser or light-emitting diode (LED) generates the light at the transmitter.
• The light travels through the fibre via total internal reflection.
• A photodiode or phototransistor detects the light pulses at the receiver and converts them back to electrical signals.

Common applications:

• International submarine cables linking continents.
• Broadband internet connections and telephone networks.
• Local area networks (LANs) in buildings and data centres.