Refraction & Total Internal ReflectionOxford AQA IGCSE Physics: Revision notes
Section 1
What happens when light refracts?
Refraction is the change in speed of a wave as it crosses a boundary between two different media. Unless the wave hits the boundary at 90° (along the normal), this speed change also causes a change in direction.
- Light entering a denser medium (e.g. air to glass) slows down and bends towards the normal
- Light entering a less dense medium (e.g. glass to air) speeds up and bends away from the normal
- The normal is the construction line drawn perpendicular to the surface at the point where the ray meets the boundary
- If the ray hits the boundary exactly along the normal (0° angle of incidence), it carries straight on with no change of direction, only a change of speed
Students often bend the ray away from the normal when entering glass — remember: denser medium, slower speed, bends towards the normal.
Section 2
How do we calculate refraction with Snell's Law?
The refractive index (n) of a medium compares the speed of light in a vacuum (or air) with its speed in that medium: n = speed of light in vacuum (air) ÷ speed of light in the medium.
Snell's Law links the refractive index to the angles either side of the boundary:
n = sin i / sin r
where i is the angle of incidence (in the less dense medium) and r is the angle of refraction (in the denser medium).
The required practical for this topic is to investigate the refraction of light by different substances (e.g. shining a ray of light through rectangular glass or perspex blocks and measuring i and r for a range of angles, then plotting sin i against sin r to find n as the gradient).
A ray hits a glass block with angle of incidence 40° and refracts at 25°. n = sin(40°)/sin(25°) = 0.643/0.423 = 1.52.
Section 3
What is dispersion?
When white light passes through a prism, refraction causes dispersion: different wavelengths (colours) of light refract by slightly different amounts, so the beam splits into a spectrum. Violet light refracts the most (it slows down the most) and red light refracts the least, spreading the colours out in the familiar rainbow order.
Section 4
What is the critical angle and total internal reflection?
As the angle of incidence inside a denser medium increases, the refracted ray bends further away from the normal until, at a certain angle, the refracted ray travels exactly along the boundary (refraction angle = 90°). This angle of incidence is the critical angle (c): n = 1 / sin c
If the angle of incidence exceeds the critical angle, the light cannot refract out at all — instead all of the light is reflected back inside the denser medium. This is called total internal reflection (TIR).
| Angle of incidence | What happens |
|---|---|
| Less than c | Ray refracts out, partial reflection also occurs |
| Equal to c | Refracted ray travels along the boundary (90°) |
| Greater than c | Total internal reflection — no light escapes |
Examiners want the exact wording: TIR occurs when the angle of incidence in the denser medium exceeds the critical angle — always name both conditions (denser medium + angle > c).
Section 5
How are optical fibres used?
An optical fibre is a thin, flexible strand of glass or plastic. Light (visible or infrared) entering one end strikes the internal walls at an angle greater than the critical angle every time, so it undergoes repeated total internal reflection and travels along the fibre with very little loss, even around bends.
Applications:
- Medicine — endoscopes let doctors see inside the body without invasive surgery
- Communications — fibre-optic cables carry telephone, internet and TV signals as pulses of light over long distances, much faster and with less signal loss than electrical cables
Think of an optical fibre like a long, curved hallway of mirrors — light keeps bouncing off the inside walls instead of escaping, so it travels the whole length of the fibre.
Must Know
- Refraction is a change in speed (and usually direction) as a wave crosses a boundary
- Denser medium → bends towards the normal; less dense medium → bends away from the normal
- Refractive index: n = speed in vacuum ÷ speed in medium; Snell's Law: n = sin i / sin r
- Critical angle: n = 1 / sin c
- Total internal reflection occurs when the angle of incidence in the denser medium exceeds the critical angle
- Optical fibres use repeated TIR to carry light for medical (endoscopes) and communication (fibre-optic cables) uses
- A prism disperses white light into a spectrum because different colours refract by different amounts
That's the notes covered.
Carry on to the next subtopic.