3.2.5Waves
Total Internal Reflection — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 19 of 35 1 quiz
Lesson summary
3.2.5 Total Internal Reflection — IGCSE PHYSICS notes: Explain total internal reflection, the critical angle and its uses in prisms and optical fibres.Total internal reflection — all of the incident ray in medium 1 is reflected back into medium 1Small angles — the refracted ray i…
- Brightness
- Ordinary reflection
- Denser to less dense
- Air to glass
- Rising angle
- Critical angle c
Part 1 of 8Internal reflection and total internal reflection
1Total internal reflection
- Total internal reflection all of the incident ray in medium 1 is reflected back into medium 1
2Small angles
- Small angles the refracted ray is strong and the reflected ray is weak, so some internal reflection occurs but it is not TIR
3Brightness
- Brightness in TIR the reflected ray is brighter and more intense than in ordinary reflection
4Ordinary reflection
- Ordinary reflection occurs whatever the refractive indices of the media
Part 2 of 8Conditions for TIR
Denser to less dense
- Denser to less dense the incident material must be denser than the second material
Angle
- Angle the angle of incidence must be greater than the critical angle
Air to glass
- Air to glass can never give TIR because the ray refracts towards the normal
Part 3 of 8Critical angle
Rising angle
- Rising angle as the angle of incidence increases in the denser medium, the angle of refraction increases towards 90°
Critical angle c
- Critical angle c the angle of incidence at which the angle of refraction is exactly 90° and the light travels along the boundary
Beyond c
- Beyond c the refracted ray disappears and all the light is reflected
Equation
- Equation sin c = 1 ÷ n,
- so a larger n gives a smaller cWhat the symbols mean: c = critical angle · n = refractive index of the denser material (no unit)
- so a larger n gives a smaller c
Part 4 of 8Three cases
| Angle of incidence | What happens |
|---|---|
| Less than c | Refraction into the less dense medium with a weak partial reflection |
| Equal to c | Refracted ray runs along the boundary |
| Greater than c | Total internal reflection, with i = r inside the denser medium |
Part 5 of 8Prisms and reflectors
Right-angled prisms
- Right-angled prisms a 45°-45°-90° prism has a critical angle of about 42°, so a ray at 45° inside is totally reflected
- One prism turns light through 90°
- Two prisms turn it through 180°
Devices
- Devices periscopes, binoculars, telescopes, cameras and safety reflectors
Periscope
- Periscope two right-angled prisms, with TIR in both
Advantage over mirrors
- TIR is nearly 100% efficient and the surface does not tarnish
Part 6 of 8Optical fibres
Optical fibres
Structure
- Structure a thin glass or plastic strand with a high-index core and a lower-index cladding
Action
- Action light is totally internally reflected each time it hits the edge of the fibre, even round gentle bends
Uses
- Uses communications, endoscopes and decorative lamps
Telecommunications
- Telecommunications landline telephone, internet and cable television signals
- Electrical signals are converted to light pulses
- The pulses travel close to the speed of light
- At the receiver the digital signal is converted back to sound
Endoscope
- Endoscope light from inside the body is totally internally reflected along the fibre to an eyepiece
Advantages
- Advantages fibres are thin, light, cheap, low loss and not affected by electrical interference
Part 7 of 8Thin film effects
Thin film interference
- Thin film interference partial internal reflection at the boundaries of thin layers gives colours
- The shiny side of a CD
- Soap bubbles
- Thin oil layers on water
Result
- Result rays partly reflected at the boundaries give the observer a spectrum of colours
Part 8 of 8More from the specification
More from the specification
Conditions for total internal reflection
- Total internal reflection occurs when light travelling in an optically denser medium strikes the boundary to a less dense medium at an angle exceeding the critical angle.
Critical angle formula
- The critical angle c is related to refractive index n by the equation sin(c) = 1 / n.
Optical fibre applications
- Total internal reflection confines light within flexible glass or plastic optical fibres, enabling high-bandwidth telecommunications and medical endoscopes.
TIR in glass prisms
- 45-degree right-angled prisms reflect light internally by 90 or 180 degrees without producing faint double images.
Converging lens real image
- Converging lens real image refracts rays from an object beyond the principal focus to form an inverted, real image.
Diverging lens virtual image
- Diverging lens virtual image spreads incident rays outward, forming an upright virtual image located between the lens and principal focus.
Distance
- Total length of path travelled, a scalar.
Virtual image
- Image formed where rays only appear to meet, not projectable on a screen
Principal focus
- Point where parallel rays meet or appear to come from
Focal length
- Distance from the lens centre to the principal focus
Plane mirror
- Flat mirror.
Key terms
| Term | Meaning |
|---|---|
| Total internal reflection | Complete reflection of light inside a denser medium at a boundary |
| Critical angle | Smallest angle of incidence that gives total internal reflection |
| Optical fibre | Thin transparent strand that guides light by TIR |
| Cladding | Lower-index layer around a fibre core |
| Endoscope | Fibre device used to look inside the body |
Avoid these mistakes
1
Saying TIR happens from less dense to more dense — it needs the denser medium first.
2
Naming only total reflection — give the whole name total internal reflection.
3
Saying all the light is reflected when i is less than c — only a weak partial reflection occurs.
4
Naming the device but not the object that reflects — state the right-angled prism then the periscope.
Marked practiceQuizzes for this lesson
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