Total Internal Reflection (Cambridge (CIE) IGCSE Physics): Revision Note

Total internal reflection

• Total internal reflection (TIR) occurs at the boundary between two media when:

All the incident ray in medium 1 is reflected back into medium 1

• When light passes between the boundary of an optically dense to a less dense medium and the angles of incidence are small

  • The refracted ray is strong

  • The reflected ray is weak

• The weak ray is reflected back into the denser medium

  • This means some internal reflection occurs

• It is not TIR because not all of the ray is reflected, only some of it

Comparing internal reflection and total internal reflection

• Normal reflection produces a less intense light compared to TIR

  • In TIR the light ray is brighter and more intense

• Normal reflection occurs independent of the refractive indices of both media

  • For TIR to occur, the incident material must be denser than the second material

Conditions for internal reflection

Internal reflection examples

• Thin film interference is an example of internal reflection

• An example of this is the shiny side of a CD

Example of internal reflection

• Other examples of thin film interference include:

  • soap bubbles

  • thin layers of oil on water

• In these examples, internal reflection occurs at the boundaries between:

  • air and water

  • water and oil

• A spectrum of colours will be seen by the observer due to the rays partially reflected at the boundary

A ray diagram of an example of internal reflection

Total internal reflection examples

• Total internal reflection is used to reflect light along optical fibres, meaning they can be used for:

  • communications

  • endoscopes

  • decorative lamps

• Light travelling down an optical fibre is totally internally reflected each time it hits the edge of the fibre

Total internal reflection example: optical fibre

• Prisms are used in a variety of optical instruments, including:

  • periscopes

  • binoculars

  • telescopes

  • cameras

  • safety reflectors

• A periscope is a device that can be used to see over tall objects

  • It consists of two right-angled prisms

Total internal reflection example: a periscope

• The light totally internally reflects in both prisms

Critical angle

• At the boundary between a more dense and a less dense medium, as the angle of incidence is increased, the angle of refraction also increases until it gets closer to 90°

• When the angle of refraction is exactly 90° the light is refracted along the boundary

  • At this point, the angle of incidence is known as the critical angle c

• When the angle of incidence is larger than the critical angle, the refracted ray is now reflected

  • This is total internal reflection

Refractive index & critical angle equation

Extended Tier Only

• The critical angle, $c$, of a material, is related to its refractive index, $n$

• The relationship between the two quantities is given by the equation:

$\sin c=\frac{1}{n}$

• This can also be rearranged to calculate the refractive index (opens in a new tab), n:

$n=\frac{1}{\sin c}$

• This equation shows that:

  • The larger the refractive index of a material, the smaller the critical angle

  • Light rays inside a material with a high refractive index are more likely to be totally internally reflected

Optical fibres

Extended Tier Only

• Optical fibres have many uses, particularly in telecommunications

• Endoscopes are used to look within the human body

  • A camera on the end of an optical fibre is placed down the throat and moved into the stomach

  • Light from inside the stomach is captured by the camera, is totally internally reflected along the optical fibre and viewed by doctors through the eyepiece

Total internal reflection using telecommunication

• Optical fibres can be used to transmit:

  • home (landline) telephone signals

  • internet signals

  • cable television signals

• In phone calls from landline phones:

  • Electrical signals are converted to light pulses

  • That travel close to the speed of light along optical fibres

  • At the receiving end, the digital signal is converted into sound

Total internal reflection example: landline telephone signal path

• Optical fibres are installed:

  • in cables attached to telephone (or telegraph) poles in the street

  • underground from the service box to the telegraph pole or under the sea

Total internal reflection example: telegraph poles