Total Internal Reflection (Cambridge (CIE) IGCSE Physics)

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 refraction and total internal reflection

Refraction happens when the angle of incidence is smaller and total internal reflection happens when the angle of incidence equals the angle of reflection

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

Total internal reflection happens with the angle of incidence is bigger than the critical angle

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

The colourful pattern observed on a CD is a result of thin film interference

• 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

Light is reflected and transmitted at the boundary from a less dense to a more dense material. Light is transmitted only at the boundary from a more dense to a less dense material. Hence, in this diagram P and Q exist but the third unlabelled ray does not.

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

Optical fibres utilise total internal reflection for communications

• 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

Reflection of light through 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

Obtaining total internal reflection examples

As the angle of incidence increases it will eventually surplus the critical angle and lead to total internal reflection of the light

• 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:

• This can also be rearranged to calculate the refractive index, n:

• 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 example: endoscope

Endoscopes utilise total internal reflection to see inside a patient's body

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

Sound from a landline telephone travels through optical fibres to the landline of the person listening

• 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

Fibre optic cables can be found in the phone cables attached between the telephone poles and the street