Total Internal Reflection | Physics for GCSE/IGCSE - Year 11 PDF Download

Total Internal Reflection

• Sometimes, when light transitions from a denser to a less dense medium, instead of refraction, all light reflects back. This occurrence is termed total internal reflection.
• Total internal reflection happens when the angle of incidence exceeds the critical angle, and the incident material is denser than the second material.
• The conditions for total internal reflection are as follows:
  • The angle of incidence must be greater than the critical angle.
  • The incident material should be denser than the second material.

• Total internal reflection finds applications in various optical instruments such as:
  • Optical fibers (e.g., endoscopes)
  • Prisms (e.g., periscopes)

Prisms Usage

• Prisms play a crucial role in different optical tools, including:
  • Periscopes
  • Binoculars
  • Telescopes
  • Cameras
• Moreover, prisms are utilized in safety reflectors for bicycles and cars, as well as in markers for roads.
• A periscope is an optical device designed to enable viewing over obstacles by using mirrors set at angles.
  • A periscope consists of two right-angled prisms arranged in a specific manner.

Reflection of light through a periscope

• Light undergoes total internal reflection in both prisms of the periscope.
  

Critical Angle

• When the angle of incidence increases, the angle of refraction also increases until it approaches 90°.
• When the angle of refraction reaches precisely 90°, the light undergoes refraction along the boundary.
  • At this juncture, the angle of incidence is referred to as the critical angle (c).

• When the angle of incidence surpasses the critical angle, the refracted ray is no longer refracted but reflected instead. 
  • This phenomenon is termed total internal reflection.

Refractive Index & Critical Angle Equation

• The critical angle, denoted as 'c', of a material is interconnected with its refractive index, symbolized as 'n'.
• The equation linking these two aspects is expressed as follows:
  
• This equation can be rearranged to determine the refractive index 'n' as well:
  
• This equation shows that: 
  • A higher refractive index corresponds to a smaller critical angle. 
  • In materials with a high refractive index, light rays are more likely to experience total internal reflection.

Optical Fibres

• Total internal reflection in optical fibres is utilized for various applications such as communications, endoscopes, and decorative lamps.
• Light traveling within an optical fibre undergoes total internal reflection each time it encounters the edge of the fibre.

• Optical fibres are crucial components of endoscopes, allowing for non-invasive internal examinations within the human body.
  Endoscopes utilise total internal reflection to see inside a patient's body