- When light is moving from a denser medium towards a less dense one, most of the light is refracted, but a small amount of it can be internally reflected
- However, if the angle of the light is great enough then instead of being refracted, ALL of the light is reflected
- This is called Total Internal reflection and happens when the angle of the incident ray is greater than the critical angle for that material
- Two conditions are necessary for total internal reflection to occur:
- The light must be going from a more dense medium towards a less dense one
- The angle of incidence must be greater than the critical angle
If asked to name the phenomena make sure you give the whole name – Total Internal Reflection.
Remember: Total Internal Reflection occurs when going from more dense to less dense and ALL of the light is reflected.
If asked to explain what is meant by the critical angle, you can draw the diagram above (showing the three semi-circular blocks).
- 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:
When calculating the value of the critical angle using the above equation:
- First use the refractive index, n, to find sin c
- Then use the inverse sin function (sin-1) to find the value of i
- Total internal reflection is also used to reflect light along optical fibres, allowing the high-speed transmission of data on the internet
- Additionally, optical fibres can be used in medicine in order to see within the human body
Diagram showing the application of total internal reflection in medicine (using an endoscope)
When drawing light reflecting down an optical fibre, make sure that each time it reflects the angle of reflection is equal to the angle of incidence.