5.3 Total internal reflection Flashcards
When a light ray travels from glass into air, it refracts ______ ______ the normal
Away from
What happens if the angle of incidence is increased to a certain value known as the critical angle
The light ray refracts along the boundary
What happens if the angle of incidence is increased further past the critical angle
The light ray undergoes total internal reflection at the boundary
In general, total internal reflection can only take place if…
The incident substance has a larger refractive index than the other substance
The angle of incidence exceeds the critical angle
What is the angle of refraction at the critical angle
90 degrees
Why is the angle of refraction at the critical angle what it is
Because the light ray emerges along the boundary
Since sin90 = 1 then sinθc=n2/n1
What happens when white light enters a diamond
It is split into the colours of the spectrum. Diamond has a very high refractive index of 2.417 so it separates the colours more than any other substance does.
What are optical fibres used in
Medical endoscopes to see inside the body and in communications to carry light signals
What does a communications optical fibre allow for
Pulses of light that enter at one end, from a transmitter to reach a receiver at the other end. These need to be highly transparent to minimise absorption of light, which would otherwise reduce the amplitude of the pulses progressively the further they travel
What does a communications optical fibre consist of
A core surrounded by a layer of cladding of lower refractive index to reduce light loss from the core. Light loss would also reduce the amplitude of the pulses
Where does total internal reflection in an optical fibre take place
At the core-cladding boundary
At any point where two fibres are in direct contact
What would happen if there was no cladding in an optical fibre
Light would cross from one fibre to the other if there were no cladding. This would mean signals would not be secure as they would reach the wrong destination
Why must the core of an optical fibre be very narrow
Prevent modal dispersion
Why does modal occur in a wide core
Because light travelling along the axis travels a shorter distance per metre of fibre than light that repeatedly undergoes total internal reflection.
What would happen in a wide core
A pulse of light would become longer than it ought to be. If it was too long, it would merge with the next pulse
