5.3- TOTAL INTERNAL REFLECTION Flashcards
When a light ray travels from glass into air, where does it refract towards?
refracts away from the normal
What happens if the angle of incidence is increased to a certain value known as the critical angle?
light ray refracts along the boundary
What does the light ray undergo if the angle of incidence is increased further than the critical angle?
total internal reflection at the boundary, same as if boundary were replace by plane mirror
In general, when does total internal reflection only happen if…? (2)
incident substance has a larger refractive index than the other substance
angle of incidence exceeds the critical angle
How is the angle of refraction 90° at the critical angle?
as the light ray emerges along the boundary
What equation can be given as sin90 = 1?
sinθ(small c) = n2 / n1
n1- refractive index of incident substance
n2- refractive index of other substance
What happens to light when in enters a diamond?
it’s split into the colours of the spectrum
What is the refractive index of diamond like?
very high refractive index of 2.417
What can diamond do as it has a very high refractive index?
separates colours more than any other substance does
What does the high refractive index of diamond give it?
critical angle of 24.4°
What does diamond having a critical angle of 24.4° mean and how does it cause it to look?
light ray in diamond may be totally internally reflected many times before it emerges, which means its colours spread out more and more
so diamond sparkles with different colours
Example of where optical fibres are used in?
medical endoscope
What are optical fibres used in medical endoscopes for?
to see inside the body
What else is optical fibres used for other than endoscopes?
in communications to carry light signals
What happens to a light ray when it reaches a fibre boundary in an optical fibre?
light ray totally internally reflected each time it reaches fibre boundary, even where fibre bends, unless radius of bend too small
What is the angle of incidence like at each point where the light ray reaches the boundary? (optical fibres)
at each point where light ray reaches boundary, angle of incidence exceeds critical angle of fibre
What does a communication optical fibre allow a pulse of light to do?
allows pulse of light that enters at one end, form transmitter, to reach receiver at other end
What do communication optical fibres need to be like?
highly transparent
Why do communication optical fibres need to be highly transparent?
to minimise absorption of light
What would happen if communication optical fibres were not highly transparent, and so didn’t minimise absorption of light?
amplitude of pulses reduces progressively further they travel in fibre
What does each fibre consist of? (optical fibres)
core surrounded by layer of cladding of lower refractive index to reduce light loss from core
What would light loss do? (optical fibres)
would reduce amplitude of pulses
Where does the total internal reflection take place? (optical fibres)
core-cladding boundary
What happens at any point where two fibres are in direct contact if there were no cladding? (optical fibres)
light would cross from one fibre to the other
What would it mean if light was to cross from one fibre to another due to there being no cladding? (optical fibres)
signals would not be secure, as they would reach the wrong destination
Why must the core be very narrow? (optical fibres)
to prevent modal dispersion
Where does modal dispersion occur? (optical fibres)
in a wide core
Why does modal dispersion occur in a wide core? (optical fibres)
as light travelling along the axis of the core travels a shorter distance per metre of fibre than light that repeatedly undergoes total internal reflection
What would happen to a pulse of light that’s sent along a wide core? (optical fibres)
would become longer than it ought to be
What would happen if a pulse of light sent along a wide core becomes too long than it ought to be? (optical fibres)
it would merge with the next pulse
When else does pulse dispersion happen?
if white light used instead of monochromatic light (light of single wavelength)
Why does the material dispersion happen when white light is used instead of monochromatic light?
speed of light in glass of optical fibre depends on wavelength of light travelling through it
How does violet light travel in comparison to red light in glass?
violet light travels more slowly than red light in glass
As violet light travels more slowly than red light in glass, what would happen when white light is used instead of monochromatic light? (optical fibres)
difference in speed would cause white light pulses in optical to become stronger, as violet component falls behind faster red component of each pulse
So why must the light used be monochromatic? (optical fibres)
prevent pulse merging
How many bundles of fibres does a medical endoscope contain?
two bundles of fibres
Where is the medical endoscope inserted?
into a body cavity
How is the medical endoscope illuminated once inserted into a body cavity?
using light sent through one of the fibre bundles
What does the light that forms the image do the (medical endoscope)?
light that forms image travels along fibres to other end of fibre bundle where image can be observed
What does the fibre bundle that forms the image have to be like? (medical endoscope)
needs to be coherent bundle
What does it means coherent bundle?
fibre ends at each end are in same relative positions
