5.3 Total internal reflection Flashcards
What is the critical angle?
» When the incident ray angle reaches the critcal angle, the light ray refracts along the boundary
What happens if the angle of incidence is greater than the critical angle?
» The light ray undergoes total internal reflection at the boundary
What are the 2 requirements for total internal reflection to take palce?
» The incidence 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, and why?
» 90 degrees, as the light emerges along the boundary
» Therefore n1.sin(theta1) = n2
What happens if incident angle is less than the critical angle?
» Refraction happens
What happens when the angle of incidence is exactly equal to the critical angle?
» The light ray refracts along the boundary
Why do diamonds sparke so bright?
» Diamond has a very high refractive index - so seperates the colors more than any other substance does
» Critical angle is also low, so a light ray may be totally internally reflect many times before it emerges, which means colors spread out more and more
Where are optical fibres used in?
» Meidcal endoscopes
» Communications to carry light signals
How do optical fibres work?
» The light ray is totally internally reflected each time it reaches the fibre boundary
» At each point where the light ray reaches the boundary - the angle of incidence exceeds the critical angle of the fibre
When does partial internal reflection occur?
» When the angle of incidence is less than or equal to the critical angle
What does each fibre consists of ?
» A core surrounded by a layer of cladding of a lower refractive index to reduce light loss from the core
» Light loss will also reduce the amplitude of the pulses
What does each fibre consists of ?
» A core surrounded by a layer of cladding of a lower refractive index to reduce light loss from the core
» Light loss will also reduce the amplitude of the pulses
Where does total internal reflection take palce in the optical fibres?
» Takes place at the core-cladding boundary,
» At any point where the 2 fibres are in direct contact, light would cross from one fibre to another - if there was no cladding
» Signals would not be secure
What is modal dispersion?
» Each beam undergoes TIR, a different number of times
» Therefore each beam reaches the end at a slightly different time, resulsts in pulse broadning
Why must the core be very narrow?
» To prevent modal dispersion
» Which would reduce bandwith/cause pulse broadning
How is modal dispersion caused in a wide core?
» Light travelling along the axis of the core travels a shorter distance per meter of a fibre
» A pulse of light sent along a wide core would become longer, than it ought to be
» If it was too long, it would merge with the next pulse
Why must the light in optical fibres be monchromatic?
» Pulse dispersion can occur if white light is used
» Speed of light in the glass depends on the wavelength
» Violet light travels slower then red lights, causes wavelengths to become longer
» Leads to pulse merging
How does a medical endoscope work?
» Contains 2 bundlesof fibres
» Endoscope is inserted into a body cavity - which is illuminated using light
» A lens over the end of the other fibre bundle is used to form an image of the body cavity
» Needs to be a coherent bundle
How does a medical endoscope work?
» Contains 2 bundlesof fibres
» Endoscope is inserted into a body cavity - which is illuminated using light
» A lens over the end of the other fibre bundle is used to form an image of the body cavity
» Needs to be a coherent bundle
What is a coherent bundle?
» Which means that fibre ends at each end are in the same relative positions
What are the 4 properties of lasers?
» Monochromatic
» Collimated
» Coherent
» Polarised
What is monochromatic?
» Waves of single frequency/wavelength
What is collimated?
» Produces approximately parallel beams
What is coherent?
» Waves produced have a constant phase difference
What is Polarised?
» Vibrations in one plane only
Explain why the fibre bundle needs to be coherent?
» The positions of ends of the fibres at the internal end need to map exactly to the same positions at the viewing end. In other words, the fibres cannot be allowed to “tangle up” as they go through the cable.
» Otherwise, the image would also be “tangled up”.
What is one application of optical fibres?
» Communications
» Provide high-speed internet
