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
