Optics (Waves) Flashcards
What is the critical angle?
• The angle of incidence that results in the angle of refraction of 90 degrees.
• For a critical angle to occur, the refractive index of the refracted material must be greater than the refractive index of the incident material.
What is an optical fibre?
• An application of total internal reflection.
• Consists of: a core and a cladding.
* Refractive Index of core MUST be bigger than the refractive index of the cladding.
Where does TIR occur in an optical fibre?
At the core-cladding boundary (where the core meets the cladding).
What is the importance of the cladding layer in an optical fibre?
• Prevents light crossing from one part of the fibre to another in situations where two fibres come into contact.
What would happen if a crossover between two optical fibres occurred?
The signal wouldn’t be secure as it wouldn’t reach its destination.
What is the law of total internal reflection?
• The angle of incidence must be greater than the critical angle.
• The angle of incidence = The angle of refraction.
What does a communication optical fibre do?
• Allows light to enter the fibre at one end, from a transmitter, to reach a receiver at the other end.
• The fastest broadband systems use optical fibre links.
What must the core of an optical fibre be very narrow?
To prevent multi-path dispersion.
Why does multi-path dispersion occur in a wide core?
Because light travelling along the axis of the core travels a shorter distance per meter of fibre than light than repeatedly undergoes total internal reflection.
What is the effect of dispersion on the input pulse entering an optical fibre?
Causes the input pulse to lengthen (broaden) as it travels along the fibre.
What is a medical endoscope?
• A medical instrument containing two bundles of fibres.
- One set of fibres transmits light into a body cavity.
- The other set returns an image for observation.
What is diffraction?
When waves spread out after passing through a gap or round an obstacle.
When do you get the most diffraction?
When the size of the aperture/gap is the same/similar size to the wavelength of the wave.
What is interference?
When two waves of the same type occupy the same space.
What is the law of interference?
• The resultant displacement is equal to the vector sum of the individual displacements.
What does wave superposition result in?
The formation of an interference pattern made up of regions of reinforcement and cancellation.
For an interference pattern to be observable, the two waves must be……
Coherent.
What does it mean if two waves are coherent?
- Same frequency
- Constant phase difference
How can two coherent waves be produced?
By a single wave passing through a double slit.
What is path difference?
The difference in distance two waves have travelled (measured in wavelength).
What is the path difference of two waves in phase?
A whole number of wavelengths.
What is the phase difference of two waves out of phase?
A half number of wavelengths e.g. 1/2 or 3/2
