2. Waves and Optics Flashcards
Definition of Amplitude
The maximum displacement of a particle from equilibrium position.
Definition of frequency
The number of complete waves per second
Definition of wavelength
The smallest distance between two adjacent vibrating particles that are in phase
Definition of wave speed
v = λf
Definition of phase
Measurement of the position of a certain point along the wave cycle
Definition of phase difference
The difference between the two positions of certain points along the wave cycle
Longitudinal Waves
Waves where the vibrations are parallel to the direction of energy transfer
E.g: sound waves, strings
Transverse Waves
Waves where the oscillations of the particles are perpendicular to the energy transfer
E.g: strings, EM waves
Definition of polarised wave
A wave that can only oscillations in one plane, and can only work if the oscillations are perpendicular to the energy propagation.
Definition of polarised wave
A wave that can only vibrate in one plane, and can only work if the oscillations are perpendicular to the energy propagation.
Define stationary waves
The superposition of two progressive waves travelling in opposite directions and are coherent and have the same amplitude
What is the principle of superposition?
The displacement of the wave is the resultant vector sum of the individual displacement of the two waves.
What is constructive interference?
When the two waves have displacement in the same direction
What is destructive interference
When the two waves have displacements in the opposite direction
What are antinodes
When the two waves meet in phase and constructively interfere to create regions of maximum amplitude
What are nodes
When the two waves meet completely out of phase and destructively interfere to create regions of zero displacement
What is the formula for the first harmonic?
f = 1/2l*(T/μ)^0.5
Describe the formation of stationary waves on a string
A wave travelling down the string that’s fixed on one end will be reflected back at the same wavelength, frequency and amplitude, forming a stationary wave.
What is the first harmonic on a string.
The lowest frequency at which a stationary wave is formed forming two nodes and one antinode, which is half a wavelength.
Describe the formation of stationary wave on a microwave.
Created by reflecting a microwave beam on a metal plate.
Definition of coherence
When a two waves have the same wavelength and frequency and a constant phase difference
Why are lasers used in interference experiments
Because lasers are monochromatic and have a small range of wavelengths and a fixed (small range of frequencies) to create clear interference patterns.
What is the equation for single slit diffraction?
W = λD/2s
Describe Single slit diffraction
The greatest diffraction occurs when the gap is roughly the same as the wavelength
There’s a bright central fringe that’s twice as wide as the other fringes
Bright fringes are caused by constructive interference when the waves meet in phase as the path difference is a whole number.
Dark fringes are caused by destructive interference when the waves meet π out of phase as the path difference is 1/2 nλ
Describe Young’s double-slit experiment
The slits act as a coherent point source. The light fringes occur due to constructive interference as the waves are in phase due to the path difference being a whole number of wavelengths. The dark fringes occur due to destructive interference of the waves which are π out of phase due to the path difference being a whole number and a half wavelengths.
What is the young’s double-slit equation?
w = λD/s
Interference pattern with a white light source
White light is a mixture of multiple wavelengths, so each of the wavelengths would be diffracted by a different amount. The central maxima would be white, but there would be a spectrum for the other fringes, and the fringes would be wider.
Diffraction grating equation
dsinθ = nλ
Explain total internal reflection
When the angle of incidence is greater than the critical angle, where the angle of reflection is 90° and the incident refractive index is greater than the refractive index of the boundary material
Define refractive index
Measures how much the light slows down when passing through the material
Explain Optical fibres
The optically dense core is surrounded by cladding with a lower refractive index for TIR to occur. Cladding protects the core from damage and prevents signal degradation from light escaping the core.
Explain absorption
Where part of the signal’s energy is absorbed by the fibre and reduces the amplitude of the signal
Explain modal dispersion
The signal received is broader than the original signal due to the light entering at different angles so there’s a range of time travel for the signals causing pulse broadening
Explain material dispersion
Caused by using light of different wavelengths so they travel at different speeds.
