Chapter 4 and 5 - Waves and Optics Flashcards
Transverse waves
Particles vibrating perpendicular to the direction of energy transfer
Transverse waves can be polarised so the vibrations are in just one plane. Crossed polaroids are able to completely block a transverse wave
Longitudinal waves
Vibrates parallel to the direction of energy transfer
Can’t be polarised
Mechanical waves
Require a medium to travel through e.g. water waves need water and sound waves need air
Electromagnetic waves
Can travel through a vacuum and most mediums. All electromagnetic waves are transverse.
Radian
180 degrees = pi radian
1 degree = pi/180 rad
55 degrees = 55 pi/180 rad
Amplitude
The maximum displacement from the equilibrium position (metres)
Frequency
The number of waves per second (Hz)
Time period
The time taken for 1 complete wave cycle
T = 1/f
f = 1/T
Wave speed equation
Frequency x wavelength
Progressive waves
Carries energy from one place to another without transferring any material
Phase
A measurement of the position of a certain point along the wave cycle
Phase difference
The amount one wave lags behind another. Phase and phase difference are measured in angles (in degrees or radians) or as fractions of a cycle.
Phase difference equation
2 x pi x distance / wavelength
Diffraction
Spreading out of a wave when it passes through a gap or past the edge of an object
Smaller gap = more diffraction
Smaller wavelength = less diffraction
Superposition
Occurs when 2 or more waves overlap. Their amplitudes combine at all points where they are overlapping.
The 2 wares at a node are always in antiphase with one another, so they always cancel each other out
At the anti-node, they are always in phase, so they combine to produce a larger wave
Stationary waves
Produced when 2 waves are moving at the same speed but opposite directions, and with the same amplitude and frequency
Waves on strings
For a stationary wave to form on a string, the length must be a whole number of half-wavelengths
Length of string equation
l = n x wavelength/2
Amplitude of harmonics
The amplitude of each harmonic decreases, so the main note heard when plucking a string is the first harmonic. The higher harmonics are superposed onto the first harmonic
Describe the frequency of a stationary wave
All particles except those at the nodes vibrate at the same frequency
Maximum = antinodes
Minimum = nodes
Wavelength of a stationary wave
2x the distance of adjacent nodes
Interference
Formation of points of cancellation and reinforcement where two coherent waves pass through each other
Laser
Produces coherent, parallel beam of monochromatic light
Modal dispersion
Lengthening of a light pulse as it travels along optical fibre due to continual total internal reflection of waves meaning some rays travel longer distance than others in fibre
Single slit diffraction
The central fringe is double the width of the other fringes and much more intense
Particle displacement
The distance of a particle from its equilibrium position in any given direction
Refraction
Wave changes direction when its speed changes as it crosses a boundary
Polarisation
(Transverse) wave oscillation is in just one plane
How does polarisation work in sunglasses
- light reflected from surfaces is (weakly) polarised in one plane (horizontal)
- polaroid in sunglasses can be orientated to remove this reflected light, reducing glare
Monochromatic
Single wavelength
Properties of a laser
Monochromatic - only one wavelength
Coherent - waves have a constant phase difference
Collimated - produces an approximately parallel beam
