Waves (Unit 2) Flashcards
Longitudinal wave
Particle vibration is parallel to direction of wave propagation
Examples of a longitudinal wave
Sound waves, seismic p-waves
Transverse wave
Particle vibration is perpendicular to direction of wave propagation
Only transverse waves can be polarised
Examples of a transverse wave
Electromagnetic radiation, seismic s-waves
Particle displacement
The distance of a particle from its equilibrium position in given direction
Amplitude
the maximum displacement of a particle (wave) from its equilibrium (or rest) position
Frequency
Number of oscillations (of a particle) per second
Time period
The time for one complete oscillation
Wavelength
Shortest distance between two points in phase
Diffraction
Spreading out of a wave (when it passes through a gap or past the edge of an object)
Refraction
Wave bends/changes direction when its speed changes
Polarisation
(transverse) wave oscillation is in one plane
Application of polarisation 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
Application of polarisation in tv transmitters and aerials
- Signals from tv transmitter (radio waves) are polarised
- Aerials need to be orientated (rotated) so they are in same plane as the transmitted signal
- For maximum signal strength
Superposition
Where two or more waves meet, the resultant displacement equals the vector sum of the individual displacements
Conditions for formation of stationary waves
- Two waves travelling past each other in opposite directions
- With the same wavelength (or frequency)
- Similar amplitudes
Nodes and antinodes
Nodes – points of no oscillation / zero amplitude
Antinodes – points of maximum amplitude
Coherent sources
waves (from two sources) that have:
• a constant phase difference
• same wavelength (or frequency)
Monochromatic
Single wavelength
Safety with a laser
- Avoid looking along the beam of a laser
- Wear laser safety goggles
- Avoid reflections
- Put up a warning sign that a laser is in use
Properties of laser light
- Monochromatic – only a single wavelength
- Coherent – waves have a constant phase difference
- Collimated – produces an approximately parallel beam
Appearance of interference fringes from two vertical slit illuminated with yellow light
- Vertical or parallel
- Equally spaced
- Black and yellow bands
Fringe width, w, changes
Slits closer together w – increases
Screen further away w – increases
Shorter wavelength (eg blue light) w - decreases
Explanation of formation of fringes with Young’s slits
- Interference fringes formed
- Where light from two slits overlaps
- The light from the two slits is coherent
- Bright fringes formed where constructive interference
- because light from the two slits is in phase (path difference equals a whole number of wavelengths)
- Dark fringes formed where destructive interference
- Because light from the two slits is in anti-phase (path difference equals a whole number + 0.5 wavelengths)
Appearance of white light through Young’s slits
- Central fringe would be white
- Side fringes are (continuous) spectra
- Bright fringe would be blue on the side nearest the central fringe.
- Bright fringes merge further away from centre.
appearance of diffraction pattern from a single slit
- Central bright fringe has twice width of other bright fringes
- The other bright fringes have a much lower intensity
- and are equally spaced
Single slit pattern changes
Narrower slit width • Wider pattern / increased separation
• Reduced intensity
Shorter wavelength • Narrower pattern / reduced separation
Lines per mm of a grating
Spacing, d, of slits on a diffraction grating given by:
d = 1/(number of lines per mm) in mm
Applications of gratings to spectral analysis of light from stars
- Dark lines in spectrum from a star (absorption spectrum)
* Reveal the composition of (elements present in) the star’s atmosphere
How does light change moving from air to glass
- speed – decreases (slows down)
- wavelength – decreases (gets shorter)
- frequency – remains constant (stays the same)
Conditions for total internal reflection
- Angle of incidence is greater than the critical angle
- The refractive index of the material light is going from is greater than the refractive index of the material the light is going to.
Total internal reflection
Where all the light is reflected back into the material
Critical angle
Angle of incidence which produces an angle of refraction of 90 degrees.
Structure of an optical fibre
Central core, surrounded by cladding. Refractive index of core must be greater than refractive index of cladding (to ensure total internal reflection)
Purpose of cladding
- prevents crossover of signal/data to other fibres
- prevents scratching of the core
- reduces pulse broadening/dispersion
Use of optical fibres
- Communication – improve transmission of data/high speed internet
- Endoscopes – improved medical diagnosis
How do pulses of light change travelling down optical fibres
- reduced amplitude due to absorption/energy loss and scattering within fibre
- pulse broadening due to multipath dispersion from rays taking different paths and different times to travel down same fibre
How is multipath dispersion reduced
Core of fibre is made very narrow/thin.
Sketches of stationary waves for first 4 harmonics
See sheet
Derivation of n(lambda) = d sin(theta)
See sheet
