What are mechanical waves?
Waves that pass through a substance, causing particles in the substance to vibrate in such a way that causes nearby particles to also begin to vibrate
What are electromagnetic waves?
Oscillating electric and magnetic fields that progress through space without the need for a substance
What is a longitudinal wave?
Waves in which the direction of vibrational the particles is parallel to the direction in which the waves travel
What types of waves are longitudinal waves?
- Sound waves
- Primary seismic waves
- Compression waves
What is a transverse wave?
Waves in which the direction of vibration is perpendicular to the direction in which the wave travels
What types of waves are transverse waves?
- Electromagnetic waves
- Secondary seismic waves
- Waves on a string or wire
When are transverse waves plane-polarised?
The vibrations stay in one plane only
What happens to unpolarised light when it passes through a Polaroid filter?
The transmitted light is polarised as the filter only allows through light which vibrates in a certain direction, according to the alignment of its molecules
When are Polaroid filters said to be 'crossed'?
When the transmitted intensity is a minimum
How do Polaroid sunglasses work?
Reflected light is polarised and the intensity is reduced
What is displacement?
A particles distance and direction from its equilibrium position
What is the amplitude of a wave?
The maximum displacement of a vibrating particle
What is the definition of wavelength?
The least difference between two adjacent vibrating particles with the same displacement and velocity at the same time
What is the period of a wave?
The time for one complete wave to pass a fixed point
What is the definition of frequency for a wave?
The number of cycles of vibration of a particle per second
What is the definition of phase difference?
The fraction of a cycle between vibrations of two particles
How do you calculate phase difference?
2 x pi x distance / wavelength
What is reflection?
Straight waves directed at a certain angle to a hard flat surface reflect off at the same angle
What is refraction?
If a wave approaches a boundary at an angle it will change direction as well as changing speed
What is diffraction?
When waves spread out after passing through a gap or around an object
What causes waves to spread out more in diffraction?
- Narrower gap
- Longer wavelength
What is the principle of superposition?
When two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point
What is interference?
When cancellation and reinforcement occurs at fixed positions
How are stationary waves formed?
When two progressive waves pass through each other
How much energy do freely vibrating stationary waves transfer to their surroundings?
When is the phase difference between two vibrating particles zero?
If the two particles are between adjacent nodes or separated by an even number of nodes
When is the phase difference between two vibrating particles 180° (pi radians)?
If the two particles are separated by an odd number of nodes
What is the difference between two adjacent nodes?
What is the equation to calculate the frequency of the first harmonic of a stationary wave?
f = c / 2L
How do you calculate the time taken for a wave to travel along a string and back?
- 2L / c
- Where c is the speed of the waves on the string
How do you calculate the time taken for the vibrator to pass through a whole number of cycles?
- m / f
- Where m is a whole number
How does an oscilloscope work?
An electron gun at one end of the glass tube emits electrons in a beam towards a fluorescent screen at the other end of the tube. Light is emitted from the spot on the screen where the beam hits the screen
If a light wave travels at an angle into a denser substance, what way will the light bend?
Towards the normal
How do you calculate the refractive index of a substance, n?
sin i / sin r
How do you calculate the refractive index of a substance, Ns?
- C / Cs (speed of light in substance)
- Lambda / lambda in substance
What is Snell's law?
What factors are needed for total internal reflection to take place?
- The incident substance has a larger refractive index than the other substance
- The angle of incidence exceeds the critical angle
What must optical fibres have to work successfully?
- Highly transparent - to minimise absorption of light
- Cladding - to reduce light lose which would reduce the amplitude of the pulses
- Very narrow - to prevent modal (multi path) dispersion
What is modal (multi path) dispersion?
- If an optical fibre has a wide core light travelling along the axis of the core travels a shorter distance than light that repeatedly undergoes total internal reflection
- Therefore a light pulse would become longer, possibly therefore merging with the next pulse
What is material or spectral dispersion?
- When white light is used in an optical fibre instead of monochromatic light
- The different coloured waves would be different speeds, causing the pulse to become longer
When using medical endoscopes, what does the term coherent bundles mean?
The fibre ends at each end of the bundle are in the same relative position
What does coherent mean?
Waves with a constant phase difference and the same frequency
What is happening when a bright fringe is formed in Young's double slit experiment?
The light from one slot reinforces the light from the other slit - the light waves arrive in phase with each other
What is happening when a dark fringe is formed in Young's double slit experiment?
The light from one slit cancels the light from the other slit - the light waves are 180° out of phase
How do you calculate fringe separation, w, in Young's double slit experiment?
- Lambda x D / s
- D = distance from the slit to the screen
- s = slit spacing, distance between the centres of the slits
Why is fringe separation greater for red light compared to blue light?
Red light has a longer wave length
What makes laser light different to other forms of light?
- It is monochromatic
- It is coherent
What do white light fringes look like?
- Central white fringe
- Tinted red on the outside
- Tinted blue on the inside
- Outer fringes become fainter
- Outer fringes merge, different colours reinforce and therefore overlap
What makes diffracted waves spread out more?
- The gap is made narrower
- The wavelength is made larger
Why do diffracted wavelengths have breaks on either side of the centre?
Waves diffracted by adjacent sections on the gap are out of phase and therefore cancel each other out in certain directions
With single slit diffraction, how do you make the fringes wider?
- Greater wavelength of light
- Make the slit narrower
How do you calculate the width of the central fringe in single slit diffraction?
Wavelength of the light / width of single slit x 2D
Why does a diffraction grating transmit light in only certain directions?
- Light passing through each slit is diffracted
- The diffracted light waves from adjacent slits reinforce each other in certain directions only
What factors increase the angle of diffraction between each transmitted beam?
- Light of a longer wavelength is used
- A grating with closer slits is used
How do you calculate the angle of diffraction of the nth order beam?
d sin = n lambda
How do you calculate the number of slits per meter on a diffraction grating?
- Where d is the grating spacing
What increases the angle of diffraction in a diffraction grating experiment?
Increasing the number of slits per meter
How do you calculate the maximum number of orders in a diffraction grating experiment?
The value of d/lambda rounded down to the nearest whole number