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Physics Waves
Physics Autumn Y12
| Question | Answer |
|---|---|
| How to mark wavelength on graph | Use wavelength arrows (T ends) for precision |
| Symbol for wavelength | λ |
| Progressive wave definition | Transfers energy *without transferring matter* through oscillations |
| Amplitude definition | Maximum displacement from equilibrium |
| Frequency definition | Number of waves passing a point per unit time |
| Time period definition | Time taken for one complete oscillation |
| Wavelength definition | the distance between 2 adjacent points on a wave that are *in phase* |
| In phase meaning | Doing the same thing - i.e. on a sin wave, the two adjacent points where it crosses the equilibrium are out of phase |
| Displacement distance graph | Shows one point in time of the profile of the wave. No time information. |
| Displacement time graph | Shows motion of one point on the wave over time. No wavelength information. |
| If asked to explain how to measure time period of a displacement time graph | 'Find the length of one OSCILLATION' - saying wavelength loses marks |
| How to find wave speed equation | Speed = distance over time The distance travelled by a wave in one time period is the wavelength. Then show the maths steps |
| In phase | Phase difference is zero degrees |
| Antiphase | Phase difference is 180 degrees |
| Phase difference symbol | Φ |
| Phase difference between A and C | (Distance between A and C/Distance for complete oscillation) x 360 |
| For phase difference to be particularly valid | Frequency has to be the same |
| Intensity | Power transmitted by a wave per unit area. Watts per square metre |
| Watch out between amplitude and area | Both capital A |
| What property of a wave affects its power? | The greater the amplitude the more power it delivers |
| Relationship between intensity and amplitude | Intensity is proportional to the square of amplitude Be careful with reversing - amplitude is proportional to root of intensity |
| Spherical point source area and intensity | Area over which power is spread is 4 pi r^2 where r is the distance between the wave source and the object detecting the waves I = P/4(pi)r^2 |
| Snell's law | n1sin1 = n2sin2 |
| Where are angles always measured | Between ray and normal (which is perpendicular to the boundary at point where wave is incident) |
| What is refraction | A change of speed of a wave as it enters a medium of a different density |
| What is n? | Refractive index (n = c/v) |
| Critical angle | Must be going more -> less dense. Angles >= critical angle give total internal reflection. At c, angle of refraction is 90 degrees. sinC = n2/n1 |
| Why do materials with high n sparkle? | 1/n, so when n high, low critical angle, so light incident on it more easily totally internally reflected |
| Optical fibre | Light sent through it always at higher than critical angle, so totally internally reflected even when bent slightly |
| Polarisation | Only transverse waves can be polarised. Intensity of unpolarised through polarising filter always half of original. Intensity of polarised light after going through another filter shown by Malus' Law. |
| Malus' Law | Intensity after filter = Original intensity x cos^2(theta) theta is the angle between light electric field and filter transmission axis |
| How to phrase the angle at which light is to filter | Polariser has transmission axis at angle of x with electric field of incident light/second polariser transmission axis |
| How to state intensity of polarised light through filter | I of polarised light through filter decreases from 0 - 90 to zero then increases 90 - 180 |
| What are the wave phenomena | Refraction, diffraction, reflection, polarisation. |
| For all waves | Draw arrows, draw mirror lines, draw normal, label angles, state equations (e.g. i = r) |
| Diffraction on ray diagram (shows direction of energy transfer) | Incident rays parallel go through holes, then spread out in triangle |
| Diffraction on wave front diagram (normal way of showing diffraction) | Wave going towards hole straight. After hole, becomes semicircles going out. (does grow longer each time but doesn't go to end I think) |
| UHF and VHF radio waves | UHF -ultra high frequency (shorter wavelength end of spectrum) VHF - very high frequency |
| Radio waves wavelength | >10^2 m |
| Microwaves wavelength | ~10^-3 |
| Infrared wavelength | ~10^-5 m |
| Visible light wavelength | 400 (violet) - 700 (red) nm Assume even split of ROYGBIV when asked of colour of certain wavelength |
| UV wavelength | ~10^-9 |
| X-rays wavelength | ~10^-10 m |
| Gamma rays | <10^-12 m |
| UVA | Longest wavelength lowest energy. Causes wrinkles and skin aging |
| UVB | Higher energy. Causes skin cancer + sunburn |
| UVC | Absorned by ozone layer. Highest energy. Not blocked by sunscreen. Got through hole in ozone layer |
| EM waves | Electric and magnetic fields perpendicular |
| Question about proportionality of amplitudes based on distance | Amplitude isn't intensity. Remember that I is proportional to A^2 |
| Name for parts of longitudinal waves | Compressions and rarefactions |
| Oscilloscope | oscilloscope is fed a signal, usually using a microphone. The timebase on the oscilloscope can be set on the x axis to represent time and on the y axis to represent the amplitude |
| Polarisation | oscillation of a wave is restricted to one plane only – plane polarised Based on electric field for EM - metal grille free electrons absorb any electric fields not perpendicular. |
| Why only transverse polarised | direction of energy transfer is already in one plane only, whereas in transverse waves, the oscillations occur in many planes, at right angles to the direction of travel |
| Intensity definition | The radiant power passing at right angles through a surface per unit area |
| Speaker behind other speaker. When quiet? | Lambda path difference |
| Speakers facing each other. When quiet? | 1/2 lambda between each because at nodes, which are only every 1/2 lambda (just know this - can draw too) btw nodes only a thing for standing waves |
| Experiments with speakers | Improve sound dampening. Measure total distance between 6 loud points to find 3 wavelengths for standing wave. Reduces measurement uncertainty. |
Created by:
Pyrogearos2