The Nature of Waves
- A progressive wave moves energy away from it's source.
- Can be reflected and Refracted
- Intensity is a measure of How much energy a wave is Carrying (The rate og flow of energy per unit area at right angles to the direction of travel of the wave. It's measured in Wm-2)
- Intensity is proportional to the amplitude of the wave.
- Frequency is the inverse of the period. 1Hz = 1 S-1
- Speed = Distance/Time OR Speed = Wavelength xFrequency
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Longitudinal & Transverse Waves.
- In Transverse Waves the Vibration is at Right Angles to the direction of travel
- Longitundinal waves the Vibrations are along the direction of travel. (Compressions - The waves clump together, Rarefaction - the waves are spread out)
- A polarised wave only ossiclates in one direction - the other direction waves are filtered out. Can pnly happen to transverse waves.
- When light reflects it's partially polarised. The intensity changes when partially polarised light is reflected.
- Materials can rotate the plane of polarisation (the pplane in which a wave moves and vibrates) EXPERIMENT: put the material between two polarising filters.
- Rotating the Plane of Polarisation affects the intensity.
- Amplitude = A0Cos0 - A0 is the amplitude of the initial wave, Cos0 is the angle the plane has rotated.
- The intensity is proportional the the amplitude squared.
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The Electromagnetic Spectrum
- In a vacuum, electromagnetic waves travel at 2.998x10^8 ms2, but slower in other medias.
- They have electric and magnetic fields are at right angles to each other and the direction of travel
- The longer the wavelength the more obvious it's characteristics.
- Energy is proportional to the Frequency.
- The higher the energy, the more dangerous it is.
- UVA has the lowest frequency and is the least damaging. Can age skin.
- UVB - more dangerous, absorbed by DNA molecules causing mutations & Cancer. SUNBURN.
- UVC - Ionising, causes cell mutations and destruction and cancer - blocked from the ozone layer.
- UVA triggers the release of Melanin - tanning agent.
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Superposition and Coherence.
- Happens when two waves pass through each other. Grow or shrink. The displacement Combines.
- Constructive = Growing
- Distructive = Flat line
- To get interference pattern the two sources must be coherent
- COHERENT = Same wavelenght, frequence and a fixed phase difference.
- Constructive interference depends on path difference.
- Constructive interference happens when there's a whole number of wavelenghts
- Destructive is when there's a half wavelenght.
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- Progressive Wave reflected off a Boundary
- The superposition of two progressive waves with the same wwavelenght moving in opposite directions.
- NO energy is transmitted
- Resonant frequencies is when you get a standing wave.
- Node = No amplitude
- Antinode = Yes amplitude.
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- The amount of diffreaction depends on the wavelenght of the wave compared to the size of the gap.
- You get the most diffraction when the gap is the same size as the wavelenght.
- If the gap is smaller the waves are just reflected back
- Wave meets obstacle - shadow behind the obstacle where there are no waves. The wider the obstacle the longer the shadow.
- when the Wavelenght = hole you get a diffraction pattern of dark and light. The narrower the slit, the wider the diffraction pattern.
- You need a coherent light source
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Two Source Interference.
- Double Slit Experiment - LAser light is coherent and monochromatic (only one wavelength). THe slits have to be around the same size as the wavelength.
- Dark & Light depends on whether it's constructive or destructive.
- Work out the wavelength with YOung's Double slit formula
- Fringe spacing = distance from the slits to the screen x wavelength/spacing of the slits.
- The experiment was evidence for the wave nature of light.
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- Interference Patterns get sharper when you diffract through more slits.
- Monochromatic light gives sharp lines
- slit distance x sin0 = nth prder maximum x wavelength
- The larger the wavelength the more the pattern will spread out,
- The coarser the grating the less the pattern will spread out.
- If sin0 is more than one that order doesn't exist.
- Shining white light light through grating produces a spectra
- White light is made up of different colours so when it's diffracteed the different colour wavelenghts spread out
- Each order is a spectrum - red on the outside, violet on the inside.
- Zero order stays white.
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