# Physics - Unit 2

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• Created by: charloj2
• Created on: 31-05-15 09:43

## 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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## Standing/Stationary Waves.

• 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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## Diffraction

• 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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## Diffraction Gratings.

• 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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