# AS Physic Unit 2 Waves

• Waves
• The nature of waves
• Progressive wave
• A pattern of disturbances travelling through a medium and carrying energy with it.
• It involves the particles of the medium oscillating about equilibrium position.
• It does not involve the transfer of matter.
• Transverse Waves
• Particle oscillations are at right angles to the direction of travel of the wave.
• Example of a transverse waves is light.
• Longitudinal waves
• The particles oscillations are parallel to the direction of travel of the wave.
• An example of a longitudinal wave is a sound waves.
• Displacement - distance graph is a snapshot of the wave at a particular time.
• A displacement-time graph follows the movement of one particle over a given time.
• Characteristic
• Wavelength
• the minimum distance between two points on the wave oscillating in phase.
• Frequency
• the number of cycles of a wave that pass a given point in one second
• Amplitude
• distance from the middle of the wave to the crest
• Speed
• the distance traveled by the wave one second
• Phase
• The relationship between the pattern of vibration of two points on a wave.
• Points oscillating in phase  will be multipliers of 360
• Points oscillating in anti phase will be 180  out of phase.
• Wave Equation
• Speed = frequency x wavelength
• Speed is in ms                   Frequency is in Hz               Wavelength is in m
• Polarization
• A transverse wave in which the particle oscillations occur in only one of the directions at right angles to wave propagation.
• If you try to view polarized light through a second polarizer filter rotating 360  the what you would see would be alternating light and dark.
• Wave poperties
• Reflection
• As the waves strike a plane barrier they are reflected. This is very similar to a beam of light reflecting on a plane mirror.
• If a cruve barrier is used, the waves can be made to converge at a point. the angle of incidence is equal to the angle of reflection.
• Refraction
• the change in direction of a wave at a boundary between two materials. This is caused by the change in speed.
• Diffraction
• The spreading out of a wave when it meets an obstacle into regions where it would not be seen if it moved only in straight lines.
• principle of Superposition
• If the waves from two sources occupy the same region then the total displacement at a point is the vector sum of the displacements of the individual waves at that point.
• there are two types of interference when this takes place. Constructive and Destructive interference.
• Constructive Interference - if two waves arrive at a point in phase, have the same frequency and equal amplitude.
• destructive Interference - if the peak of one wave arrive at the same time as the toughs fro the other. There will be a smaller amplitude and sometimes cancel out
• Coherence
• There is a constant phase difference and same frequency.
• An example is a laser light.
• Path Difference
• A measure of the distance between two waves arriving at a point in terms of their wavelength.
• If we consider the effect of superposition at many points, an interference pattern will develop showing areas with constructive and destructive interference.
• if the path difference is S2P - S1P equals a whole number of wavelengths then the waves arriving at point P in phase will produce constructive interference
• If S2P - S1P is equal to multipliers of half wavelengths then waves arriving in anti-phase at point P will produce destructive interference.
• Young's Double Slit Experiment
• Two light sources at the double slits are produce. Because these two light sources originate from the same primary source, they are coherent and create an interference pattern.
• Bright fringes can be created when the diffracted waves overlap and are caused by constructive interference.
• Dark fringes can also be created from destructive interference.
• The diffraction grating
• Its a plate on which there are a very large number of parallel, identical and closely spaced slits.
• If monochromatic light is incident n this plate, a pattern of narrow bright fringes are produces.
• As there are many slits, the bright fringes are extremely narrow, and usually much further apart than the double slit experiment.
• dsin0 = n
• d = slit width n= order        number           = wavelength 0 = angle
• A very small d makes the orders much further apart and a large number of slits makes the beams much brights and sharper.
• With white light
• Each wavelength making up the white light is diffracted differently. Red is diffracted the most and violet is diffracted the least.