Manipulating Waves Flashcards
How do polarised waves oscillate?
-The polarised wave only oscillates in the plane of polarisation
-Polarising a wave into one direction is called plane Polarisation
-Ordinary light waves are a mixture of different directions of vibration
-For polarised light, the direction of the vibrations is perpendicular to the direction of the propagation of light
-Only happens to transverse waves, which is seen by polarising light
What are polarising filters?
-Light waves can be polarised by a polarising filter
-There are two filters, if they are at different angles to one another, less light gets through
-If they are at right angles, no light goes through as the direction if cut off
-When it is 45 degrees, it has half the intensity
How can you use polarising filters to remove reflections?
-If you direct an unpolarised light at a reflective surface, then view through a polarising filter, the intensity changes with the orientation of the filter
-The intensity changes because at angle, light is partially polarised
-Used to remove unwanted reflections in photography and in polaroid sunglasses to remove glare
How does polarisation help television and radio signals?
-On aerials, the rods are horizontal, as the tv signals are polarised by the orientation of the rods on the broadcasting aerial
-To receive a strong signal, you have to line up the rods on the receiving signal with them on the transmitting erika
-If you tune an aerial, it has to align
How are microwaves polarised?
-As microwaves wavelength’s are too long, metal grills are used to polarise them
-The plane of polarisation here is at right angles instead of parallel
-The intensity drops to 0, when the plane of polarisation is at right angels, because the grille absorbs the energy
What is an interface?
-It is the boundary between two different media
-When a wave passes through an interface, some energy is reflected and some is transmitted
-If the media have two very different densities, more energy is reflected
How do ultrasound scans work?
-Use short pulses of ultrasound radiation to form images inside the body
-Is directed using a transducer, a gel is used to make the densities more similar, and more is transmitted
-When they reach then interface inside your body, a computer calculated the distance inside the body
-It uses the information to build up an image inside out body
How do you make an ultrasound more efficient?
-The pules must be short, so the reflected waves do not reach the transducer while it is transmitting
-The gap between pulses must be long
-Shorter wavelengths diffract less, so spread out less
What is superposition?
-When two or more waves pass through each other
-The displacements due to each wave combine, and then split again
-The resultant displacement equals the vector sum of the individual displacements
What is interference?
-When two or more waves superpose, it is called interference
-Constructive interference is when it is both a crest or both a trough
-Destructive interference is when it is a crest and a trough and it cancels
What is it to be in phase?
-If both points are in phase they are at the same point in the wave cycle
-Have the same displacement and velocity
-Two points with a phase difference of zero or a multiple of 360 or 2pi
-Two different waves can be in phase, if they come from the same oscillator
What is it to be out of phase?
-A phase difference of odd-number multiples of 180 or 1pi, are exactly out of phase
What is it to be a coherent wave?
-In order for interference patterns to be clear, the sources must be coherent
-This means they have the same wavelength, frequency and fixed phase difference
-When waves are coherent, in phase and travelling the same direction, they create wavefronts, which are imaginary planes which cut across them all, and joining up the points which are in phase, the distance is one wavelength
What is path difference?
-Interference depends on how much further one wave has travelled the the other to get to that point
-The amount one travelled more than the other, is the path difference
-At any point an equal distance from both sources, you get constructive interference, or at a whole number of wavelength, as they arrive in phase
-At points where the path difference is an odd number of half wave lengths, the waves arrive out of phase and you get destructive interference
How can you observe interference?
-Connect two speakers to the same oscillator, so the sound waves are coherent and in phase, and place them in line
-Walk slowly in front
-Where the sound is loudest, the path difference is whole
-The sound when quiet, the path difference is an odd number of half wavelengths
