2.4 Superposition, Interference and Diffraction Flashcards
What is the principle of superposition?
When waves interfere, the resultant displacement of the medium at any point is the vector sum of the displacements that each wave would cause at that point in time
Define constructive interference
when the crests and troughs of the waves overlap in phase, producing a wave of greater amplitude
Define destructive interference
when the crest of one wave coincides with the trough of the other, the displacements are in opposite directions
Define a standing wave
A wave produced by the interference if two waves, of the same type, wavelength, amplitude and speed, but moving in opposite directions
What apparatus can be used to show standing waves
Melde’s apparatus
How does Melde’s apparatus work?
A signal generator is used to generate vibrations up and down w small amplitude
- waves travel along the string
- when the waves meet at pulley end of string they are reflected back, creating 2 similar waves travelling in opposite directions
Define a node
a point along a standing wave at which the resultant displacement is always zero
Define an antinode
midway between two nodal points where the amplitude of vibration is a maximum
Define coherence
Coherence means that there is a constant phase difference between the waves
Define coherent light
light waves that are in constant phase
eg. of a source of coherent light
laser light
Define incoherent light
random stream of energy bursts
eg. of source of incoherent light
incandescent sources eg. hot filament
define diffraction
the spreading out of waves as they pass thru a small aperture
As the size of the gap increases, diffraction decreases/increases
decreases
For the same slit width, does blue or red light have a narrower diffraction pattern, and why?
Blue light has a narrower diffraction patter because it has a smaller wavelength
As the width of the slit thru which diffraction takes place decreases, what 2 effect are observed?
width of diffraction pattern increases ( fewer maxima observed)
heights of the maxima decrease (less energy arrives on the screen)
Compared to subsidiary maxima, central maximum is (2)
- many times more intense
- twice the width
Name three precautions for laser usage
- never look directly into ANY laser beam
- pin a notice to lab door ‘LASER IN USE: DO NOT ENTER’
- remove any unnecessary shiny, reflective surfaces ( so laser light not reflect into eye)
In terms of energy, describe the difference between a stationary wave and a progressive wave
In a progressive wave there is a flow of energy in the direction of propagation; no point is permanently at rest.
In a stationary wave, there is no flow of energy; certain pts called nodes are permanently at rest
What are the necessary conditions to produce a standing wave?
Stationary waves are produced by the superposition of two waves, of the same type and having the same amplitude, wavelength and speed but moving in opposite directions
