5.4 Double slit interference Flashcards
What type of waves do the double slit need to emit in order to observe the interference of light?
» Coherent source of waves
What are coherent sources of waves?
» Which means that they emit light waves with a constant phase difference
» And the same frequency and wavelength
What can you observe in the double slit experiment?
» Young’s fringes
» Spaces of dark fringes and bright fringes
What happens if the single slit which is used to illuminate the double slit becomes too wide?
» Dark fringes of the double slit pattern become narrower than the brige fringer
» Due to each part of the slit being displace slighting from the pattern
How are the fringes formed?
» Due to the interference of light
How is a bright fringe formed?
» Superposition of the 2 waves
» Diffraction from waves overlap
» Waves from each slit will arrive in phase
» Causing points which interfere construcitvely
How is a dark fringe formed?
» Superposition of the 2 waves
» Diffraction from waves overlap
» Waves from each slit will arrive 180 degrees out of phase
» Causing points which interfere destructively
What is the fringe seperation?
» The distance from the centre of a bright fringe to the centre of the next bright fringe
What is the equation for fringe seperation?
» w = (wavelength x D)/s
» w = fringe seperation
» wavelength = wavelength
» D = The distance from the slit to the screen
» s = The slit spacing - distance between the centres of the slits
How can you measure the fringe seperartion, w?
» Measure across several fringes from the centre of a dark fringe to the cetnre of another dark fringe
» Divide by the number of fringes you saw
How can 2 loudspeakers demonstrate interference?
» Must be connected to the same signal generator
» As they are coherent source of sound waves
» You can detect poitns of cancellation and reinforcement as you move across in front of the speakers
What is path difference?
» The difference in the distance travelled by two waves
What is the purpose of putting a single slit before the double slit in the young’s experiment?
» To make sure the light has a fixed path difference
» Also acts as a single source, diffraciton light to both slits
What is the purpose of the filter in the young’s experiment?
» To make light monochromatic
What is the consequence of having the path difference has a whole number?
» Having a difference of a whole number of wavelengths, such as M x lambda
» Means the 2 sources of light interfere constructively
What is the consequence of having the path difference has a whole number and a half?
» Having the difference as a whole number and a half of wavelength such as (n+1/2) x lambda
» Means the waves interfere destructively
How can the appartus be changed to obtain an accurate value of the wavelength?
» Increase D
» Measure across more than 2 maxima
» Therefore reduces percentage error
If a microwave receiver was placed at the points of dark fringes, explain why the signal strength falls to a minimum?
» Path difference for the 2 waves
» Give rise to phase difference
» Destructive interference occurs
How can you find the frequency of a wave by having the path difference?
» Path difference = 2 x wavelength
» Find wavelength
» Use C/wavelength = frequency
Why at some points about the fringes, the minimum intensity is not zero?
» Intensity decreases with distance
» One wave may travel further than the other wave
» Amplitudes do not cancel out
What is the importance of the double slit experiment?
» Suggests that the nature of light is a wave rather than a particle
Explain what happens when a laster emitting visible light has a shorter wavelength, how will this affect the spacing of the maxima on the screen?
2 Marks
» Shorter wavelength means less diffraction due to w being directly proportional to wavelength
» Therefore the spacing of the maxima would decrease as the maxima would become closer together
