3.3.2.1 Waves: Interference Flashcards
What is two-source interference?
When waves from two sources interfere to produce a pattern.
What are the conditions needed to get clear interference patterns from two sources?
Two source must be monochromatic and coherent (same wavelength and frequency and fixed phase difference)
What is coherent?
Two waves are coherent if they have the same wavelength and frequency and a fixed phase difference
On the pattern what determines whether you get constructive or destructive interference?
How much further one wave has travelled than the other wave to get to that point (path difference)
What is path difference?
The amount by which the path travelled by one wave is longer than the path travelled by the other wave
What is a maxima?
Points at an equal distance from two sources, where constructive interference occurs. Also occurs at any point where the path difference is a whole number of wavelengths
What is minima?
At points where the path difference is half a wavelength/ 1.5 wavelengths etc. waves arrive out of phase and total destructive interference occurs.
When does constructive interference occur?
When there is a path difference of nlambda
When does destructive interference occur?
When there is a path difference of (n+1/2)lambda
How to see interference patterns with sound waves?
Have two speakers connected to an amplifier. The interference pattern is produced at a black line. This can be observed by having someone walk in a straight line parallel to speakers, they’ll observe quiet areas where destructive interference occurs and louder areas where constructive interference occurs.
How to see interference patterns from microwaves?
You can use two microwave transmitter cones attached to the same signal generator - you also need a microwave receiver probe (you move it parallel to the transmitter cones to see a pattern)
How can you see two-source interference of light?
Use two coherent light sources or shine a laser through two slits.
Why is a laser used to generate interference patterns?
It is monochromatic and coherent
What must be true about the slits used to shine a laser?
They must be about the same size as the wavelength of the laser so that it is diffracted, making the light from the slits act like two coherent point sources.
What happens if you use white light instead of a laser?
Bright central maxima, with alternating bright fringes and dark fringes on either side, of lower intensity, but are spectra. The bright fringes are more blue nearer to the central fringe, and more red further away
How can you create two coherent light sources using a single laser?
Shine a laser through two slits. They have to be about the same size as the wavelength in order for light to be diffracted, making the light from the slits act like two coherent point sources.
Describe some laser safety precautions
Don’t shine laser towards a person
Wear laser safety goggles
Avoid shining laser beam at reflective surface
Have a warning sign on display (?)
Turn off when not needed
What is the double slit formula?
w=lambdaD/s
((fringe spacing = wavelength*distance between slits and screen)/distance between slits)
How can you derive the double slit equation?
Consider path difference between the waves from each slit when light interferes constructively to form the first bright fringe. Path difference between the light waves at the first bright fringe is lambda
What is the evidence for the wave-like nature of light?
Young’s double slit experiment proved that light could diffract and interfere, hence wave
