Diffraction and Two source interference Flashcards
What is diffraction?
The spreading of waves when they pass through a gap of a similar magnitude to their wavelength
What happens if the gap is a lot bigger than the wavelength?
The diffraction is unnoticeable
What happens if the gap is several wavelengths wide?
Noticeable diffraction
What happens if the gap is the same size as the wavelength?
Most diffraction occurs
What happens if the gap is smaller than the wavelength?
The waves mostly reflect back
What happens when a wave meets an obstacle?
- Diffraction happens around the edges
- A shadow is formed around the obstacle where there are no waves
- The wider the obstacle compared to the wavelength the less diffraction you get
What is a monochromatic light?
A light with a single frequency
What pattern do you get when a laser diffracts through a slit?
A central maximum and alternating dark and bright fringes on either side
Why are bright fringes produced? Comment about their phase relationship
constructive interference, where waves arrive in phase
Why are dark fringes produced? Comment about their phase relationship
Destructive interference, where waves arrive out of phase
What effects the width of the central maximum?
Width of the slit and the wavelength of the light
What happens if you increase the width of the slit? (Wavelength was smaller than the width previously)
Decrease diffraction, central maximum is narrower, intensity of the central maximum is higher
What happens if you increase the wavelength? (The wavelength was smaller than the slit previously)
Increase diffraction, central maximum is wider and intensity of the central maximum is lower
Why do you need a monochromatic light source?
Because if the light isn’t monochromatic, different wavelengths will diffract by different amounts and the pattern produced won’t be very clear
Why do houses in valleys or next to mountains get bad TV and radio signals?
Because the shorter wavelength waves don’t diffract much around the wide obstacles
What happens if you shine white light through a slit?
Bright central maxima with dark and spectra fringes on either side
What is intensity?
Power per unit area
What does an increase in intensity of light mean?
An increase in the number of photons per second
Why is the central maxima brighter than the other bright fringes?
There are more photons per unit area hitting the central maximum per second than the other bright fringes
What does coherent mean?
Two waves that have the same wavelength and amplitude and a fixed phase 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 does the path difference have to be to get constructive interference? Why?
A whole number of wavelengths, because the two waves are in phase
What does the path difference have to be to get total destructive interference?
An odd number of half wavelengths
What is the path difference of two waves that produce constructive interference?
1 wavelength
What is the path difference of two waves that produce destructive interference?
lambda/2
