3.7 Diffraction Flashcards
What is diffraction?
The way that waves spread out as they come through a narrow gap or go around obstacles
What does the amount of diffraction depend on?
The wavelength of the wave compared with the size of the gap
What happens when the gap the wave is travelling through is a lot bigger than the wavelength?
Diffraction is unnoticeable
What happens when the gap the wave is travelling through is several wavelengths wide?
Noticeable diffraction occurs
What happens when the gap the wave is travelling through is the same size as the wavelength?
Maximum diffraction occurs
What happens when the gap the wave is travelling through is a smaller than the wavelength?
The waves are mostly just reflected back
Why do you have the shine light through a very narrow slit to get noticeable diffraction?
Light has a very small wavelength, so needs a very small gap for diffraction to occur
How are waves diffracted around an obstacle?
When a wave meets an obstacle, you get diffraction around the edges. Behind the obstacle is a ‘shadow’ where the wave is blocked. The wider the obstacle compared with the wavelength of the wave, the less diffraction you get, and so the shadow is longer.
What type of light should you use to observe a clear diffraction pattern?
A monochromatic, coherent light source
Why is monochromatic light used for diffraction patterns?
Monochromatic light is light of a single wavelength and frequency and so a single colour. If you use non-monochromatic light, different wavelengths diffract by different amounts and the pattern produced will be unclear.
What provides monochromatic light?
- laser light is monochromatic and coherent
- a colour filter in front of white light also works, but a laser is clearer
What is a downside to using laser light to get a diffraction pattern?
Laser beams are very powerful and could damage your eyesight, so you need to take precautions
How is the diffraction of light shown?
By shining a laser beam through a very narrow slit onto a screen. If the wavelength of a light wave is roughly similar to the size of the slit, you get a diffraction pattern of light and dark fringes.
How does a diffraction pattern (of monochromatic and coherent light) look?
The pattern has a bright central fringe (central maximum) with alternating dark and bright fringes on either side of it
Why do bright fringes occur on a diffraction pattern?
Constructive interference
Waves from across the width of the slit arrive at the screen in phase
Why do dark fringes occur on a diffraction pattern?
Total destructive interference
Waves from across the width of the slit arrive at the screen completely out of phase.
What happens when you diffract white light?
White light is a mixture of different colours, each with different wavelengths.
When white light is shone through a narrow slit, all of the different wavelengths are diffracted by different amounts.
This means that instead of getting clear fringes, you get a spectra of colours.
Why is the central maximum in a single-slit diffraction pattern the brightest?
The intensity of light is highest in the centre
What is the definition of intensity?
The power per unit area
What does an increase in intensity mean for monochromatic light?
All photons have the same energy, so an increase in the intensity means an increase in the number of photons per second. So there are more photons per unit area hitting the central maximum per second.
What happens to the central maximum if you increase the slit width?
Increasing the slit width decreases the amount of diffraction.
This mean the central maximum is narrower and the intensity of the central maximum is higher.
What happens to the central maximum if you increase the wavelength?
Increasing the wavelength increases the amount of diffraction.
This means the central maximum is wider, and the intensity of the central maximum is lower.
