3.3.2.2 Waves: Diffraction Flashcards
What is diffraction?
When waves spread out as they come through a narrow gap or around obstacles
What does the amount of diffraction depend on?
The wavelength of the wave compared to the size of the gap
How does diffraction varies as you vary the size of the gap?
When gap is a lot bigger than the wavelength, diffraction is unnoticeable.
You get noticeable diffraction when the gap is several wavelengths wide
Most diffraction happens when the gap is the same size as the wavelength.
When the gap is smaller than the wavelength, the wave is mostly reflected back
Describe diffraction around obstacles
When a wave meets an obstacle, you get diffraction around the edges, behind the obstacle is the ‘shadow’, where the wave is blocked.
How does diffraction vary around obstacles?
The wider the obstacle compared to the wavelength, the less diffraction, so the longer the shadow
What should you use if you want to observe a clear diffraction pattern?
Monochromatic, coherent light source.
Or use a colour filter on white light
What is monochromatic light?
Light of a single wavelength and frequency
Why is white light not good for diffraction patterns?
It has many different wavelengths that diffract by different amounts, so unclear diffraction pattern
Describe the diffraction pattern when shining a laser through a single slit
Pattern has a bright central fringe (central maximum) with alternating bright and dark fringes either side of it. This pattern is due to interference - bright fringes are due to constructive interference, where waves arrive at the screen in phase, dark fringes are due to total destructive interference, where waves across the width of the slit arrive at the screen completely out of phase
Describe what happens when white light passes through a narrow slit
Diffraction pattern produced, different wavelengths are refracted by different amounts, so pattern produced is more red away from the central maximum, and more blue towards the central maximum, because shorter wavelengths diffract less
What is intensity?
Power per unit area
How does the width of the central maximum vary?
With the width of the slit and the wavelength of light being diffracted - Increasing slit width decreases amount of diffraction so central maximum is narrower and intensity of central maximum is higher
- Increasing wavelength increases diffraction, central maximum is wider and intensity of central maximum is lower.
What is a diffraction grating?
A diffraction grating is a piece of glass or plastic with a large
number of equidistant, closely parallel ‘lines’ or slits.
Why is diffraction grating better than double slit?
Interference pattern produced is sharper (bright areas are brighter and dark areas are darker).
Why are sharper fringes better?
They make for more accurate measurements
