5.6 Diffraction Flashcards
What is diffraction
The spreading of waves when they pass through a gap or by an edge
Less diffraction occurs when waves pass through a ______ gap than a ______ gap.
Wide
Narrow
Why can we see features further away by using a telescope
This is because a telescope is much wider than the eye pupil meaning that there is much less diffraction when using a telescope
Diffraction of water waves through a gap can be observed using a:
Ripple tank
The arrangement shows that the diffracted waves spread out more if:
The gap is made narrower
The wavelength is made larger
Why are there breaks in each diffracted wavefront either side of the centre
Due to waves diffracted by adjacent sections on the gap being out of phase and cancelling each other out in certain directions
How can diffraction of light by a single slit be demonstrated
By directing a parallel beam of light at the slit
The diffracted light forms a pattern that can be observed on a white screen. The pattern shows a central fringe with further fringes either side of the central fringe.
Where is the intensity of the fringes the greatest
At the centre of the central fringe
Notes about fringes
The central fringe is twice as wide as each of the outer fringes
The peak intensity of each fringe decreases with distance from the centre
Each of the outer fringes is the same width
The outer fringes are much less intense than the central fringe
If the single slit pattern is observed:
Using different sources of monochromatic light in turn, the observations show that the greater the wavelength, the wider the fringes
Using an adjustable slit, the observations show that making the slit narrower makes the fringes wider
The width W of the central fringe observed on a screen at distance D from the slit is given by…
W = (the wavelength of the light / the width of the single slit) * 2D
The width of each fringe is proportional to…
wavelength of light / width of single slit
What happens if the slits are too wide and too far apart
No interference pattern is observed. This is because interference can only occur if the light from the two slits overlaps
What is important for the double slit experiment to work
Each slit must be narrow enough to make the light passing through it diffract sufficiently
The two slits must be close enough so the diffracted waves overlap on the screen
In general for monochromatic light:
The fringe spacing of the interference fringes w = λD / s
The width of the central diffraction fringe W = 2λD / a
