Diffraction and interference - slits, gratings, and holes Each section will open directly in a new window. Close each window as you finish. You might find it helpful to print these Essential questions.
1 Diffraction of water waves (barriers and gaps)
Read the diffraction section in the Water waves index.
Essential questions
1 The end of a barrier acts as the source of new waves. (What principle is involved).
2 What is the bending of waves into a shadow region called?
3 What part of the process do many texts leave out of the diagram showing diffraction around a barrier?
4 Under what conditions do interference bands appear in a single slit diffraction pattern?
5 Who was the first to report interference fringes in the edge of a shadow?
2 The two source interference pattern
Read the two source interference pattern section in the Water waves index.
Essential questions
1 How (briefly) may regular ripples be made in a shallow tank?
2 What exactly is happening on a nodal line in terms of the two wave sets?
3 Why are there very dark spots along lines of reinforcement in Ripple tank photographs?
4 What effect do the following have on the separation of nodal lines?
a Increasing the wavelength.
b Increasing the source separation.
c Increasing the distance from the sources.
3 Wavelength measurements using the two source interference pattern
Read the wavelength measurements section in the Water waves index.
Essential questions
1 What exactly are the variables l, d, x and l, in the equation l = dx / l?
2 Why is it not necessary to measure x and l inside the eye, when using two slits to estimate the wavelength of light?
3 What is the path difference for waves on the first line of reinforcement?
4 What shape do the lines of reinforcement have; near the sources and far from the sources?
Read about Young's double slit experiment and about double slit interference patterns in the Waves index.
Essential questions
5 What is the size relationship between red, green and blue patterns, and why are white-light fringes colored?
6 What effect does the single slit diffraction pattern have on the fringes seen with a double slit?
7 What would be the advantages of using laser light for a double slit demonstration in a lecture hall?
4 Diffraction gratings
Read the diffraction gratings section in the Waves index.
Essential questions
1 Why do we not observe single slit diffraction with a diffraction grating?
2 What are the differences and similarities between fringes made with a double slit and a grating?
3 What is a 'first order' grating?
4 What is the grating formula?
5 What could you use to make diffraction gratings for sound, and for X-rays?
5 Interference in thin films
Read the thin films section in the Waves index.
Essential questions
1 What phase changes occur at closed and open boundaries?
2 For what thickness of soap (water) film does no reflection occur?
3 What substance is coated on lenses to reduce unwanted reflection (blooming).
4 What are Newton's rings, and how are they used to test for optical flatness?
6 Rayleigh's Criterion
Reread the Rayleigh's Criterion section in the Waves index
Essential questions
1 What does the diffraction pattern of a small hole look like?
2 What is the Airy disc?
3 Interpret the symbols in the equation x'/L' = 1.22 l/d with a diagram.
4 What is meant by the resolving power of an atronomical telescope and how is it calculated?
7 Scattering
Read the scattering section in the Water waves index.
Essential questions
1 Small objects scatter waves in approximately circular (spherical) patterns. What is meant by small?
2 Why is the sky blue (dark blue in Tibet)?
3 Why is sound (not light) our 'early warning system'?
4 Can laser light be scattered?
8* Diffraction in the atmosphere
Read the essay, Diffraction Halos, in the Essays index.
Essential question
1 How exactly can the average droplet size be estimated from the size of a diffraction halo?
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