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Unformatted text preview: Chapt. 37 Diﬀraction 281
intensity I (r ) = 0: e.g., a region of shadow. What can you say about the diﬀraction
patterns cast by the two masks in regions of shadow.
037 qfull 00300 1 3 0 easy math: gyrinid beetle
Extra keywords: (HRW-916:36E)
5. Tropical gyrinid beetles (whirligig beetles) have scales whose alignment makes up a scattering (or
reﬂection) diﬀraction grating. The diﬀraction pattern from the grating may confuse predators.
Say 0.555 µm light (green light: HZ-56) is incident normal to the beetle grating and a 1st order
maximum is seen at an angle of 26◦ from the normal.
a) What is the grating spacing?
b) What angle of the 2nd order maximum?
c) Ordinarily the grating will scatter white light. What would you see if you projected this
light on a screen? What would you see if you were looking at the beetle? Assume the
beetle grating works like an ideal grating. Real beetles may be less than ideal gratings.
037 qfull 00400 1 3 0 easy math: X-ray diﬀraction, Bragg law
Extra keywords: (HRW-917:53E)
6. You shine 1.2 ˚ X-rays on a crystal of lithium ﬂoride.
a) The 2nd order reﬂection occurs at a Bragg angle of 28◦ . What is the interplanar spacing
in Angstroms for this reﬂection? HINT: Recall that the convention in X-ray diﬀraction is
to measure angle from the surface of scattering, not from the normal as in visual optics.
b) What is the angle of the 1st order reﬂection for the interplanar spacing? ...
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