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## Concept Questions with Answers 8.02 W14D2

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**W14D2:Interference and DiffractionExperiment 6**Today’s Reading Course Notes: Sections 14.4-14.9**Concept Question: Lower Limit?**Using diffraction seems to be a useful technique for measuring the size of small objects. Is there a lower limit for the size of objects that can be measured this way? • Yes – and if we used blue light instead of red light we can measure even smaller objects than the ones we measure using red light • Yes – and if we used blue light instead of red light we couldn’t even measure objects as small as the ones we measure using red light • No**Concept Q. Answer: Lower Limit?**Answer: 1. we have the condition that There is a lower limit imposed by the condition, namely that Once the feature size a is as small as the light wavelength you can’t go to an angle large enough to satisfy the above equation for any m > 0. Blue light has a shorter wavelength than red light, so you can measure smaller sizes using blue light.**Con. Q.: Interference & Diffraction**Coherent monochromatic plane waves impinge on two long narrow apertures (width a) that are separated by a distance d with d > a. • The resulting pattern on a screen far away is shown above, with distantly-spaced zeroes of the envelope, as indicated by the length X above, and closely-spaced zeroes of the rapidly varying fringes, as indicated by the length Y above. • Which length in the pattern above is due to the finite width a of the apertures? • X • Y • X and Y • Neither X nor Y**Concept Q. Ans.: Inter. & Diffraction**• Answer: 1. The ‘envelope’ length X depends on slit width. • You could infer this in two ways. • Slit width a < slit separation d. Angles and size scale inversely, so the bigger features come from a. • Interference patterns are roughly equal in magnitude while diffraction creates a strong central peak. So the envelope is from diffraction.**Concept Q.: Changing Wavelengths**You just observed an interference pattern using a red laser. What if instead you had used a blue laser? In that case the interference maxima you just saw would be • closer together. • further apart. • came distance apart.**Concept Q. Ans.: Changing Colors**Answer: 1. Closer Together Blue light is a higher frequency (smaller wavelength) so the angular distance between maxima is smaller for blue light than for red light