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