1. Concept Questions with Answers 8.02 W14D2

2. W14D2: Interference and Diffraction Experiment 6
Today’s Reading Course Notes: Sections
Class 18

3. Concept Question: Lower Limit?
Week 15, Day 1
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
Class 35

4. Concept Q. Answer: Lower Limit?
Week 15, Day 1
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.
Class 35

5. Con. Q.: Interference & Diffraction
Week 15, Day 1
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
Class 35

6. Concept Q. Ans.: Inter. & Diffraction
Week 15, Day 1
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.
Class 35

7. Concept Q.: Changing Wavelengths
Week 15, Day 1
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.
Class 35

8. Concept Q. Ans.: Changing Colors
Week 15, Day 1
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
Class 35