Question

Interference with light We aim a red (620 nm) laser onto a small screen that has two slits that are 0.1 mm apart. Each slit has a width of 0.03 mm. The light coming out of the two slits is projected onto a big screen a distance X from the slits. In the photo on the right you can see the pattern that’s visible on the big screen. Using a ruler, we determine that the distance between two adjacent bright spots is equal to 12 mm.

[1 point] What can you say about the two slits? [ ] Since the pattern is horizontal, the slits are horizontal as well. [ ] Since the pattern is horizontal, the slits must be vertical. [ ] To obtain this pattern we must have one horizontal and one vertical slit. [ ] Not enough information is given to make any conclusion about the slits.

[5 points] Determine the distance X between the slits and the big screen.   The red laser has a wavelength of 620 nm and its beam has a circular cross section with a diameter of 0.8 mm. The power rating of the laser “1.5 mW“ (i.e. 1.5 milliwatts) which we may assume is the power of the light coming out of the laser.

[4 points] Calculate how many photons are being emitted by the laser each second. When we aim the 620-nm laser on a special piece of glass, we find that none of the light is being reflected! That’s because the glass has been coated with a very thin layer of MgF2 which has a refractive index of 1.37, while the glass itself has a refractive index of 1.52. [5 points] Determine the minimum thickness of the MgF2 layer. Give your answer in nanometers.  

Answer 1