| Date | May 2013 | Marks available | 3 | Reference code | 13M.3.SL.TZ1.21 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 21 | Adapted from | N/A |
Question
This question is about laser light.
Laser light is monochromatic and coherent. Explain what is meant by
(i) monochromatic.
(ii) coherent.
A beam of laser light is incident normally on a diffraction grating which has 600 lines per millimetre. A fringe pattern is formed on a screen 2.0 m from the diffraction grating.
The fringe pattern formed on the screen is shown below.
Determine the wavelength of the laser light.
Markscheme
(i) single frequency/wavelength / narrow range of frequencies/wavelengths;
(ii) in phase;
constant phase difference/relationship;
Award [2] for any correct reference to constant phase difference.
\(\theta = {\tan ^{ - 1}}\left[ {\frac{{0.65}}{{2.0}}} \right]\left( { = 18^\circ } \right)\);
recognition that n=1;
\(d = \frac{1}{{600}}\left( { = 0.0017{\rm{mm}}} \right)\);
λ=(=d sin θ = 0.0017 x sin18º) = 520(mm);
Examiners report
(i) was very well done.
Many candidates got one mark for “in phase” in (ii), but few got two marks for “constant phase difference” even though this is the standard definition for coherence. This is a standard question that has been in several papers over the last few years. Teachers and candidates should take note of the following clarification - there can be two sources which are in phase (that is, have exactly the same phase) and so are coherent. There can also be two sources which have different phases at any one instant but have a constant phase difference/relationship and so are coherent. Thus “constant phase difference” is a better answer than “in phase” because “in phase” is a special case of “constant phase difference” (that is, the constant phase difference is zero).
Most candidates used the equation for double-slit interference rather than the equation for diffraction grating. Candidates appear to be far more comfortable with the double-slit case than the diffraction grating case.
