| Date | May 2011 | Marks available | 1 | Reference code | 11M.3.SL.TZ2.21 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | State | Question number | 21 | Adapted from | N/A |
Question
This question is about interference of light.
Two coherent narrow beams of light pass through two identical evacuated tubes, as shown below.
The two coherent narrow beams are brought to a focus at point P on a screen.
State what is meant by coherence.
State, with reference to the wavelength, the condition that must be satisfied for a bright fringe to be formed on the screen at point P.
Air is allowed to enter gradually into one of the evacuated tubes. The brightness of the light at point P is seen to decrease and then increase again repeatedly.
(i) State the effect on the wavelength of the light in the evacuated tube as the air is introduced.
(ii) Suggest why there is a variation in the brightness of the light at point P.
Markscheme
constant phase difference;
path difference between beams=nλ, where n is an integer/is one wavelength;
(i) wavelength decreases;
(ii) (effective/optical) path/phase difference changes;
