| Date | May 2014 | Marks available | 2 | Reference code | 14M.3.HL.TZ1.13 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Calculate and Sketch | Question number | 13 | Adapted from | N/A |
Question
This question is about thin-film interference.
A thin air wedge consists of two flat glass plates that form an angle θ of 1.0×10–3rad.
When illuminated with monochromatic light from above, the fringe pattern below is observed in the reflected light. The distance D between two consecutive fringes is 0.30mm.
Calculate the wavelength of the light.
The upper glass plate is now replaced with a curved glass plate. The dotted line represents the upper glass plate used in (a).
Sketch the new fringe pattern in the space below. The fringe pattern of (a) is given for comparison.
Markscheme
\(\lambda = \left( {2D\tan \theta = } \right)2 \times 0.30\tan {10^{ - 3}}\) or \(2 \times 0.30\sin {10^{ - 3}}\);
\(\lambda = 6.0 \times {10^{ - 7}}\left( {\rm{m}} \right)\);
Award [1 max] for use of degrees instead of radians giving \(\lambda = 1.0 \times {10^{ - 8}}\left( {\rm{m}} \right)\).
decreasing distance from left to right;
distance larger than original at left and shorter than original at right;
Examiners report
This part of syllabus does not appear to be well understood. Some candidates calculated the wavelength but many used the formula from the data booklet without explanation. Only a few analysed the situation well. Even the well prepared candidates had a problem with the radian angle unit. The majority of candidates found the change in shape of one of the plates very difficult. Only a few candidates realized that the number of fringes must not change.
This part of syllabus does not appear to be well understood. Some candidates calculated the wavelength but many used the formula from the data booklet without explanation. Only a few analysed the situation well. Even the well prepared candidates had a problem with the radian angle unit. The majority of candidates found the change in shape of one of the plates very difficult. Only a few candidates realized that the number of fringes must not change.
