| Date | May 2015 | Marks available | 2 | Reference code | 15M.3.SL.TZ2.3 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Estimate | Question number | 3 | Adapted from | N/A |
Question
This question is about diffraction and resolution.
Monochromatic light is incident on a narrow rectangular slit.
The light is observed on a screen far from the slit. The graph shows the variation with angle θ of the relative intensity for light of wavelength 7.0×10–7m.
Estimate the width of the slit.
On the graph, sketch the variation of the relative intensity with θ when the wavelength of the light is reduced.
State and explain, with reference to your sketch in (b), whether it is easier to resolve two objects in blue light or in red light.
Markscheme
diffraction angle=0.05 rad;
\(b = \left( {\frac{\lambda }{\theta } = \frac{{7.0 \times {{10}^{ - 7}}}}{{0.050}} = } \right)1.4 \times {10^{ - 5}}\left( {\rm{m}} \right)\); (do not accept use of 1.22)
Award [2] for a bald correct answer.
same shape with narrower central maximum;
Ignore height of intensity peak.
blue light gives better resolution;
blue light has shorter wavelength than red light;
giving smaller angle of diffraction;
Allow reverse argument for red light.
Examiners report
Parts (a) and (b) on single slit diffraction were well answered.
Parts (a) and (b) on single slit diffraction were well answered.
However there were fewer correct answers for part (c) where effect of the different wavelengths of red and blue light were sometimes confused and the smaller θ interpreted as poorer resolution. Often the ability to resolve was explained incorrectly in terms of the intensity of the graphs drawn.
