A student is investigating the reflection of light from a compact disc (CD), as shown in Fig. 1.1. [Figure 1.1] The student observes on the screen a pattern of maxima and minima which is similar to that produced by a diffraction grating. The distance h is measured to one of the maxima. It is suggested that the relationship between h and the wavelength λ of the incident light is h = nλ/d + B where n is the order of the maximum and d and B are constants. Design a laboratory experiment to test the relationship between h and λ. Explain how your results could be used to determine values for d and B. You should draw a diagram, on page 3, showing the arrangement of your equipment. In your account you should pay particular attention to • the procedure to be followed, • the measurements to be taken, • the control of variables, • the analysis of the data, • any safety precautions to be taken.

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

1. [1] λ is the independent variable and h is the dependent variable or vary λ and measure h
2. [1] keep angle of incidence of light on to CD constant
3. [1] labelled diagram of workable experiment including: light source, CD and screen
4. [1] method to support light source
5. [1] light source labelled and at least one other label
6. [1] method to change wavelength, e.g. use filters, different lasers/LEDs
7. [1] method to determine λ e.g. read from filter/label or experiment e.g. Young's slits, diffraction grating
8. [1] measure h with a rule or labelled rule shown in diagram close to screen with h indicated
9. [1] plot a graph of h against λ or plot a graph of h against nλ
10. [1] d = n/gradient or d = 1/gradient (consistent with graph)
11. [1] B = y-intercept
12. [1] safety precaution linked to exposure of eyes to (intense) light, e.g. avoid looking (directly) at the (intense) light source or wear dark glasses or wear goggles to protect eyes from light
13. [1] keep distance from CD to screen constant
14. [1] keep n constant
15. [1] method to identify same order, e.g. identify the zero order and count the number of maxima to where h is measured
16. [1] equation to determine λ e.g. Young's slits/diffraction grating formula with λ as subject
17. [1] use intense light source/collimated light source
18. [1] perform experiment in a dark room
19. [1] clean surface of CD/check that there are no scratches
20. [1] detail on measuring h (e.g. measure to the top and bottom of the maximum and average)
21. [1] relationship valid if a straight line (with a y-intercept)