Quick answer: Superposition means waves add their displacements where they meet. This Physics 11-12 lab on wave interference gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How can you measure the wavelength of light without a spectrometer? Use a diffraction grating of known spacing and a laser pointer, measure the positions of several orders on a wall and combine them to find λ.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB C.3
Aim
To measure the wavelength of a laser pointer using a diffraction grating of known line spacing.
Equipment
- Low-power laser pointer (class 2 or lower)
- Diffraction grating, 300 lines per mm, in a holder
- Metre rule and a plain white wall or screen
- Clamp stand and a spirit level
Safety
Never shine the laser at anyone’s eyes or at shiny surfaces. Switch it off when not taking readings, remove jewellery from the beam path and keep other students out of the line of the beam.
Method
- Clamp the grating at right angles to the beam, 1.000 m from the wall.
- Switch on the laser and mark the central spot (n = 0).
- Mark the first-, second- and third-order spots on each side of the centre.
- Measure the distance x from the centre to each marked spot, and average the left and right values for each order.
- Record the distance L from grating to wall.
- Calculate θ = tan⁻¹(x ÷ L) and λ = d sin θ ÷ n for every order.
Results
Sample measurements, averaged left and right, for L = 1.000 m and 300 lines per mm:
| Order n | x / m | θ / ° | sin θ |
|---|---|---|---|
| 1 | 0.200 | 11.31 | 0.1961 |
| 2 | 0.423 | 22.93 | 0.3896 |
| 3 | 0.723 | 35.87 | 0.5859 |
Analysis questions
Question 1 Slit spacing
Calculate d for a grating of 300 lines per mm in metres.Question 2 Angle of the first order
Use the table to calculate θ for n = 1, and check the sin θ column.Question 3 Wavelength from each order
Calculate λ from each order and find the mean.Question 4 Graph of sin θ against n
Plot sin θ against n and find the gradient. State how the gradient relates to λ.Question 5 Uncertainty in x
The uncertainty in each distance x is ±2 mm. Find the percentage uncertainty in x for n = 1 and say which order gives the best precision.Question 6 Small-angle approximation
A student uses sin θ ≈ x/L for the third order. Calculate the error this causes in λ.Question 7 Comparison
The laser label states 650 nm. Compare your mean with this value and suggest a reason for any difference.Question 8 Improving the experiment
Suggest two improvements.
