Question 19M.3.SL.TZ2.3
| Date | May 2019 | Marks available | [Maximum mark: 4] | Reference code | 19M.3.SL.TZ2.3 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Determine, Explain | Question number | 3 | Adapted from | N/A |
A student uses a Young’s double-slit apparatus to determine the wavelength of light emitted by a monochromatic source. A portion of the interference pattern is observed on a screen.
The distance D from the double slits to the screen is measured using a ruler with a smallest scale division of 1 mm.
The fringe separation s is measured with uncertainty ± 0.1 mm.
The slit separation d has negligible uncertainty.
The wavelength is calculated using the relationship .
When d = 0.200 mm, s = 0.9 mm and D = 280 mm, determine the percentage uncertainty in the wavelength.
[2]
Evidence of used ✔
«add fractional/% uncertainties»
obtains 11 % (or 0.11) OR 10 % (or 0.1) ✔
A very easy question about percentage uncertainty which most candidates got completely correct. Many candidates gave the uncertainty to 4 significant figures or more. The process used to obtain the final answer was often difficult to follow.
Explain how the student could use this apparatus to obtain a more reliable value for λ.
[2]
ALTERNATIVE 1:
measure the combined width for several fringes
OR
repeat measurements ✓
take the average
OR
so the «percentage» uncertainties are reduced ✓
ALTERNATIVE 2:
increase D «hence s»
OR
Decrease d ✓
so the «percentage» uncertainties are reduced ✓
Do not accept answers which suggest using different apparatus.
The most common correct answer was the readings should be repeated and an average taken. Another common answer was that D could be increased to reduce uncertainties in s. The best candidates knew that it was good practice to measure many fringe spacings and find the mean value. Quite a few candidates incorrectly stated that different apparatus should be used to give more precise results.