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 λ = sd/
When d = 0.200 mm, s = 0.9 mm and D = 280 mm, determine the percentage uncertainty in the wavelength.
Evidence of AND used ✓ «add fractional/% uncertainties» obtains 11% (or 0.11) OR 10% (or 0.1) ✓
Explain how the student could use this apparatus to obtain a more reliable value for λ.
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 «hence s» OR Decrease so the «percentage» uncertainties are reduced ✓
