A student carries out a Young's double-slit experiment. Laser light passes through two slits 0.40 mm apart. The fringes are observed on a screen 2.5 m from the slits. The student measures the distance across 10 fringe spacings as 34 mm.
(a) Calculate the wavelength of the light. [3]
(b) Explain why the student measures across 10 fringe spacings rather than one. [1]
(c) State one safety precaution when using a laser. [1]
- (a) $w = 34/10 = 3.4$ mm(1)
- (a) $\lambda = \dfrac{ws}{D} = \dfrac{3.4\times10^{-3}\times0.40\times10^{-3}}{2.5}$(1)
- (a) $5.4\times10^{-7}$ m allow $5.44\times10^{-7}$ m / 540 nm / 544 nm(1)
- (b) the larger distance has a smaller percentage (fractional) uncertainty (same absolute uncertainty from the ruler) allow 'reduces the uncertainty in the fringe spacing' / 'the uncertainty is shared over 10 spacings'; reject 'more accurate' alone(1)
- (c) e.g. never look directly into the beam / avoid reflective surfaces / wear laser safety goggles / display a warning sign(1)
- Answer: (a) $5.4\times10^{-7}$ m (b) reduces percentage uncertainty (c) e.g. do not look into the beam
- Examiner tip: Convert both mm values to metres; leaving s in mm gives an answer 1000 times too big.