Outline what is meant by the principle of superposition of waves.

Red laser light is incident on a double slit with a slit separation of 0.35 mm.
A double-slit interference pattern is observed on a screen 2.4 m from the slits.
The distance between successive maxima on the screen is 4.7 mm.

Calculate the wavelength of the light. Give your answer to an appropriate number of significant figures.

Explain the change to the appearance of the interference pattern when the red-light laser is replaced by one that emits green light.

One of the slits is now covered.

Describe the appearance of the pattern on the screen.

when 2 waves meet the resultant displacement

is the «vector» sum of their individual displacements

Displacement should be mentioned at least once in MP 1 or 2.

λ = \(\frac{{4.7 \times {{10}^{ - 3}} \times 0.35 \times {{10}^{ - 3}}}}{{2.4}}\)

= 6.9 x 10^–7 «m»

answer to 2 SF

Allow missed powers of 10 for MP1.

green wavelength smaller than red

fringe separation / distance between maxima decreases

Allow ECF from MP1.

bright central maximum

subsidiary maxima «on either side»

the width of the central fringe is twice / larger than the width of the subsidiary/secondary fringes/maxima

OR

intensity of pattern is decreased

Allow marks from a suitably labelled intensity graph for single slit diffraction.