State the condition necessary to observe interference between two light sources.

The diagram below shows an arrangement for observing a double slit interference pattern. A parallel beam of coherent light of wavelength 410 nm is incident on two parallel narrow slits separated by 0.30 mm. A screen is placed 1.60 m beyond the slits.

Calculate the fringe spacing on the screen.

The slits in (b) are replaced by a large number of slits of the same width and separation as the double slit. Describe the effects that this change will have on the appearance of the fringes on the screen.

the light from the sources must be coherent / phase difference must be constant (allow “in phase”) / the electric fields must have the same polarization;

fringe spacing=\(\frac{{1.60{\rm{m}} \times {\rm{410nm}}}}{{0.30{\rm{mm}}}}\);
2.2 mm;
Award [2 max] for a response that makes use of nλ in the double slit formula
Award ECF [1 max] if answer is for a value of n greater than 1.
Award [2] for a bald correct answer.

sharper fringes / OWTTE;
brighter;
same spacing;