Two light sources are used to produce an interference pattern.
Interference fringes appear when the two sources emit waves that are coherent.
What is meant by coherent waves?
Correct Answer: \( \boxed{\mathrm{B}} \)
Coherent waves maintain a constant phase difference and have the same frequency.
A zero phase difference is not required; any constant phase difference will produce a stable interference pattern.
Therefore, the correct answer is (B).
A student carries out a double-slit experiment using a laser emitting red light of wavelength \( 680\,\mathrm{nm} \). The light is incident normally on a double slit.
The diagram shows part of the pattern of bright fringes visible on a screen at a distance of \( 2.4\,\mathrm{m} \) from the slits. The distance across five bright fringes is measured as \( 34\,\mathrm{mm} \).
What is the slit separation?
Correct Answer: \( \boxed{\mathrm{C}} \)
The fringe spacing is
\( w=\dfrac{34\,\mathrm{mm}}{4}=8.5\times10^{-3}\,\mathrm{m} \).
Using the double-slit equation
\( w=\dfrac{\lambda D}{d} \), so
\( d=\dfrac{\lambda D}{w}=\dfrac{(680\times10^{-9})(2.4)}{8.5\times10^{-3}}\approx1.9\times10^{-4}\,\mathrm{m} \).
Therefore, the correct answer is (C).
Light waves are emitted from two sources.
What is a necessary condition for observable interference fringes to be produced?
Correct Answer: \( \boxed{\mathrm{C}} \)
Observable interference requires the two sources to be coherent, meaning they maintain a constant phase difference and have the same frequency.
Equal amplitudes are not essential, although they produce higher-contrast fringes. Polarisation is also not a necessary condition.
Therefore, the correct answer is (C).
