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Diffraction refers to various phenomena which occur when a wave encounters an obstacle or a slit. Diffraction arises because of the way in which waves propagate; this is described by the Huygens–Fresnel principle and the principle of superposition of waves. The above shows a single-slit diffraction in a ripple tank, where the vibrations of the water were observed at positions $P$ and $Q$ but not at position $R.$ If we want to observe the vibrations of the water at position $R,$ which of the following methods should be used?

a) Make the slit narrower.

b) Make the water of the ripple tank deeper.

c) Increase the frequency of the water wave.