Question

Diffraction is common in sound but not in light waves, why ? .

Answer 1

Because Light Wavelength is actually less than a sound wave. And Diffraction is more in longer wavelength waves, as is less in wider slits. The wavelength of sound is of the order of 1 meter. So any objet visible to the eye can deflect it.

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Answer 2

$\underline{\mathfrak{Answer}}$

$\bold{For\: diffraction}$ of a wave, the size of particle/obstacle/aperture should be of the order of warve length of wave. Since the average wavelength of visible light is of the order of 10-6 m and obstacle/aperture of this size are rare, therefore, diffraction of light is not common.
But in case of sound, the wave length of waves is 1 -2 m and obstacle/aperture of this size are readily available, therefore diffraction of sound is quite often.$\bold{quite\: often}$

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$\bold{hope\: it\: helps}$