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Huygens's principle is a fundamental concept in wave theory that describes the behavior of waves, and it is indeed valid for both transverse waves and longitudinal waves.

This principle can be applied to both transverse waves and longitudinal waves:

1. Transverse Waves: In a transverse wave, the oscillations of the particles are perpendicular to the direction of wave propagation. Huygens's principle is used to explain how the wavefronts of transverse waves propagate by considering each point on the wavefront as a source of secondary circular wavelets. The new wavefront is formed by the envelope of these circular wavelets.

2. Longitudinal Waves: In a longitudinal wave, the oscillations of the particles are in the same direction as the wave's propagation. Huygens's principle can also be applied to longitudinal waves. In this case, each point on the wavefront is considered as a source of secondary spherical wavelets, and the new wavefront is determined by the envelope of these spherical wavelets.

So, Huygens's principle is a versatile concept that can be applied to various types of waves, including both transverse and longitudinal waves, to help explain how wavefronts propagate.