Reflection and Transmission of Waves

When reflection takes place from a denser medium, the wave undergoes a phase change of π, that is, it suffers a phase reversal. On reflection from a rarer medium, no phase change takes place.

When reflection takes place from a denser medium, the longitudinal waves are reflected without change of type but with change in sign. And on reflection from a rare medium, a longitudinal wave is reflected back without change of sign but with change of type.