Q Is it true that the speed of sound remains constant whether in darkness or light?

A This statement is generally true but, sometimes, you can find exceptions.

Let's consider a potential experiment, as follows.

In the daytime, you and I go to Bukit Timah Hill. You shout loudly at the top, and I, standing a few hundred metres away, measure the speed of sound. Then, we do the same measurement at night.

We will find that the speed of sound in the daytime is faster than at night.

This is because sunshine in the daytime increases the air temperature, which, subsequently, increases the speed of sound.

Sound is the propagation process involving the mechanical vibration of air molecules. Its speed depends on a few factors that determine how easily these molecules vibrate, such as the density and stiffness of air... Temperature will affect sound speed mainly by changing the density of air.

Light, in general, does not affect the speed of sound.

• Assistant Professor Zhang Baile, Nanyang Technological University, School of Physical and Mathematical Sciences.

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