Waves: common misconceptions

If your child says… “Every wave needs matter.”

Ask together

Would removing air from a bell jar also block its lamp’s light?

What’s true

Mechanical waves need matter; electromagnetic waves do not.

If your child says… “The plus sign means rightwards.”

Ask together

If ωt grows, what must happen to kx to keep their sum unchanged?

What’s true

The time sign alone does not name the travel direction.

If your child says… “Faster shaking makes faster waves.”

Ask together

Which changed here: the source frequency or the string’s tension?

What’s true

The source sets frequency, while the string sets speed.

If your child says… “Air stays at the same temperature as sound compresses it.”

Ask together

Can heat escape fast enough to keep each rapid compression at constant temperature?

What’s true

Sound compressions are too rapid for much heat exchange, so temperature changes.

If your child says… “Up three and down two make five.”

Ask together

Does the opposite bend increase the displacement or reduce it?

What’s true

Opposite displacements subtract rather than add their sizes.

If your child says… “Any point at zero is a node.”

Ask together

Does that point stay still through the whole cycle?

What’s true

A node stays at zero throughout the motion.

If your child says… “Closed pipes have every harmonic.”

Ask together

Can identical standing-wave patterns satisfy those different end conditions?

What’s true

Displacement amplitude is zero at the closed end and greatest at the open end.

If your child says… “Beats occur at half the frequency difference.”

Ask together

Would skipping alternate loudness peaks count all the beats you hear?

What’s true

Count every loudness peak, because reversing the pressure swings does not reduce their size.