Types of Waves — Question 2

Question 2

Removing matter tests a proposed mechanical transmission path

A sealed chamber contains a ringing source and a flashing lamp. A detector outside responds to both when the chamber contains air. The gas is then removed. Assume the lamp remains operating, and isolate solid supports so they transmit negligible vibration.

1. Predict which direct signal can still cross the evacuated gap, and identify its wave category.

2. If a weak sound-like signal remains, what alternative path must be checked before concluding that sound propagates through vacuum?

Question 2 - Solution

Removing matter tests a proposed mechanical transmission path

Method and mathematical working

Separate source operation, propagation through the gap, and detector response. Removing the gas changes the mechanical path but does not remove the electromagnetic field.$$\begin{aligned}\rho_{\mathrm{gas}}=0&\Rightarrow\text{no gas-borne mechanical transmission},\\\text{remaining signal}&\Rightarrow\text{test light, walls and supports separately}.\end{aligned}$$

(a) Calculation and conclusion

Light is electromagnetic and can cross the evacuated gap. Ordinary airborne sound is a mechanical disturbance of matter; without gas, that direct acoustic path is removed. The maintained source operation matters: loss of a source would not test propagation.

(b) Interpretation and limits

Check residual gas and vibrations carried through wires, walls or supports, followed by sound generated outside. A detector response alone does not identify the intervening propagation mechanism. A valid vacuum test must rule out material bypasses.

Physical check

A residual detector signal establishes only that some coupling remains. Tracing each possible path through gas, supports and chamber walls is necessary before assigning that signal to vacuum propagation.