Problem — The remote control and the sheet of paper

Problem

When using a remote control to change a TV channel (even from very close up), it still works if there is a thin sheet of paper in front of the TV’s sensor. Why? What if there were a sheet of aluminium foil instead? Explain in terms of transmission and absorption of IR waves.

Solution

The remote control uses infrared (IR). The beam is infrared light, an electromagnetic wave with a wavelength slightly longer than visible light.

Thin sheet of paper. Paper is partially transparent to IR: a fraction of the radiation passes through it (by transmission and scattering) and still reaches the sensor. Enough gets through to trigger the command, especially since the remote control emits a fairly intense pulse and the sensor is very sensitive. The sheet absorbs and scatters some of it, but does not block everything.

Aluminium foil. Metal is an excellent conductor: it reflects (and absorbs) almost all of the IR electromagnetic waves, without letting them through. The signal does not reach the sensor and the remote does not work.

$\ev{\text{Paper: IR partly transmitted} \Rightarrow \text{works}; \quad \text{Aluminium: IR reflected/absorbed} \Rightarrow \text{does not work}}$

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