A student uses a Geiger-Müller (GM) tube and counter to measure the radioactivity of a sample. Before placing the sample near the GM tube, the student records a reading of 25 counts per minute25\text{ counts per minute}25 counts per minute.
What does this reading measure?
the visible light entering the detector from room lighting
the infra-red radiation emitted by the student and the detector
the ionising radiation present in the environment when the sample is not nearby
the ionising radiation that has leaked from the sample's protective container
44 exam-style questions on CIE IGCSE Physics 5.2.1 Background radiation and measuring count rate. Each one has a worked solution and a mark scheme showing where the marks go.