An astronomer records visible light from the same faint region of the sky twice using one telescope. During observation 1, a circular mask covers the outer part of the mirror. The mask is removed before observation 2. The brightness of the region and the detector settings remain unchanged.
The signal accumulated per minute is directly proportional to the light-collecting area. For a circular mirror, the light-collecting area varies with the square of its effective diameter. The detector adds the signals when separate exposures are combined.
| Observation | Effective mirror diameter | Exposure information |
|---|---|---|
| 1 | 0.72 m | Total exposure time of 15.0 min |
| 2 | 1.20 m | Six exposures of equal duration |
Calculate the duration of each exposure in observation 2 needed to give the same combined signal as observation 1.
75 exam-style questions on Edexcel GCSE Physics 7.3 Stars, covering 7.3.1 Evolution of stars similar to the Sun, 7.3.2 Evolution of massive stars, and 7.3.3 Observing the Universe. Each one has a worked solution and a mark scheme showing where the marks go.