申論題

2. A star located 6.00 ly from Earth undergoes a super-nova explosion. At that distance the star behaves as an isotropic point source of light. A ground-based telescope with a circular objective of radius 1.20 m registers photons from the event at a rate of 4.50 × photons/s. The combined optical-detector efficiency of the instrument is 80.0%. Assume, as a rough approximation, that all the emitted light has the single visible wavelength 500 nm and that the Sun's total luminosity is 3.90 × W.

(a) Using the measured photon rate, calculate the radiant power that reaches the face of the telescope objective.