Date | November 2020 | Marks available | 2 | Reference code | 20N.3.SL.TZ0.15 |
Level | Standard level | Paper | Paper 3 | Time zone | 0 - no time zone |
Command term | Explain | Question number | 15 | Adapted from | N/A |
Question
Show that the apparent brightness b∝AT4d2, where d is the distance of the object from Earth, T is the surface temperature of the object and A is the surface area of the object.
Two of the brightest objects in the night sky seen from Earth are the planet Venus and the star Sirius. Explain why the equation b∝AT4d2 is applicable to Sirius but not to Venus.
Markscheme
substitution of L=σAT4 into b=L4πd2 giving b=σAT44πd2
Removal of constants σ and 4π is optional
equation applies to Sirius/stars that are luminous/emit light «from fusion» ✓
but Venus reflects the Sun’s light/does not emit light «from fusion» ✓
OWTTE