B.1.15—Apparent brightness
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- HL
Interpret Apparent Brightness
Apparent brightness
Apparent brightness, b, describes how much power from a distant source is received per unit area at the observer. It is an observation-dependent quantity.
Why distance matters
Radiation from an approximately point-like source spreads over larger spherical areas as it travels outward. The same emitted power is distributed over more area, so the received power per unit area decreases.
Do not confuse the quantities
Luminosity is the source’s total emitted power. Apparent brightness is what reaches a specified observer per unit area. A source can be intrinsically luminous but appear faint when it is far away.
Common trap
Apparent brightness is not simply the source’s total power. Always ask whether the question concerns emission by the source or reception at a distance.
B.1.15 Exam Analysis
Assessment in practice
1 marksHow it is assessed
One direct fallback item asks what apparent magnitude measures: apparent brightness. The other packet item concerns parallax uncertainty and is not used as direct evidence for this objective.
Command terms
State / Define
What earns marks
Define apparent brightness as the received power per unit area at the observer. Distinguish it from luminosity, the source’s total emitted power. If a question uses apparent magnitude, connect it to apparent brightness rather than treating it as a direct measure of luminosity.
Watch for
Calling apparent brightness the total emitted power of the source.
Representative question
Question 1
[Maximum number: 1]
what apparent magnitude is a measure of.
(apparent) brightness;