A-Level Physics May/June 2024 Q10(a)(ii): Explain how a standard candle in a distant galaxy can be used to determine the distance…

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  A standard candle is an astronomical object that has a known luminosity, $L$. An observer on Earth measures the radiant flux intensity, $F$, received from the standard candle. The relationship between these quantities and the distance $d$ to the galaxy is given by the inverse square law for flux, $F = \frac{L}{4\pi d^2}$. Since both $L$ is known and $F$ is measured, the distance $d$ can be determined by rearranging this equation.

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  How the marks are awarded

  • B1 — The first mark is for stating that a standard candle is an object of known luminosity ($L$).
  • B1 — The second mark is for explaining that the radiant flux intensity ($F$) of the standard candle is measured by the observer.
  • B1 — The final mark is for stating the inverse square law formula, $F = L / 4\pi d^2$, and explaining that it is used to calculate the distance ($d$).

  Common mistakes

  • Confusing luminosity (the known intrinsic power) with radiant flux intensity or 'brightness' (the measured power per unit area).
  • Forgetting to state that the radiant flux intensity must be measured from Earth, which is a crucial step in the process.
  • Mentioning the 'inverse square law' in general terms without explicitly writing the formula $F = L / 4\pi d^2$.
  • Incorrectly introducing concepts like redshift or Hubble's Law, which are not directly used in this specific distance determination method.

  Examiner tip: For 'explain how' questions, clearly state the known physical principle or quantity, what is measured, and the specific formula that connects them to find the unknown.

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