AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Classification by luminosity (A-level only): Apparent magnitude, m.
Apparent magnitude, m, is a logarithmic measure of the irradiance received from a star or other astronomical object.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 1 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Apparent magnitude, m, is a logarithmic measure of the irradiance received from a star or other astronomical object. It describes how bright the object appears from Earth, not how much light it emits. The scale runs backwards: smaller m means brighter. A difference of 5 magnitudes corresponds to a brightness ratio of exactly 100, so a difference of 1 magnitude corresponds to a brightness ratio of 100^(1/5), about 2.51. You use the relationship between magnitude difference and irradiance ratio, and you must handle negative magnitudes for very bright objects. Remember that apparent magnitude depends on both the object's luminosity and its distance, so it is not an intrinsic property.
What earns the marks
- Define apparent magnitude as a logarithmic measure of the irradiance received from an object, describing how bright it appears from Earth.
- State that the scale is inverted: a smaller magnitude corresponds to a brighter object, and values can be negative.
- Apply the convention that a difference of 5 magnitudes corresponds to a factor of 100 in irradiance.
- Use the relationship between magnitude difference and irradiance ratio, including the fifth root of 100 as the ratio for a one-magnitude difference.
- Distinguish apparent magnitude from absolute magnitude and luminosity, recognising that apparent magnitude depends on distance.
- Perform calculations involving magnitude differences and irradiance ratios, keeping signs consistent.
Common mistakes and how to put them right
Treating a larger magnitude as brighter.
Put it right: Remember that smaller and negative magnitudes indicate brighter objects.
Assuming a 1-magnitude difference means a brightness ratio of 100.
Put it right: 5 magnitudes corresponds to a ratio of 100, so 1 magnitude corresponds to about 2.51.
Confusing apparent magnitude with luminosity or absolute magnitude.
Put it right: Apparent magnitude measures received irradiance and depends on distance.
Exam tips
- Write the magnitude–irradiance relationship clearly and substitute values with signs before calculating.
- Check whether a question asks for a ratio or a magnitude difference, and give the answer in the requested form.
- Use the anchor that 5 magnitudes equals a factor of 100 in brightness to sanity-check any result.
Try the lesson
The start of the Classification by luminosity (A-level only): Apparent magnitude, m. lesson, made from checked teaching notes for this specification point.
Have a go first
What does the apparent magnitude, m, of a star measure?
Listen
Apparent magnitude, m, is a logarithmic measure of the irradiance received from a star or other astronomical object. It describes how bright the object appears from Earth, not how much light it emits. The scale runs backwards: smaller m means brighter. A difference of 5 magnitudes corresponds to a brightness ratio of exactly 100, so a difference of 1 magnitude corresponds to a brightness ratio of 100^(1/5), about 2.51. You use the relationship between magnitude difference and irradiance ratio, and you must handle negative magnitudes for very bright objects. Remember that apparent magnitude depends on both the object's luminosity and its distance, so it is not an intrinsic property. A strong answer earns marks for these points. Define apparent magnitude as a logarithmic measure of the irradiance received from an object, describing how bright it appears from Earth. State that the scale is inverted: a smaller magnitude corresponds to a brighter object, and values can be negative. Apply the convention that a difference of 5 magnitudes corresponds to a factor of 100 in irradiance. Use the relationship between magnitude difference and irradiance ratio, including the fifth root of 100 as the ratio for a one-magnitude difference. A common slip: Treating a larger magnitude as brighter. Instead, remember that smaller and negative magnitudes indicate brighter objects. Exam tip: Write the magnitude–irradiance relationship clearly and substitute values with signs before calculating.
See it
Key words
Say what each word means before you turn the card.
Apparent magnitude
Try answering aloud or in your head before turning the card.
Irradiance
Try answering aloud or in your head before turning the card.
Luminosity
Try answering aloud or in your head before turning the card.
See every definition
- Apparent magnitude
- A logarithmic measure of the irradiance received from a star or other astronomical object, describing how bright it appears from Earth.
- Irradiance
- The quantity received from an object that apparent magnitude is a logarithmic measure of, and which depends on both luminosity and distance.
- Luminosity
- How much light an object emits, which apparent magnitude does not describe, so apparent magnitude is not an intrinsic property.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Classification by luminosity (A-level only): Apparent magnitude, m.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge