ASPhysicsEdexcelRevision Notes5. Waves & Particle Nature of LightRefraction, Reflection & PolarisationMagnification

Magnification (Edexcel AS Physics): Revision Note

Exam code: 8PH0

Lindsay Gilmour

Last updated

Magnification

Magnification as a Ratio of Heights

  • Magnification means how much larger the image is than the object

    • This is the ratio of the image/object height

m space equals space h subscript i over h subscript o

  • Where:

    • m = magnification

    • h subscript i = image height (m)

    • h subscript o = object height (m)

Magnification as a Ratio of Distances

  • A diagram of an object and its real image will produce similar triangles

    • Therefore, the ratio of magnification is also represented by comparing the distance from the lens to the object and the image

5-23-magnification-derivation-edexcel-al-physics-rn-1

  • This also works for virtual images

    5-23-magnification-derivation-2_edexcel-al-physics-rn

     m space equals space v over u

  • Where:

    • m = magnification

    • v = distance from lens to image (m)

    • u = distance from lens to object (m)

  • Since magnification is a ratio, it has no units

  • Values of m can be positive or negative

    • If m is positive, then the image formed is virtual and upright

    • If m is negative, then the image formed is real and inverted

Worked Example

A magnifying glass has a focal length of 15 cm. It is held 5 cm away from a component which is being examined. 

Determine the magnification of the image.

Answer:

Step 1: Write the known values

  • Focal length, f = 15 cm

  • Distance between object and lens, u = 5 cm

Step 2: Use the lens formula and rearrange to make v the subject

1 over f space equals space 1 over u space plus space 1 over v

v space equals space open parentheses 1 over f space minus space 1 over u close parentheses to the power of negative 1 end exponent

v space equals space open parentheses 1 over 15 minus 1 fifth close parentheses to the power of negative 1 end exponent space equals space minus 7.5 space cm

Step 3: Use the magnification formula to find the magnification of the image

m space equals space v over u

m space equals space fraction numerator negative 7.5 over denominator space 5 end fraction space equals space minus 1.5

  • The negative sign indicates a virtual image (expected for a magnifying glass)

Worked Example

A person sees an image through a magnifying glass.

Magnification Worked Example Questions, downloadable IGCSE & GCSE Physics revision notes

Calculate the magnification of this image. Clearly show your working on the diagram.

Answer:

Magnification Worked Example Solution, downloadable IGCSE & GCSE Physics revision notes

Step 1: Measure the height of the object from the scale

Height of the object: h subscript o = 10 cm

Step 2: Measure the height of image from the scale

Height of the image: h subscript i = 20 cm

Step 3: Substitute values into the magnification equation

Magnification: m space equals space h subscript i over h subscript o space equals space 20 over 10 space equals space 2

Examiner Tips and Tricks

The most common mistake with magnification calculations is to get the formula upside down.

Do a 'sanity check' by looking at the answer to make sure that magnified objects have got bigger (m > 1) and diminished ones smaller (m < 1).

Since we are working with ratios (so the units get cancelled out), this is one of those rare times when you don't need to convert everything to SI units, but do check that your units are all the same - for example, all distances in cm.

You've read 0 of your 5 free revision notes this week

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

  • I would just like to say a massive thank you for putting together such a brilliant, easy to use website.I really think using this site helped me secure my top gradesin science and maths. You really did save my exams! Thank you.

    Beth
    IGCSE Student

  • This website is soooo useful and I can’t ever thank you enough for organising questions by topic like this. Furthermore, the name of the website could not have been more appropriate as it literally did SAVE MY EXAMS!

    Fathima
    A Level Student
Test yourself

Did this page help you?

Previous:The Lens EquationNext:Plane Polarisation
Lindsay Gilmour

Author: Lindsay Gilmour

Expertise: Physics Content Creator

Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc at Imperial College London. Now with many years’ experience as a Head of Physics and Examiner for A Level and IGCSE Physics (and Biology!), her love of communicating, educating and Physics has brought her to Save My Exams where she hopes to help as many students as possible on their next steps.

Download notes on Magnification