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Magnification (Oxford AQA IGCSE Physics)

Revision Note

Ann Howell

Magnification

  • The magnification produced by a lens may be calculated using the equation:

magnification space equals space fraction numerator image space height over denominator object space height end fraction

  • This equation can be rearranged with the help of a formula triangle:

Formula triangle for magnification

The magnification formula triangle, from IGCSE & GCSE Physics revision notes
Cover up the quantity you are trying to find and use the visible formula remaining

  • The magnification of a lens depends on:

    • The distance of an object from the lens

    • The power of the lens

  • Magnification is a ratio and has no units

    • Both the object and image must be measured in the same units

    • For example, both in cm, or both in mm

  • If the magnification is:

    • > 1, then the image is magnified

    • = 1, then the object and image are the same size

    • < 1, then the image is diminished

Worked Example

A person sees an image through a magnifying glass.

A lens diagram showing a converging lens on vertical and horizontal axes. The object is drawn behind the lens at the half the distance between the focal point F and the lens. An arrow is drawn from the top of the object sloping down through the origin of the axes. This arrow is extrapolated backward with a dotted line. A second arrow is drawn parallel to the horizontal axis from the top of the object to the top of the lens. The second arrow then diverges sloping downward to the focal point F in front of the lens. The diverging portion of the second arrow is extrapolated backward with a dotted line. The image is drawn as an arrow with its base at the focal point behind the lens and its tip at the point where the two dotted lines cross. The image arrow is larger than the object arrow, and both are pointing upward. The scale on the diagram is 2 large squares (4 cm) = 20 cm

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

Answer:

The height of the image and object arrows are measured. Using the scale, they are determined to be image height = 20 cm and object height = 10 cm

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

The object is 10 cm

Step 2: Measure the height of the image from the scale

The image is 20 cm

Step 3: Substitute values into the magnification equation

magnification space equals space fraction numerator image space height over denominator object space height end fraction

magnification space equals space fraction numerator 20 space cm over denominator 10 space cm end fraction

magnification space equals 2

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    Ann Howell

    Author: Ann Howell

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    Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students, no matter their schooling or background.

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