Numford

OpticsClass 12, Ray Optics and Optical Instruments

Dimensional formula of Focal length

[L]

Symbolf SI unitmetre (m)

The distance from the lens or mirror to its focus. It is a length.

Derivation

How to get the dimensional formula of focal length

  1. Start from the definition: focal length = length.
  2. Put in the dimension of each part: [L].
  3. Add the powers of each base quantity: [L].
Same dimensions

Quantities with the dimensions [L]

Equal dimensions do not make them the same quantity. Compare focal length with the others in the full grid.

Practice

Three questions on focal length

1Which of these has the same dimensions as focal length?

2What is the SI unit of focal length?

3Which one has different dimensions from the others?

Questions

Focal length: quick answers

What is the dimensional formula of focal length?

The dimensional formula of focal length is [L]. It comes from focal length = length.

What is the SI unit of focal length?

The SI unit of focal length is the metre (m).

Which quantities have the same dimensions as focal length?

Focal length shares [L] with length, displacement, radius of gyration, mean free path, wavelength, amplitude, fringe width, de Broglie wavelength. Equal dimensions do not make them the same quantity.

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