# Wavelength, Frequency

## Main Question or Discussion Point

Hello, I confused as there are 2 very similar equation but I do not know when to use each of them. They are:

$f = \frac{v}{\lambda}$ and $f = \frac{c}{\lambda}$.

What is the difference between $c$ and $v$ and when can the appropriate one be used?

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ZapperZ
Staff Emeritus
2018 Award
Er... one is for a particle or wave at any velocity v, while the other is when v=c (i.e. light in vacuum).

Zz.

Also, can $c$ also be used when we are considering other source of waves (e.g. electromagnetic spectrum)?

ZapperZ
Staff Emeritus
2018 Award
When I use the word "light", I do mean the EM wave, not just "visible light".

Zz.

GT1
Why do short wavelengths usually penetrate deeper then long wavelengths ?
(I know it has more energy, but I'm looking for more detailed explanation after reading the FAQ).

Why do short wavelengths usually penetrate deeper then long wavelengths ?
(I know it has more energy, but I'm looking for more detailed explanation after reading the FAQ).
Said that way it's not true, in general: it depends on material, its surface conditions, and on the range of frequencies; in some cases it could be the opposite.

GT1
Said that way it's not true, in general: it depends on material, its surface conditions, and on the range of frequencies; in some cases it could be the opposite.
So if choose randomly 10000 materials only on 50% of the cases the short wavelengths will penetrate deeper ?

ZapperZ
Staff Emeritus