When to use c vs. v in the wavelength-frequency equation?

[Air]

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?

 

[ZapperZ]

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.

 

[Air]

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

 

[ZapperZ]

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).

 

[lightarrow]

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]

So if choose randomly 10000 materials only on 50% of the cases the short wavelengths will penetrate deeper ?

Look at one of the most common material on hand - ordinary, transparent glass that you can buy at a store. It allows for the transmission of almost all visible light spectrum, but it doesn't allow UV to penetrate. And UV has a shorter wavelength than visible light.

Your question can't be answered because almost all materials have a finite bandwidth of absorption and/or transmission. This means that there isn't usually a "trend". While some wavelengths smaller than something may get transmitted, other that are smaller or longer may not.

Zz.