# Need some help with the multiple choice [EMR]

1. Nov 30, 2011

### Nitrate

1. The problem statement, all variables and given/known data
which are properties of all Electromagnetic radiation
I the travel at the speed of light in space
II they propagate through the creation of mutually perpendicular electric and magnetic fields
III they are constant and do not vary with time
IV they have the same frequency and wavelength

2. Relevant equations

3. The attempt at a solution
solutions:
a) I only
b) I and II
c) I and III
d) I and IV

the only one im sure about is II
and the only answer that has that is b) but i'm not sure if I is correct
ive also ruled out IV as an answer
any relevant links that can help me out would be appreciated

Last edited: Nov 30, 2011
2. Dec 1, 2011

### PeterO

Note: If you are sure about II, then option b) is the only one that includes it. [I must be correct since it is part of every option]

3. Dec 1, 2011

### Simon Bridge

You've done the usual approach - go with what you know for sure, and if there is only one option that has it, that one must be correct :) But you seem to be unsure of yourself, which suggests you are trying to do this from memory alone.

Memory can be unreliable - so how to check?
How about: by reasoning it out - a good way is to turn the key statements into questions. eg.

Look at each of the four properties supposedly shared by all EM radiation.
If you can come up with one counter-example, then you have ruled out the property.
So:

I: can you come up with any EM radiation that does not travel at the speed of light?
(A=No. And it appears in all the choices - so the question is actually a choice between the other three.)

II: can you find an EM wave that does not have this arrangement of E and M fields?

III: is there any way to have time-varying EM radiation?

IV: can you come up with examples of EM radiation with different wavelengths?