# Homework Help: Which electromagnetic waves travel the fastest in a vacuum?

1. Feb 3, 2009

### stickplot

1. The problem statement, all variables and given/known data

Which electromagnetic waves travel the fastest in a vacuum?

a. Violet light.
b. Red light.
c. Ultraviolet light.
d. Red, violet, and ultraviolet light travel at the same speed.

ok so i thought that in a vacuum every wave traveled at the same velocity, so only the frequency could vary. I know the ultraviolet light has the highest frequency, so i was assuming d was the correct answer. But im not so sure because d doesnt really specify they travel the same only in a vacuum but it says the travel at the same speed in general so i dont know whether it is c or d.

2. Feb 3, 2009

### Ouabache

Do you know the relationship between wavelength and frequency of light in a vacuum? (a basic equation). Armed with that information, you won't have any doubt as to which is the correct answer.
If i was considering d, I would state an assumption along with my answer: that the phrase travel at the same speed, refers to the conditions described in the question.

3. Feb 3, 2009

### stickplot

are you talking about velocity= wavelength(frequency)
but im just confused because i know all waves travel the same in a vacuum but i know frequency is higher in ultraviolet which would make the velocity greater so i dont know if its c or d

4. Feb 4, 2009

### Ouabache

You are on the right track, that is the general equation. In a vacuum, it becomes $$c= \lambda f$$.
You should be familiar with the units of this expression from your https://www.physicsforums.com/showthread.php?p=2061710,#2" about the electromagnetic spectrum.
so we have;
$\lambda$ wavelength (m)
$f$ frequency (1/s)
$c$ velocity of light (m/s), c= 299,792,458 m/s

So if the frequency is higher (as you say for ultraviolet light), and you know the velocity of light,
what must change, for this equation to hold true?

Last edited by a moderator: Apr 24, 2017
5. Feb 4, 2009

### stickplot

the wavelength needs to change, so it would be d? because the wavelength of ultraviolet light is smaller but the frequency is larger? so they would all turn out to be the speed of light right?

6. Feb 4, 2009

### Ouabache

Yes, as I stated earlier, if you understand the relationship we just discussed, you won't have any doubt what the correct answer is.

7. Feb 4, 2009

### stickplot

ok i understand thank you