Help with Hi School Physics question

I’m stumped about this question in my 10th grader’s homework.

The energy of light wave can be found from the formula E = hv, where h = 6.63 X 10^-34 Joules/hertz. What energy is associated with each of the 3 colors?

Blue:__________

Green:_________

Yellow:_________

Red:___________


Yep, that’s really how it’s phrased. I’ll refrain from commenting on the “3 colors”. The only constants she’s given are the wavelengths of those colors:

Blue: 450nm
Green: 500nm
Yellow: 580 nm
Red: 700 nm

…and c = 3.00 X 10^17 nm/sec

Any help would be appreciated. What on earth is “v”? If it’s velocity (of light, I presume), then E would be a constant, right?

See if this helps.

BTW, it’s a nu, not a v.

v = fλ

Ooooh! What I thought was a “v”, is [some unintelligible symbol by teacher who cannot count to 3] representing frequency? Or c over the wavelength (right)?

Now I get it.

Thanks much, everyone.

No, I don’t think you do.

v = fλ, where

v = wave velocity (m/s)
f = frequency (cycles/second or Hz)
λ = wavelength (m)

Also note that 1 nm = 1X10[sup]-9[/sup] m

For an electromagnetic wave traveling in a vacuum, v = c = 299792458 m/s

I think he does. Planck’s equation is E = h[symbol]n[/symbol], where “[symbol]n[/symbol]” is the Greek nu for frequency, not a vee-for-velocity.

As pullin says, the frequency [symbol]n[/symbol] = c/[symbol]l[/symbol], and E = h[symbol]n[/symbol].

Oops, my bad. You’re right.

I hate when they do that… :stuck_out_tongue:

So the guy says, “What’s new?” and the physicist says, “E/h.”

As a side note, you could have figured this out just by looking at the units of the formula you were given (E = hv, where h = 6.63 X 10^-34 Joules/hertz).

Since “E” is going to be in Joules, and “h” is given in Joules/hertz, then “v” has to be a frequency value (hertz) for the units to work out. (Joules= [Joules/hertz]*hertz)

Of course, just knowing that it has units of frequency doesn’t tell you the whole store, since frequencies are sometimes measured in cycles per time (usually given as nu or f), and sometimes measured in radians per time (usually given as omega, which looks like a curly w). So, for instance, instead of saying E = hnu, you might say E = hbaromega. The former (h and nu) is usually introduced first to students, but the latter (hbar and omega) is, in practice, more often useful in real physics.

You guys are such a caution.:stuck_out_tongue:

Seriously, Thanks to everyone for their help. I got my youngster’s homework finished. Now on to scatter plots, and curve-fitting. (This is 10th grade? I don’t think I covered this stuff 'till college)

I’m curious where you are in N. Texas, because I swear my 10th-grade son (in Arlington ISD) just had this same homework assignment, or something very much like it …

Far northwestern corner. Maybe the school’s are using the same handouts?

Could be. I’d kind of forgotten that the whole state shares a unified curriculum–it makes sense that they’d be sharing other materials besides just books.