# Help with Hi School Physics question **URL:** \ **Category:** Factual Questions\ **Created:** [October 28, 2010, 1:39am UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607 "2010-10-28T01:39:09Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![pullin](https://avatars.discourse-cdn.com/v4/letter/p/c57346/32.png) [@pullin](https://boards.straightdope.com/u/pullin)\ **Post date:** [October 28, 2010, 1:39am UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/1 "2010-10-28T01:39:09Z")
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? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 28, 2010, 1:44am UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/2 "2010-10-28T01:44:30Z")
See if [this](http://wiki.answers.com/Q/What_is_the_equation_used_to_determine_the_energy_content_of_a_packet_of_light_of_specific_frequency) helps. BTW, it’s a nu, not a v. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 28, 2010, 1:44am UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/3 "2010-10-28T01:44:58Z")
v = fλ ---
**Author:** ![pullin](https://avatars.discourse-cdn.com/v4/letter/p/c57346/32.png) [@pullin](https://boards.straightdope.com/u/pullin)\ **Post date:** [October 28, 2010, 1:51am UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/4 "2010-10-28T01:51:34Z")
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. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 28, 2010, 10:30am UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/5 "2010-10-28T10:30:25Z")
> [@pullin](#): > > 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. 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 ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [October 28, 2010, 10:54am UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/6 "2010-10-28T10:54:39Z")
> [@Crafter\_Man](#): > > No, I don’t think you do. I think he does. [Planck’s equation](http://www.gcisd-k12.org/17342093082559793/lib/17342093082559793/5-2PlancksEq.pdf) 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]. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 28, 2010, 12:29pm UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/7 "2010-10-28T12:29:57Z")
> [@zut](#): > > I think he does. [Planck’s equation](http://www.gcisd-k12.org/17342093082559793/lib/17342093082559793/5-2PlancksEq.pdf) 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… 😛 ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [October 28, 2010, 1:13pm UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/8 "2010-10-28T13:13:04Z")
So the guy says, “What’s new?” and the physicist says, “E/h.” ---
**Author:** ![YamatoTwinkie](https://avatars.discourse-cdn.com/v4/letter/y/cc9497/32.png) [@YamatoTwinkie](https://boards.straightdope.com/u/YamatoTwinkie)\ **Post date:** [October 28, 2010, 5:37pm UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/9 "2010-10-28T17:37:41Z")
> [@pullin](#): > > Ooooh! What I thought was a “v”, is [some unintelligible symbol by teacher who cannot count to 3] representing frequency? 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) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 28, 2010, 8:59pm UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/10 "2010-10-28T20:59:29Z")
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 = h_nu, you might say E = hbar_omega. 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. ---
**Author:** ![pullin](https://avatars.discourse-cdn.com/v4/letter/p/c57346/32.png) [@pullin](https://boards.straightdope.com/u/pullin)\ **Post date:** [October 29, 2010, 1:46am UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/11 "2010-10-29T01:46:04Z")
> [@Bill\_Door](#): > > So the guy says, “What’s new?” and the physicist says, “E/h.” You guys are such a caution.😛 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) ---
**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\ **Post date:** [October 29, 2010, 3:09am UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/12 "2010-10-29T03:09:28Z")
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 … ---
**Author:** ![pullin](https://avatars.discourse-cdn.com/v4/letter/p/c57346/32.png) [@pullin](https://boards.straightdope.com/u/pullin)\ **Post date:** [October 29, 2010, 2:06pm UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/13 "2010-10-29T14:06:31Z")
Far northwestern corner. Maybe the school’s are using the same handouts? ---
**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\ **Post date:** [October 29, 2010, 6:49pm UTC](https://boards.straightdope.com/t/help-with-hi-school-physics-question/558607/14 "2010-10-29T18:49:50Z")
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.