1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: LEDs different colours and their forward(working) voltage

  1. Nov 24, 2009 #1
    1. The problem statement, all variables and given/known data
    hi i've just been doing labs today on data aquisition looking at current voltage characteristics, one thing that i did was look at the forward(working) voltage for a red, green and white LED.

    Red = 1.5V Green = 1.8V White = 2.6V

    when i asked the demonstarter why this was he said that we arent asked to know this to write in our lab books and so it didnt matter, but i do want to know why.

    ive looked on the web and i think it has something to do with the wavelength produced and that each one might need slightly more energy to get the lower wavelength [cause E=hc/[tex]\lambda[/tex]] and white light needs alot more at it isnt monochromatic.

    is any of this thinking right?
  2. jcsd
  3. Nov 24, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Roughly correct.
    The energy of the photon emitted depends on the bandgap of the material as does the forward voltage.
    Red, green and blue LEDs use different semiconductor material (or simialir material with different doping) with different bandgaps.
    To slightly confuse things white LEDs are generally blue LEDs with a phosphor that is excited by high energy blue light and emits roughly white light
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook