Energy of a light quantum

1. Jun 2, 2005

Jchem

I'm looking for an expression for the "energy of a light-quantum in eV when the wavelength is in nanometers"

and I'm kind of stumped

anyone know this formula?

thanks

2. Jun 2, 2005

Nylex

You can use $E = \frac{hc}{\lambda}$, where h is Planck's constant, c is the speed of light and $\lambda$ is the wavelength (in metres). This will give you an answer in Joules, all you need to do then is convert to eV (1 eV = 1.6 x 10^-19 J).

3. Jun 2, 2005

Jchem

ok so I can use $E = \frac{hc}{\lambda}$ X 1eV/1.6 x 10^-19 J

What about the "in nanometers" part?

The formula will work with a wavelength in nanometers, it will also work with any other size wavelengths..

not sure what they are asking here.

thanks

4. Jun 2, 2005

Andrew Mason

The formula applies regardless of the units. You just have to use consistent units. If you use MKS, the energy is measured in Joules (m^2kg/sec^2), c is in m/sec, distance in m and h in Jsec. A nanometer is $10^{-9} metres$.

AM

5. Jun 5, 2005

ConceptuallyInept

Is MKS just the same as SI units?

6. Jun 5, 2005

DieCommie

I think MKS is meter-kelvin-seconds which is basically SI units.

7. Jun 5, 2005

Andrew Mason

MKS is metre-kilogram-seconds or SI units (Système International d'Unités) as opposed to CGS (=centimetre-gram-seconds) or FPS (=foot pound seconds).

AM