# Planck's Constant Question

1. Nov 29, 2005

### Erwin Schrodinger

When using the equation E = hf to find the energy of a photon, do I convert the energy and Planck's constant so they contain the same energy unit? So if I use eV for energy, I convert Planck's constant to eVs? If so, then Planck's constant would be about 4.14(10^-15)eVs? Thanks for the help in advance.

2. Nov 29, 2005

### Staff: Mentor

You need to use the same units on both sides of any equation.

What you're doing is perfectly OK. You can either convert the energy from eV to standard units (Joules) and then use Planck's constant in J-s. Or just use Plank's constant in eV-s, as you did.

3. Nov 29, 2005

### mezarashi

Your conversion is perfectly fine, and your new units for the Planck constant would be Joules Coulomb per second or eV/s. For convenience, the approximation

$$E(eV) = \frac{1.24}{\lambda (\mu m)}$$

is often used as well.

Last edited: Nov 29, 2005