# Need some help with solving this Physics exercise ? (Planks constant )?

If on a metal plate of Cesium is lighted with light which wavelenght is 400nm, the photoelectrons will stop if there's a voltage of 1V, and if it's used ultraviolet light with wavelenght of 200 nm, the stopping voltage is 4.11V. Calculate the value of Planks constant ?

### 1 Answer

- nyphdinmdLv 72 months agoFavorite Answer
Assume the photon energy is given by E = h*f = hc/wl where f = frequency, wl = wavelength of photon, c = speed of light and h = unknown Planck constant.

Now when wl = wli1 = 400 nm, the electron has 1 eV of kinetic energy. 1eV = 1.6x10^-19 J so we can write

hc/wl1 = W + 1.6x10^-19 J where W = work function of Cesium

For wl = wl2 = 200nm, the electron has 4.11 eV of kinetic energy = 6.576x10^-19 J

and hc/wl2 = W + 6.576x10^-10 J

Subtract the first energ balance from the second:

hc/wl2 - hc/wl1 = 4.976x10^-19 J now simplify let dE = 4.976x10^-19 J

use f2 = c/wl2 and f1 = c/wl1 then

h*(f2 - f1) = dE ---> h = dE/(f2-f1) = 6.634x10^-35 J-s

Isn't it supposed to be ^-34 J*s ?