'The maximum energy of photoelectrons emitted from potassium is 2.1 eV when illuminated by light of wavelength 3 * 10^-7 m and 0.5 eV when the light wavelength is 5 * 10^-7 m. Use these results to obtain a value for Plank's constant.'

i can use E = hf - w where w = work function, f = frequency and E is energy

if i rearange this to -w = E - hf

This gives two equations

w will be the same for both equations so these can be put equal to each other. since i know f and E for both equations i should be able to work out h.

however whenever i put the numbers in, i dont get the right answer. Could someone else give it a go, and see if they can do it. im begining to thikn there is a mistake in the question. But more than likely, ive done something wrong. Does my method look right?

Plotting E against f, h will be the gradient and -w will be the y intercept.

Finding h is simply a matter of using the two data points given to you to find the gradient, h. (You will have to convert wavelengths into frequencies and eV into Joules).

I get 6.4*10^-34 using this method.