Chem Problem help!, threshold frequency!?

Question

Find the velocity of an electron emitted by a metal whose threshold frequency 2.10×10^14 s^-1, when it is exposed to visible light of wavelength 4.89×10^−7

I have been trying to figure this out for so long! can some one Please give me a hand?
Thank you

Answer 1

I'm not very sure, anyway, try this:

General light wave function:
velocity = nu (frequency) x lambda (wavelength)
= (2.10×10^14 s^-1) x (4.89×10^−7)

(Note: Specific light wave function:
c = nu x lambda
where "c" indicates the velocity of light.)


Threshold frequency:
1/ No light, no current

2/ Shine light but not reaches the threshold frequency yet: no current because electrons can't store energy (quantized).

3/ Shine light @ threshold frequency: the intensity increases, the current occurs and increases.

Answer 2

Photoelectric effect. By the way, this is what Einstein got his Nobel prize for sorting out. They never gave him one for relativity.

Conservation of energy. Energy of light photon = threshold energy + k.e. of the electron (1)

To find the energy of the light photon used, and of the threshold energy, use

E = Planck's constant x frequency

You know the threshold frequency. You still need to find the frequency of the light used, and you can get this from

frequency x wavelength = speed of wave = c

You can now use Eq. (1) to calculate k.e.

I expect you know the formula for kinetic energy:

k.e.= 1/2m v^2

Where in this case m is the mass of the electron and v is the speed you want to find.

Care with units; since k.e. is in J, and v is in m s-1, m has to be in kg.

Good luck!