Problem with photoelectric effect

[ant284]

Hi,

I'm have a problem with photoelectric effect
it states that we have a wavelength of 2.08*10^-7 falls on the photosurface, a voltage of 1.40V is required to stop the emitted electrons from reaching the anode.
What is the largest wavelength of light which will result in emission of electrons from this photosurface.?

so we have the equation
eV=hf-W
W=h* critical frequency and
K=hf-W
and
f=speed of light/wavelength.

v=1.40
f=(3*10^8)/(2.08*10^-7)

And wouldn't the Kinetic energy be 0 for a wavelength of 2.08*10^-7
So i don't know where to go from here! I want to do it but I can't it is frustrating!
Any help is appreciation

 

[ant284]

Hi again,

I think i got it

we have
K(kinetic)= eV
eV=hf-W
1.4 * (1.6*10-19) = h c/wavenlength -work
we get work =
7.7*10^-19 (About)
Now when it is the largest value above this value no electron will be emitted therefore KE=0
so we would have
0= Hc/waveleng-W
or
hc/w=wavelength
and we get 2.6*10^-7

But my question what would be the minimum value of wavelength?? if it is possible to find

 

[Mulder]

Your answer to the question is right (I got 272 nm without rounding). As for a minimum wavelength - there isn't really a restriction on how short wavelength the light is aslong as it is shorter than the threshold (maximum) wavelength. It's just that a higher potential would be needed to stop emission.

 

[ant284]

Hi,

one more quick question, as long as the frequency is below the threshold electrons are emitted, and above the threshold, the electrons are not emitted? Am i correct

 

[Mulder]

Damn sorry I've probably just confused you now. I've editted my original to say wavelength instead of frequency (oops). Aslong as the photons are above the threshold frequency, or below the wavelength at this frequency will there be emission. Sorry I kept swapping wavelength and frequency and probably confused us both!

Anyway to make it clear again - aslong as the photons are above the threshold frequency, or have a shorter wavelength than the wavelength at this frequency there will be electron emission. So there is really no minimum wavelength value. lol

editted: about 10 times for spelling and grammar