Question

A certain metal has a threshold wavelength of 6525 Å. Find the stopping potential when the metal is irradiated with

(a) monochromatic light having a wavelength of 4000 Å. 

(b) light having twice the frequency and three times the intensity of that in (a) above.

(c) If a material having double the work function were used, what would be the answer to (a) and (b) above?

Answer 1

(b) Stopping potential is independent of the intensity of the incident light but varies directly as frequency f provided it is more than f₀ .

Since frequency is twice, the wavelength of the light is half i.e. 2000 Å

(c) If work function is double, then λ₀ is reduced to half i.e λ₀ = 6525 / 2 =3262.5 Å. Since the incident light  has λ = 4000 Å, it would not be able to produce photo emission. 

In the second case, λ = 2000 Å.