Ultraviolet light of wavelength 2271 Å from a 100 W mercury source irradiates a photo-cell made of molybdenum metal. If the stopping potential is –1.3 V, estimate the work function of the metal. How would the photo-cell respond to a high intensity (∼105 W m–2) red light of wavelength 6328 Å produced by a He-Ne laser?
Given:
Wavelength of light, λ = 2271 Å = 2271 × 10-10m
Power of mercury source, E = 100 Js-1
Stopping potential, Vs = -1.3 V
Let frequency of light = v
Work function, Φ0 is given by,
Φ0 = hv-eVs
Φ0 = 
Where,
h = Planck’s constant = 6.6 × 10-34Js
c = speed of light = 3 × 108m
λ = wavelength of light
e = charge on each electron = 1.6 × 10-19C
Φ0 = 
Φ0 = 6.64 × 10-19J
Φ0 = (6.64/1.6) × 10-19 eV = 4.15 eV
Let v0 be the threshold frequency of the metal,
Φ0 = hv0
v0 = Φ0/h
→ v0 = 6.6 × 10-19/ 6.6 × 10-34
→ v0 = 1.00. × 10-15 s-1
Wavelength of red light, λ’ = 6323 × 10-10m
Frequency of red light can be given as,
v' = c/λ’
v’ = 
v’ = 4.74 × 1014 Hz
Since the threshold frequency is greater than the frequency of red light, the photocell will not respond to the red light produced.