Question

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?

Answer 1

Wavelength of ultraviolet light, λ = 2271 Å = 2271 × 10⁻¹⁰ m

Stopping potential of the metal, V₀ = 1.3 V

Planck’s constant, h = 6.6 × 10⁻³⁴ J

Charge on an electron, e = 1.6 × 10⁻¹⁹ C

Work function of the metal =

Frequency of light = ν

We have the photo-energy relation from the photoelectric effect as:

= hν − eV₀

Let ν₀ be the threshold frequency of the metal.

∴= hν₀

Wavelength of red light, = 6328 × 10⁻¹⁰ m

∴Frequency of red light,

Since ν₀> ν_r, the photocell will not respond to the red light produced by the laser.