Question

Ultraviolet light of wavelength 2271 Å from a 100 W mercury sourceirradiates a photo-cell made of molybdenum metal. If the stoppingpotential is –1.3 V, estimate the work function of the metal. Howwould the photo-cell respond to a high intensity (∼105 W m–2) redlight of wavelength 6328 Å produced by a He-Ne laser ?

Answer 1

It is given that the wavelength of ultraviolet light, λ=2271  A o or 2271× 10 −10  m, stopping potential of the metal, V=1⋅3 V.

The formula of photo energy relation can be written as,

ϕ= hc λ −eV

Substitute the values in above expression.

ϕ= 6⋅626× 10 −34 ×3× 10 8 2271× 10 −10 −1⋅6× 10 −19 ×1⋅3 ϕ=8⋅72× 10 −19 −2⋅08× 10 −19 ϕ=6⋅64× 10 −19  J

The formula of work function is,

ϕ=h υ o

Substitute the values of h and ϕ in the above equation.

6⋅64× 10 −19 =6⋅626× 10 −34 × υ o υ o =1⋅006× 10 15  Hz

It is given that the wavelength of red light, λ r =6328  A o or 6328× 10 −10  m.

The formula of frequency is,

υ r = c λ r

Substitute the values in above expression.

υ r = 3× 10 8 6328× 10 −10 υ r =4⋅74× 10 14  Hz

Hence, υ o > υ r which means the photocell will not respond to the red light produced by the laser.