Question

Electrons are ejected from sodium metal by any light that has a wavelength shorter than 544 nm. What is the kinetic energy of an electron (in J) that is ejected from sodium by light with a wavelength of 385 nm?

Answer 1

cutoff wavelength is 544 nm

we have below equation to be used:

Energy = Planck constant*speed of light/wavelength

=(6.626*10^-34 J.s)*(3.0*10^8 m/s)/(5.44*10^-7 m)

= 3.654*10^-19 J

This is cut off energy. Eo = 3.654*10^-19 J

Now find the energy of photons which ejects an electron

we have below equation to be used:

Energy = Planck constant*speed of light/wavelength

=(6.626*10^-34 J.s)*(3.0*10^8 m/s)/(3.85*10^-7 m)

= 5.163*10^-19 J

E = 5.163*10^-19 J

Kinetic energy,

KE = E - Eo

= 5.163*10^-19 J - 3.654*10^-19 J

= 1.51*10^-19 J

Answer : 1.51*10^-19 J