In the photoelectric effect, electrons are ejected from a metal surface when light strikes it. A certain minimum energy, Emin, is required to eject an electron. Any energy absorbed beyond that minimum gives kinetic energy to the electron. It is found that when light at a wavelength of 540 nm falls on a cesium surface, an electron is ejected with a kinetic energy of 260 x 10-20 1 When the wavelength is 400 nm, the kinetic energy is 1.54 x 10-19 J. (a) Calculate Emin for cesium in joules. (b) Calculate the longest wavelength, in nanometers, that will eject electrons from cesium.

The minimum energy of the electron is

The longest wavelength of the electron is 549 nm.

Given that, in the photoelectric effect, electrons are ejected from a metal surface when light strikes it. A certain minimum energy, Emin, is required to eject an electron. Also given wavelength of the light is 540 nm. The kinetic energy ejected by the electron is 260 x 10-20 J.

When the wavelength is 400 nm, the kinetic energy is 1.54 x 10-19 J.

The energy of the electron can be calculated as,

Where, is the energy of the electron, is plank's constant, is the speed of light that is and is the wavelength.

So the energy of the electron is,

The energy of the electron is  .

The Emin can be calculated as given below.

Where is the kinetic energy of the electron that is given as

So

The minimum energy of the electron is

For the longest wavelength, the electron will have its minimum energy that is Emin.

Hence, the longest wavelength can be calculated as given below.

The longest wavelength of the electron is 549 nm.

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A) E_min = 36.21 × 10^(-20) J

B) 549 nm

Explanation:

A) The formula for energy of a photon is given as;

E = hc / λ

Where;

h is Planck's constant = 6.626 x 10^(-34) J.s

c is the speed of light = 3 × 10^(8) m/s

λ is wavelength

Wavelength is given as; 540 nm = 540 × 10^(-9) m

Thus;

E = (6.626 × 10^(-34) × 3 × 10^(8))/(540 × 10^(-9))

E = 36.81 × 10^(-20) J

We are given kinetic energy as;2.60 x 10^(-20) J

Now formula for E_min is;

E_min = E - K.E

E_min = (36.81 × 10^(-20)) - (2.60 x 10^(-20))

E_min = 36.21 × 10^(-20) J

B) the longest wavelength, in nanometers, that will eject electrons from cesium would have an energy that would be equal to E_min.

Thus,

36.21 × 10^(-20) = (6.626 × 10^(-34) × 3 × 10^(8))/λ

Making λ the subject gives;

λ = (6.626 × 10^(-34) × 3 × 10^(8))/36.21 × 10^(-20) = 549 x 10(-9) = 549 nm

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