Photoelectric effect

p. 275

The photoelectric effect is when light shines on metal and electrons are released from the metal.

Valensband

Conduction band

-

Valence band-

Effect of intensity

Valensband

Conduction band

-

Valence band-

-

-

-

-

Threshold frequencyThe minimum frequency of light required to release electrons from a certain metal surface. (f0)

Work functionThe working function of a metal is the minimum energy required by an electron in the metal to be released from the metal surface. (W0)

PhotonsLight is emitted as elementary particles (quantum) and is called photons.

Energy of photons

Eαf

E = hf = ℎ𝑐

𝜆

Planck's constant: h = 6,63 x 10-34 J.s

Energie

E = hf

e-

e-

E = W0 + Ek

Energy E = W0 + Ek

hf = hf0 + ½mv2

Kinetic energy of photoelectrons released is determined by the frequency of the light.

or ℎ𝑐

𝜆= hf0 + ½mv2

What is the meaning of the photoelectric effect?

• Establish the quantum theory• Demonstrate the particle of light.

0 1

A

e ee

e

ee

e

e

e

e

e

e

e

e

e

ee

e

ee

Wo = hfo

E = hf

E =ℎ𝑐

𝜆

Ek = 1

2𝑚v2

E = Wo + Ek

0 1

A

e ee

e

ee

e

e

e

e

e

e

e

e

e

ee

e

ee

Low fLong λ

0 1

Ae

e

e

e

Low f

High intensity

e ee

ee

e

e

e

e

e

e

e

e

e

ee

0 1

eAHigh fshort λ

e ee

e

ee

e

e

e

e

e

e

e

e

ee

e

ee

0 1

e

e

e

e

A

High intensity

High f

e ee

ee

e

e

e

e

e

e

e

e

ee

Homework

Exercise 1 p 3242, 5, 7, 12, 22

Emission and absorption spectra

p. ...

Spectra

Light ray is refracted to demonstrate the frequencies of all the colours in a continuous spectrum.

Diffraction gratingSpectroscope

Spectra

Spectra

Emission spectra Absorption spectra

Continuous

Veroorsaak deur ligbron

Line emission

Spectra

Emission spectrum of gasses - Hydrogen

n=1

n=2

n=3

n=4

n=5

n=6

ground state

1st excited state

--

--

Infrared series

Ultravioletseries

Internal energy of an atom

n=1 (E1)

n=2 (E2)

n=3 (E3)

Energy levels

-

When internal energy is lost, the energy is emitted as a photon:E = E3 – E2 = hf

Example: For the

electron crossing of energy levels E3

to E2 calculate the energy of an emitted photon.

n=1 (E1)

n=2 (E2)

n=3 (E3)

-1,2 x 10-19 J

-

E = E3 – E2 = hf= -2,614 x 10-19 - (-5,424 x 10-19)= 3,81 x 10-19 J

-2,614 x 10-19 J

-5,424 x 10-19 J

-21,76 x 10-19 J

Example: For the

electron crossing of energy levels E3

to E2 calculate the frequency of an emitted photon.

n=1 (E1)

n=2 (E2)

n=3 (E3)

-1,2 x 10-19 J

-

E = hf3,81 x 10-19 = 6,63 x 10-19 x ff = 4,24 x 1014 Hz

-2,614 x 10-19 J

-5,424 x 10-19 J

-21,76 x 10-19 J

Example: It is found that the wavelength of the indigo line in the hydrogen spectrum is

approximately 434 nm. Calculate the energy of a quantum (photon) of the indigo light.

E = ℎ𝑐

𝜆

= (6,63×10−34)(3×108)

4,34×10−7

= 4,58 x 10-19 J

Absorption spectra

When light passes through a cold gas light (photons) is absorbed so that electrons move to a higher energy level. The light is emitted again (in other directions). The light that passes through the gas shows a spectrum without the absorbed light.

Absorpsiespektrum Emisiesiespektrum

Huiswerk

P 3422, 6, 9