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CHAPTER 28 a

PHOTOELECTRIC EFFECT

When light (ultraviolet) falls on a metal

surface electrons are observed coming off

with various

velocities.

To study the effect we need an apparatus

like

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What we find is

The voltage necessary to stop all electrons

is called the stopping voltage, V0

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The photocurrent depends on the intensity

of the light.

The stopping voltage is the same for all

intensities of light.

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Change the frequency

The photocurrent is the same for all

frequencies.

The stopping voltage is more negative for

higher frequencies.

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Plot stopping potential times e for various

frequencies of light, f

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Cannot explain with electromagnetic wave.

Einstein Solution

1. Light made up of pulses or quanta of

energy called photons

2. Photons appear or disappear in units of 1

or more.

3. Each photon has energy ℎ�

Photon has energy ℎ�and gives this energy

to one electron in metal. Electron uses

certain amount of energy to escape. Left

over energy goes into kinetic energy

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Note: �� ≝ �

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Think: Conservation of Energy

Energy available from photon less energy to

remove electron from metal equals energy

of escaping electron.

ℎ� − � = � = 1

2��� = ���

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The electrons with the highest velocity will

have come from the top of the energy

levels in the metal – near the Fermi level.

Note in figure �� ≝ �

The energy needed to escape will be the

work function.

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Therefore

ℎ� − � = ���

Also:

��� = ℎ� − �

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Einstein theory explains all data.

Increase intensity – number of electrons

increases and current increases

Increase frequency – energy of photons is

greater and give more energy to electrons

and need larger ��.

No time delay – photon gives all of energy

to one electron.

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WHAT IS LIGHT

WAVE?

PARTICLE?