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Contents:

Photons 2

Photoelectric effect 4

Bohr Atom 11

Line Emission Spectra 14

Types of Spectra 18

Stimulated Emission and Lasers 19

Photons, Spectra and Lasers

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Red :

f= 4.76 x 10 14 Hz = 630 x 10 -9 m

Blue :

f= 7.90 x 10 14 Hz = 380 x 10 -9 m

Light travels in packets of energy called photons.

Photons have different frequencies and wavelengths

Blue light has photons with a higher frequency and

shorter wavelength than red light

Photons

3

Photons of light carry energy.

The energy is proportional to the frequency

E = h f

frequency of photon / Hz energy of photon / J

Planck’s constant6.63 x 10-34 J s

Blue photons have a higher frequency

and greater energy than red photons.

4

The Photoelectric Effect

Electromagnetic radiation can remove an electron

from a zinc plate if:

the radiation is ultraviolet

the plate is clean

the plate is charged negatively

Zinc plate

electroscope

ultraviolet

if the ultraviolet is more intense, the zinc discharges faster

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electron

Zinc atom photons of red light do not release electrons

photons of ultraviolet can release electrons

The photoelectric effect with zinc only works with ultraviolet

6

Photoelectric Current

ultraviolet radiation

thin quartz window

vacuum

2 kV

+

mA

anodeanodenegative zinc cathode

The photons of ultraviolet pass through the window onto the zinc

cathode

7

Photoelectric Current

ultraviolet

2 kV

+

mA

electrons

The photocurrent only flows if the frequency (and hence energy) of the photons is high

enough to knock the electrons from the zinc cathode

8

ultraviolet

2 kV

+

mA

electrons

photocurrent

f0 frequency of u.v.

The THRESHOLD FREQUENCY, f0, is the minimum frequency needed to release an electron from the surface of the zinc cathode.

E0 = hf0

The WORK FUNCTION, E0, is the minimum energy needed to release an electron from the surface of the zinc cathode.

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Kinetic Energy of the Photoelectron

zinc

Ek = hf - hf0

electron

photon

If the photon has more energy than the work function, the extra energy becomes the kinetic energy of the

electron.

Kinetic energy of electron = photon’s energy - work function

10

Photocurrent

Intensity of ultraviolet

If the intensity is increased, the photocurrent is increased.

Greater intensity means more photons per second, more ejected electrons per second and a greater photocurrent.

Doubling the intensity will double the photocurrent.

Ultraviolet

2 kV

+

mA

electrons

11

The Bohr Atom

nucleus

electronThe electrons are in fixed orbits round the nucleus

The positively charged nucleus is at the centre of the

atom

12

nucleus

electron

Electrons can drop to lower

energy orbits..

.. emitting the excess energy as photons of

light

13

E = hf

The biggest jump produces

photons with the biggest energy

….

…. and the highest

frequency

The smallest jump produces

photons with the smallest energy

….

…. and the lowest

frequency

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biggest energy jump

smallest energy jump

brightest line is the most popular jump- more transitions

occur

Line Emission Spectrum

lower frequency

longer wavelength

higher frequency

shorter wavelength

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Energy Levels

E0

E0

E1

E1

E2

E2

E3

E3

ground state

excited states

1 2 3 4 5 6

6 possible lines on the emission

spectrum

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Using numbers!

The energy levels for a hydrogen atom :

- 21.76 x 10-19 J

E0

- 5.43 x 10-19 JE1

- 2.47 x 10-19 JE2

- 1.36 x 10-19 JE3

- 0.84 x 10-19 JE4

The change in energy is:

- 2.47 x 10-19 - (- 5.43 x 10-19 )

= 2.96 x 10-19 J

An electron drops from E2 to E1

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E= 2.96 x 10-19 J

A photon of light is emitted, its frequency can be found :

E = hf

2.96 x 10-19 = 6.63 x10-34 x f

f = 2.96 x 10-19 / 6.63 x10-34

= 4.46 x 1014 Hz

- 21.76 x 10-19 J

E0

- 5.43 x 10-19 JE1

- 2.47 x 10-19 JE2

- 1.36 x 10-19 JE3

- 0.84 x 10-19 JE4

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300 400 500 600 nm

Spectra

Continuous spectrum

Absorption spectrum

Line emission spectrum

filament bulb

discharge tube

light from sun

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E3

E4

Stimulated Emission of Photons

phase direction wavelength

The stimulated photon has the same :

A photon with the same energy as the difference between the two energy levels causes an electron to fall to the lower level hence stimulates the emission of another photon.

20

Helium-Neon Laser

Helium-Neon gas is held in a tube.

There are mirrors at the ends of the tube - one lets 1% of the light pass through. The mirrors reflect the photons back into the gas and stimulate more transitions which amplifies the beam.A high frequency generator “pumps” the electrons back up to excited states.

The electrons in the gas are stimulated to emit photons of red light.

mirror99%

mirror

Helium/neon gas

generator

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mirror99%

mirror

LASER LIGHT

light amplification by the stimulated emission of radiation

Helium-Neon Laser

Helium/neon gas

Laser light is :

Monochromatic

In phase

Intense

Parallel