QUANTUM NOTES

Physics

So whoever does an atom’s weight of good will see it.

And whoever does an atom’s weight of evil will see it.

Surah Az-Zalzalah

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ƒincrease decrease 7.1 QUANTUM THEORY OF LIGHT

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ƒobjeLctOW

HOT

ƒobject

HIGH

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BLACK 7.1 QUANTUM THEORY OF LIGHT
BODY
• A surface that absorbs and emit all
emeniettregdyby electromagnetic radiation which is ideal

• emit thermal radiation (Cannot be seen by the human eye)

Ideal : absorbed = emitted

Black body emission= Black Body Radiation
An ideal electromagnetic radiation in the form of heat or light

Thermal radiation in light Oven heating Thermographic
bulbs at different element image of a car
temperatures
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1. Light intensity 7.1 QUANTUM THEORY OF LIGHT
• Object as a black body radiator
Light energy • Emit thermal radiation
emitted

2.

Temperature

3.

Temperature

ƒdecrease increase Wavelength

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Classical theory 7.1 QUANTUM THEORY OF LIGHT

Isaac Newton Thomas Young John Dalton J.J Thompson
(1643-1727) (1773-1829) (1766-1844) (1856-1940)

• Described the • Showed light as a • Atoms as the • Discovered
particle wave (Double- smallest negatively-charged
nature of slits experiment) particle of subatomic particles
light matter (electrons)
• Unable to
• Unable to explain the black • Unable to • Unable to
explain the body explain the light explain the light
refraction radiation spectrum spectrum
of light spectrum produced produced
by atoms by atoms
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QUANTUM theory 7.1 QUANTUM THEORY OF LIGHT

• Introduced the idea of • The line spectrum is produced
quantum as packet of by Hydrogen atoms
discrete energy
• Photons are emitted when electrons
• Electromagnetic wave emitted by a black transit from high energy levels
body is in discrete packet of energy to low energy levels
(quantum of energy)
Max Planck Niels Bohr
(1858-1947) • E = hƒ (1885-1962)

Albert Einstein • Developed Planck’s theory by stating Louis de Broglie • Introduced a hypothesis that
(1879-1955) that light exist in (1892-1987) particles can also show wave
quanta (photon) properties

• From E = hƒ (relate the kinetic energy • Einstein & de Broglie postulated the idea
of the photoelectrons that are emitted of wave-particle duality
with light frequency) of light and all subatomic particles

• Discovered the 6DREAM BIG AIM HIGH NEVER GIVE UP
photoelectric effect

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The visible light is considered continues 7.1 QUANTUM THEORY OF LIGHT
because there is NO gap separation
between each color in the spectrum QUANTUM OF ENERGY

CONTINUOUS
SPECTRUM

LINE The line spectrum shows the electromagnetic radiation of specific
SPECTRUM frequencies.
Every line in the line spectrum is caused by a transition, form one quantum state to another
ƒincrease quantum state involving electrons

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QUANTUM OF ENERGY: PLANCK’S QUANTUM THEORY 7.1 QUANTUM THEORY OF LIGHT

discrete energy • light exist in quanta (photon)
packet and not a • Energy size of the radiation depends on frequency
continuous energy

E = hf Speed of the electromagnet radiation
in vacuum is c (3.0 x 108 ms-1)
E = photon energy
ƒ = frequency c = f
h = Planck’s constant (6.63 x 10-34 Js) ! decrease
ƒ = "
ƒincrease Energy quantum
∴ E = !" increase
Energy quantum increase #

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De Broglie’s wavelength: WAVE-PARTICLE DUALITY 7.1 QUANTUM THEORY OF LIGHT
De Broglie’s principle:
Reflection of light : particle
! ! = wavelength Refraction of light : wave
" #$ p = momentum of particle = mv
= = m = mass of particle Experimental evidences:
v = velocity of particle
h = Planck’s constant (6.63 x 10-34 Js) Wave Particle

Wave properties of matter = De Broglie waves or matter waves Young’s double-slit Photoelectric effect
Diffraction Compton effect

E = hfPhoton energy Photon = electron + light

Photon power Wave-particle duality

P = nhf(n = number of photon - All electromagnetic spectrum
emitted per second) - Subatomic particle (proton & neutron)

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Wave-particle Duality 7.1 QUANTUM THEORY OF LIGHT

Reflection of light : particle
Refraction of light : wave

Reflection of light : particle Refraction of light : wave

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7.2 PHOTOELECTRIC EFFECT Emission of photoelectrons from
The emitted Photoelectrons a metal surface when light at a
(electrons) attracted
certain frequency illuminates the metal
to the anode
Light sensitive metal
Amazing Physics with Tcer Alina surface (cathode)
- Illuminated with a certain light

beam
- Photoelectrons (electrons)

will be emitted from surface metal

The movement of Photoelectrons
(cathode → anode) produces
current

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7.2 PHOTOELECTRIC EFFECT Relationship between the activation voltage (Va) and the light wavelength.

Graph I against V Graph Va against Gradient = = $%
&
I/A Va

Va V/V = ℎ ∴ h = &'
(
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1 V decrease
aincrease

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7.2 PHOTOELECTRIC EFFECT increaseƒ photon:
• Photoelectric effect occurs when light strikes the surface of increaseKinetic energy of Photoelectrons:

a metal. ƒo (threshold frequency): KE max / J
• The electrons in the metal absorb energy from the light
Minimum frequency of Metal
and escape from the metal surface. light needed for metal to
• CLASSICAL THEORY, light in wave form is a spectrum emit electrons

with continuous energy and photoelectric effect should be Kinetic energy of ƒo ƒ/Hz
able to occur at any light wave frequency. Photoelectrons does
• Bright light which has high energy should NOT depend on the
be able to emit electrons quickly. intensity of light
• Dim light has low energy so the electrons
need a longer time to absorb enough energy to escape Photoelectrons are emitted instantaneously
from the metal surface. when metal surface is illuminated
• The PHOTOELECTRIC EFFECT EXPERIMENTS show that by light
the emission of photoelectrons only apply to light waves
with frequencies that exceed a certain 13DREAM BIG AIM HIGH NEVER GIVE UP
value without being affected by the
intensity of the light.
• Photoelectrons are also emitted instantaneously at those
light frequencies even at low light intensities.

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Characteristics of PHOTOELECTRIC EFFECT

CLASSICAL WAVE OBSERVATIONS FROM THE
THEORY PREDICTIONS PHOTOELECTRIC EFFECT EXPERIMENT

HIGHER intensity of light LOW intensity of light (HIGHER frequency)
GREATER energy imparted to the metal surface will eject the photoelectrons
(emission of photoelectrons)
Kinetic energy of Photoelectrons does NOT
depend on the intensity of light

Emission of photoelectrons occurs for all frequencies Emission of photoelectrons ONLY when frequency of radiation
of radiation exceeds the threshold frequency, ƒo, for metal surface
Energy of radiation depends on intensity NOT Energy (kinetic energy) of radiation depends on FREQUENCY
FREQUENCY

Radiation energy is spread over the wavefront. Photoelectrons are ejected instantaneously.
The amount of energy incident on any one electron is very NO time delay between irradiation & emission
small. (even the intensity is low)
An electron MUST gather sufficient energy to be emitted (there is
a time lapse between irradiation & emission) 14DREAM BIG AIM HIGH NEVER GIVE UP

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7.2 PHOTOELECTRIC EFFECT PHOTOELECTRIC
EFFECT
THERMIONIC
EMISSION Emission of photoelectrons from a
metal surface when light at a certain
Emission of electron from
heated surface metal frequency illuminates the metal
Take some time
(longer time) Instantaneous
(shorter time)
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7.3 EINSTEIN’S PHOTOELECTRIC THEORY

Idea!! Photons are light particles that bring energy.
Photon energy is directly proportional to the frequency of light, ƒ.

E = hf E = photon energy
ƒ = frequency
h = Planck’s constant (6.63 x 10-34 Js)

Minimum frequency of light needed for metal to emit
electrons

Amazing Physics with Tcer Alina Minimum energy needed for photoelectrons to be emitted
from a metal surface

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7.3 EINSTEIN’S PHOTOELECTRIC THEORY AT THE THRESHOLD FREQUENCY, ƒo

FOR THE ELECTRONS ON THE METAL SURFACE

Photoelectrons are emitted without kinetic energy
'
MINIMUM MAXIMUM ( mv2max = 0
energy required kinetic energy of
Photon Energy = + a photoelectron
to release a
photoelectron W0 ==hhƒƒoo- W

E = W + K max

hƒ = W + ) mv2max ) mv2max = hƒ - W
* *

) mv2max = hƒ - W ) mv2max = hƒ - hƒo
* *

*Accordance with the Principle of Conservation of Energy ) mv2max = h(ƒ – ƒo)
*

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7.3 EINSTEIN’S PHOTOELECTRIC THEORY Work Function and Threshold Frequency for Photoelectric Effect

Work function, W: Minimum energy needed for photoelectrons to be emitted from a metal surface

ƒo (threshold frequency): Minimum frequency of light needed for metal to emit electrons

NO Photoelectric effect • The graph is a straight line with a KE max /J Metal B
Photoelectric EXISTS positive gradient and NOT passing Metal C
through the ORIGIN. Metal A
effect
KE max /J • Threshold frequency, f0is the value of
the intercept on the
frequency axis. Graphs with the
same gradient

0 ƒo A ƒo B ƒo C ƒ/Hz

0 ƒo ƒ/Hz

Amazing Physics with Tcer Alina Threshold frequency, f0 : increase
Work Function: increase

This means the minimum energy required for photoelectric effect to occur is higher.
Different metals have different threshold frequencies.

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7.3 EINSTEIN’S PHOTOELECTRIC THEORY Generating Photoelectric Current in a Photocell Circuit

Cathode: semi- When the photocell is illuminated by
cylindrical light, photoelectric current is produced
in the circuit
A circuit consist of a
glass vacuum tube Metal rod (anode)

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7.3 EINSTEIN’S PHOTOELECTRIC THEORY Production of PHOTOELCTRIC CURRENT by photocells coated
with CAESIUM and LITHIUM

CAESIUM LITHIUM

Work function of caesium, W = 2.14 eV Work function of lithium, W = 2.50 eV
Threshold frequency, f0 = 5.16 × 1014 Hz Threshold frequency, f0 = 6.03 × 1014 Hz

Maximum wavelength to produce Maximum wavelength to produce
photoelectric current, = 579 nm photoelectric current, = 496 nm

Work Function: increase Light Intensity: increase
Shortermax. wavelength to produce photoelectric current Photoelectric current: increase

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7.3 EINSTEIN’S PHOTOELECTRIC THEORY PHOTOELECTRIC EFFECT APPLICATIONS

LED Solar Cells NOOR COMPLEX POWER PLANT Satellite powered by solar cells
(MOROCCO)

Light detectors at the automatic doors use The image sensor is a main component
infrared beam and photocells as switches in high- resolution cameras

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‫ا ﱣ"ُ َﺧﺎ ِﻟـ ُق ﻛُ ِّل َﺷ ۡﻰ ٍء‬
ALLAH IS THE CREATOR OF ALL THINGS
SURAH AR-RA’D: AYAT 16

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