Quantum Physics Kssm

Quantum
Physics

By : Iskandar .

Table of Contents

01 02 03 04

Quantum Photoelectric Einstein’s Photoelectric
Theory of Light Effect
Photoelectric Theory Effect Applications

01

Quantum
Theory of Light

All objects emit electromagnetic radiation. Cold
objects emit waves with low frequencies, while hot
objects emit waves with higher frequency.

Black body

● an idealised body that is able to absorb all electromagnetic
radiation that falls on it.

● A black body can also emit thermal radiation depending on its
temperature.

● The radiation emitted forms a continuous spectrum and is
unaffected by the nature of the black body surface.

● The object emitting electromagnetic radiation which is
determined by its temperature is known as a black body radiator.

● The higher the light radiation intensity of a black body, the more
light energy it emits

Example

● The rays of light that enter the ear cavity will undergo

repeated reflections on the inner walls of the ear cavity.

● At each reflection, parts of the rays are absorbed by the

inner walls of the ear until all the rays are absorbed.

● Thus, the ear cavity acts like a black body.

Ideas that Sparked the
Quantum Physics Theory

Classical Theory Quantum theory

• Light is an electromagnetic wave that is • Based on the graph of radiation intensity

produced from the vibration of an electric against wavelength for black-body
radiation
charge.
• the light intensity on the left side of the
• In a hot object, electrons vibrate rapidly and
peak does not continue to increase with
randomly in any direction and produce light. the increase of wave frequency as
predicted by classical theory.
• As the object becomes hotter, the vibrations of
• This controversy in the concept of light
the electrons become more energetic and more
energy has sparked the theory of
light will be emitted. quantum physics

• According to classical theory, electrons vibrating

at the same frequency should have the same

energy content.

• The vibration frequency of the electrons also has

no limits.

• Thus, the light energy produced by the vibration

of electrons can reach unlimited high values

Classical Theory

Isaac Newton Thomas Young John Dalton J.J. Thomson

The partlicle nature Double-slit experiment Dalton Atomic Model Discovery of electron
of light
• Conducted doubleslit • Matter consists of basic • Discovered negatively
• Described light as a single experiment on light in
stream of particles or 1801 and showed that particles that cannot be charged subatomic
corpuscles in 1704. light is a wave.
further divided called particles called electrons in
• Unsuccessful in explaining • Unable to explain the
the phenomenon of light radiation spectrum atoms. 1897.
refraction due to failure in produced by black • Same elements have the • Designed experiment to
comparing the speed of light bodies
in glass and air same type of atoms. study the behaviour of
• Unable to explain the light
electrons.
• Unable to explain the line
spectrum produced by

atoms spectrum of light produced

by atoms.

Quantum Theory

Max Plank Albert Einstein Niels Bohr Louis de Broglie

• Introduced the concept • Introduced the photon • Explained the production • Introduced the
of quantum (discrete concept in 1905. of line spectrum by hypothesis on the wave
energy) in 1900 hydrogen atoms. nature of particles in
• E∝f 1924.
• E∝f • Einstein's quantum • The electrons in an atom
• The intensity of the orbit around its nucleus • Einstein and de Broglie
theory of light was on certain shells only. postulated the idea of
radiation is low for the successful in the wave-particle duality
high frequency waves explaining the • The transition of of light and all subatomic
characteristics of the electrons from a higher particles.
energy level shell to a
photoelectric effect lower energy level shell
emits photons.

Quantum of energy

discrete energy packet and not a
continuous energy.

• According to Max Planck and Albert Einstein's quantum theory,
light energy exists in the form of an energy packet known as a
photon.

• Photons are light energies transferred in quantum of energy.

E∝f
E = hf

Where ,
E = photon energy
h = Plank’s constant ( 6.63 × 10⁻³⁴ J s )
f = frequency of light waves

Example question Compare the energy of a 400 nm
and a 750 nm light photons.
1
3
Identify the problem
Identify the formula to be used
Energy of a 400nm photon
Energy of a 750 nm photon c = f λ , then f = _c_
E = hf = _h_c λ
2
λ
Identify the information given
4
Plank’s constant ,h = 6.63 × 10⁻³⁴ J s
Speed of light ,c = 3.0 × 10⁸ ms⁻¹ Solve the problem numerically
Wavelength , λ₁ = 400 × 10⁻⁹ m
Wavelength , λ₂ = 750 × 10⁻⁹ m E₁ = 6.63 × 10⁻³⁴ ( 3.0 × 10⁸ )
400 × 10⁻⁹
= 4.97 × 10⁻¹⁹ J

E₂ = 6.63 × 10⁻³⁴ ( 3.0 × 10⁸ )
750 × 10⁻⁹
= 2.65 × 10⁻¹⁹ J

Wave-Particle Duality

• Electromagnetic radiation such as light is said to have wave

properties because it exhibits the phenomena of diffraction and

interference.
• Objects like marbles are said to have particle properties because

they possess momentum and kinetic energy and can collide with
each other.

Louis de Broglie (‘de Broy’) (1892 – 1987), a quantum physicist, he

stated that the relationship between the momentum of a particle, p

and its wavelength, λ is as follows

λ =_ph_ λ =m_h_v P= _n_h_c
λ

Where
λ = wavelength
h = Plank’s constant ( 6.63 × 10⁻³⁴ J s )

p = momentum of particle
m =mass of particle
v = velocity of particle
n = number of photons emitted per second

Example question A 50 W lamp emits red light with a wavelength, λ =7.0 × 10⁻⁷m.
What is the number of photons emitted per second?
1
Identify the problem 3
Identify the formula to be used

Number of photons P= _n_h_c Then, n = _P_λ_
emitted per second λ hc

2 4
Solve the problem numerically
Identify the information given

Plank’s constant ,h = 6.63 × 10⁻³⁴ J s n = ____(7_._0_×__1_0_⁻_⁷_)(_5_0_)___
Speed of light ,c = 3.0 × 10⁸ ms⁻¹ (6.63 × 10⁻³⁴)(3.0 × 10⁸)
Wavelength , λ =7.0 × 10⁻⁷ m
Power ,P = 50 W = 1.76 × 10²⁰ s⁻¹

Difference between optical microscope and

• The de Broglie wavelength of an electron microscope

electron beam is approximately

1000 -10 000 times shorter compared

to the wavelength of light.
• This property is very important for
How it’s Diffraction
higher magnification of electron Shape Image working pattern
microscope.

Optical ..
microscope

Electron ..
microscope

02

Photoelectric
Effect

When a metal surface is illuminated by a
beam of light at a certain frequency,
electrons can be emitted from the metal

Activity to study photoelectric effect

When a light sensitive metal surface (cathode) is illuminated with a
certain light beam, electrons will be emitted from the metal surface.
These electrons are called photoelectrons.

The emitted photoelectrons are attracted
to anode which has positive potential

The movement of the photoelectrons from
the cathode to the anodeproduces a
current inside the circuit. The milliammeter
shows the current value

Activity 7.5

Aim: To determine the value of Planck's constant using the Planck's constant kit

Apparatus: Planck's constant kit ( 9 V battery, 1 kΩ potentiometer,
LEDs of different colours,milliammeter and voltmeter)

Steps : Inference : The activation voltage depends
on the LED light wavelength
1. Using the red LED, connect the Planck’s
constant kit as shown in Figure 7.8. Diagram :

2. Adjust the knob on the potentiometer to
obtain the voltage, V = 0.2 V. Record the
milliammeter reading in Table 7.2

3. Repeat step 3 for V = 0.4 V, 0.6 V,
0.8V,……3.0V

4. Draw a graph of current against
voltage.Based on the graph intercept
value on the voltage axis, determine the
activation voltage, Vaof the red LED.

5. Repeat steps 3 to 5 using orange, green
and blue LEDs.

Result : Plot graph :
Data analysis :
Gradient,m= hc
e
V-interceptis CalculatePlank’sconstant
activationvoltage , Va
using formula :

h= me
c

Conclusion :

The longer theLED light wavelength,
the lower the activationvoltage,Va

The characteristic of photoelectric

The higher the frequency of The kinetic energy of photoelectrons
the photon of light, the does not depend on the intensity of
higher the kinetic energy of light. An increase in the light intensity
the photoelectrons emitted does not produce photoelectrons
from the metal surface with a higher kinetic energy.

The minimum frequency of Photoelectrons are emitted
light needed for a metal to instantaneously when a metal
emit electrons is known as surface is illuminated by light
the threshold frequency, f₀

03

Einstein’s

Photoelectric Theory

In 1905, Albert Einstein introduced a
photoelectric theory that successfully
explained all the characteristics of
photoelectric effect in related experiments. He
was awarded the Nobel Prize in 1921 for this
achievement.

Based on Principle of Conservation of Energy

Photon energy Minimum energy required Maximum kinetic energy
to release a photoelectron of a photoelectron

E = W + Kmax At threshold frequency , f0 photoelectron are emitted
hf = W + ½mv²max without any kinetic energy , ½mv²max = 0
Then , 0 = hf0 – W
½mv²max = hf - W W = hf0

Subtitute W = hf0

½mv²max = hf - W
½mv²max = hf – hf0
½mv²max = h (f – f0 )

Work Function and Threshold Frequency for
Photoelectric Effect

work function = the minimum energy required for a
photoelectron to be emitted from a metal
surface

threshold frequency = the minimum frequency for a light photon
to produce photoelectric effect

The relationship between the maximum kinetic energy of
photoelectrons, Kmax frequency, f is shown by the graph

the x-intercept is threshold frequency • The higher the threshold frequency of a

metal, the higher the work function.

• This means the minimum energy

required for photoelectric effect to occur
is higher.

• Different metals have different threshold

frequencies

Example question A blue light with a frequency of 6.67 × 10¹⁴ Hz is shone on a
cleancaesium metal surface. What is the maximum kinetic energy of
1 photoelectrons emitted?
Identify the problem
[ Work function of caesium = 3.43 × 10⁻¹⁹ J , Plank’s constant ,h = 6.63 × 10⁻³⁴ J s ]

3

Identify the formula to be used

Maximum kinetic energy of hf = W + Kmax
photoelectrons emitted , Kmax Kmax = hf - W

2 4

Identify the information given Solve the problem numerically

Plank’s constant ,h = 6.63 × 10⁻³⁴ J s Kmax = hf - W
Work function of caesium = 3.43 × 10⁻¹⁹ J
Frequency , f = 6.67 × 10¹⁴ Hz = ( 6.63 × 10⁻³⁴ )(6.67 × 10¹⁴ ) – (3.43 × 10⁻¹⁹ )

= 9.92 × 10⁻²⁰ J

Generating Photoelectric
Current in a Photocell Circuit

The semi-cylindrical
cathode is coated with a
light-sensitiv. e metal and
connected to the
negative potential.

The anode is a metal rod
fixed at the axis of the
semi-cylindr.ical cathode
and connected to the
positive potential.

When the photocell is
illuminated by light, the
production of
photoelectric current is
produced in the circuit.

X Y1 eV= 1.602×10⁻¹⁹J
Example production of photoelectric current by
photocells coated with caesium and lithium

Diagram Work function Threshold Maximum wavelength
frequency, f0 required to produce
photoelectric current

ZCaesium 2.16 eV 5.16 × 10¹⁴ Hz 579 nm
2.50 eV 6.03 × 10¹⁴ Hz
Lithium X496 nm

04

Photoelectric Effect

Applications

For example :

Light detector of Solar cells Image sensor ISS solar panels
automatic door

• It uses infrared beam • The Noor Complex Solar • The image sensor is a main • The operation of the ISS
andphotocells as switches. Power Plant located in the component in high
Sahara Desertis the world's resolution cameras. (International Space
• When the light path is Station) depends on the
largest concentrated solar • This component is used to source of electrical energy
disturbed, photoelectric power plant. convert light into electrical generated from solar
current in the photocell • This station is expected to signals which can be panels.
circuit will be disconnected be completed in 2020 and processed to form digital
and the door will remain is capable of producing images. • The ISS has 16 wings of
open.
580MW capacity for use by solar panels and each
1 million residents. wing which measures 35
m × 12 m has 33 thousand
solar cells.

• These panels are capable

of generating 84 – 120 kW
of electricity.

Thanks

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