Quantum Physics

OVERVIEW

• Concept of Photon

• Blackbody Radiation and Planck’s Hypothesis

• Photoelectric Effect

• Compton Effect

• de Broglie Wavelength

• Relativity

Why we study this chapter?

Some application in light quantum physics

SOLAR CELL TECHNOLOGY

Theory of Light

WAVE PARTICLE

Light has both wave and particle

characteristic

Photoelectric Effect

Compton Effect

Blackbody Radiation

A black-body is any object that can absorb or emit radiation energy

Emissivity (𝑒) 0 ≤ 𝑒 ≤ 1

Perfect Reflector Perfect absorber or emitter

Reflects all radiation energy it received.

When absorbing, it will absorb all energy it receives. When emitting, it will emit all energy it has.

Example

Which of the following phrases correctly describes a perfectly

blackbody?

a. object from which neither light nor matter escapes

b. absorbs all radiation and emits no radiation

c. emits all radiation and absorbs no radiation

d. perfectly absorbs or emits all radiation

Concept of Photon

Light has an electromagnetic wave characteristic.

electromagnetic wave energy is emitted and absorbed by materials in the form of energy package called photons.

E = ℎ 𝑓

The energy of light can be stated as,

h = Plank’s Constant (6.63 x 10-34 J.s)

f = Photon’s Frequency (Hz)

𝑓 =c__λ

Example

A radio transmitter emits a wave of 15.08 MHz frequency. Calculate the energy of each photon of the wave. (h = 6.63x10-34 Js)

E = ℎ 𝑓

The energy of light can be stated as,

= (6.63𝑥10_34 Js)(15.08x106 Hz)

= 100 x 10-28 J

Example

Calculate the photon’s energy of an electromagnetic wave with 663 nm wavelength. (c = 3 × 108 m/s, h = 6.63 × 10–34 J.s)

E = ℎ𝑐

𝜆

The energy of single photon can be stated as,

= (6.63x10-34)(3𝑥108)

663𝑥10−9

= (10-34) (3x1015)

= 3x10-19 J

Photoelectric Effect

in 1887 Hertz observed a phenomenon during his experiments whichproved the existence of electromagnetic waves characteristic in light.

This phenomenon was the emission ofelectrons from a metal surface whenlight was incident upon it.

PHOTOELECTRIC EFFECT

The emitted electrons are calledphotoelectrons.

Photoelectric Effect

If the wavelength of light is held constant,an increase in light intensity causes anincrease in the number of electrons emitted.However, the kinetic energy of the emittedphotoelectrons does not change.

If the intensity of light is held constant,when the wavelength of light is changed,the number of emitted photoelectrons remainsthe same, but their kinetic energies change.

When a different metal is used,depending on the wavelength oflight, sometimes no emission isobserved.

Electrons are emitted from the surface almostinstantaneously.

Explained by Einstein in 1905

Photoelectric Effect

Energy of Photon =

Kinetic Energy of Electron

+Work

Function of Metal

The minimum energy needed to remove one electron from

the metal surface

E = KEelectron+ W

1 eV = 1.6 x10-19 Joule

Work Function

From the photoelectric equation, the minimum energy required by the photon

(to observe the photoelectric effect)must be equal to

the work function

E W=

h f =

The minimum frequency

of the photoelectric

effect

h foIf the photon’s frequency

is less thanthe minimum frequency

Then photoelectric effect cannot occurred

Photoelectric Effect

E = KEelectron+ W

KEelectron = E - Wh f h fo

For Photoelectric to occur:

• Photon’s frequency must be larger than the cut-off frequency.

• Photon’s Energy must be larger than the work function of the

metal.

f > fo

E > W

Examples (UN)

Perhatikan pernyataan berikut!

(1) Elektron dapat keluar dari logam saat permukaan logam

disinari gelombang electromagnetik.

(2) Lepas tidaknya electron dari logam ditentukan oleh frekuensi

cahaya yang datang.

(3) Fungsi kerja untuk setiap logam selalu sama.

Pernyataan yang benar berkaitan dengan efek fotolistrik adalah...

Examples (UN)

Keluarnya elektron dari logam karena radiasi gelombang

elektromagnetik disebut sebagai ....

A. Efek Compton

B. Efek fotolistrik

C. Foton terhambur

D. Fungsi kerja

E. Frekuensi ambang

Examples (UN)

Pernyataan-pernyataan berikut ini berkaitan dengan efek foto listrik:

(1) Efek foto listrik terjadi bila energi foton yang datang pada permukaan

logam lebih besar dari fungsi kerjanya

(2) Kecepatan lepasnya elektron dari permukaan logam bergantung pada

intensitas cahaya yang diterimanya

(3) Banyaknya elektron yang lepas dari permukaan logam bergantung

frekuensi cahaya yang menyinarinya

Pernyataan yang benar adalah ....

Examples (UN)

R

Examples (UN)

Grafik di samping menginformasikanenergi kinetik maksimum elektronyang disebabkan dari logam 1, 2, 3, 4 dan 5 yang disinari cahaya. Frekuensi ambang terbesar adalah ....

5

f

Examples (UN)

1Frekuensi ambang terkecil adalah ....

Compton Effect

Compton effect was first observed by Arthur Compton in 1923.

And this discovery led to his award of the 1927 Nobel Prize in Physics.

The discovery is important because it demonstrates that light cannot be explained purely as a wave phenomenon.

Compton Effect

Foton yang menumbuk electron

akan kehilangan sebagian energinya.

Sebelum Tumbukan Setelah Tumbukan

E E’

h f h f'

>>

Frekuensi

berbandingterbalikdengan

panjanggelombang

Panjanggelombangnya (λ)

bertambah

λ λ’<

Compton Effect

Efek Compton adalah terhamburnya foton setelah ditumbukkan pada

elektron.

momentum

p=

hλ

λ' = wavelength of photon after collision

m = mass of scattered electron

h = Planck’s Constant (6.63 x 10-34 J.s)

ϴ = angle of scattering

De Broglie Wavelength

Example (UN)

Examples (UN)

Panjang gelombang foton setelah menumbuk elektron semakin

besar. Peristiwa tersebut akan mengakibatkan ....

A. Energi foton berkurang

B. Energi foton bertambah

C. Kecepatan gerak foton berkurang

D. Kecepatan gerak foton bertambah

E. Energi sistem akan berubah

Examples (UN)

Jika suatu foton menabrak elektron sehingga terjadi efek

Compton, peristiwa yang tidak terjadi yaitu ....

A. Panjang gelombang foton bertambah

B. Energi foton berkurang

C. Berlaku hukum kekekalan energi

D. Elektron akan terpental dengan energi tertentu

E. Energi sistem berkurang setelah tumbukan