Quantum Physics

Question 1

What are the two ways to describe the nature of light?

Answer 1

Light behaves like a wave and like a stream of particles (photons)

This dual nature of light is a fundamental concept in quantum physics.

Question 2

What is a photon?

Answer 2

A photon is a quantum of electromagnetic radiation

Photons are considered as particles of light.

Question 3

What does Max Planck’s investigation reveal about electromagnetic waves?

Answer 3

Electromagnetic waves can only be released in discrete packets called quanta

A single packet of electromagnetic radiation is referred to as a quantum.

Question 4

What is the formula for energy carried by a wave packet?

Answer 4

E = h * f

Where E is energy, h is Planck’s constant, and f is frequency.

Question 5

What is the relationship between frequency and energy in electromagnetic radiation?

Answer 5

Higher frequency means more energy in wave packets

This is a key concept in understanding photon energy.

Question 6

What is the charge of a photon?

Answer 6

Photons have no charge; they are neutral

This is similar to neutrons in atomic structure.

Question 7

What is an electronvolt?

Answer 7

An electronvolt (eV) is defined as the kinetic energy gained by an electron when accelerated through a potential difference of 1 volt

1 eV = 1.60 × 10^-19 J.

Question 8

What is the value of Planck’s constant?

Answer 8

Planck’s constant is approximately 6.63 × 10^-34 Js

This constant is fundamental in quantum mechanics.

Question 9

What is the threshold voltage in context of an LED?

Answer 9

The minimum voltage needed for current to pass through an LED

This voltage allows electrons to gain energy equivalent to a photon.

Question 10

How can the Planck constant be estimated using LEDs?

Answer 10

By finding the threshold voltage for different wavelength LEDs and plotting a graph of threshold voltages against 1/λ

The gradient of this graph can be used to calculate Planck’s constant.

Question 11

What is the formula to find the energy of a photon using voltage?

Answer 11

E = e * V

Where E is energy, e is the charge of an electron, and V is the threshold voltage.

Question 12

What is the significance of the experiment with different colored LEDs?

Answer 12

It allows measurement of the Planck constant by using different wavelengths of light emitted

Each color corresponds to a different wavelength and energy.

Question 13

Fill in the blank: A single packet of EM radiation is called a _______.

Answer 13

quantum

Question 14

True or False: The energy of a photon can be described using the formula E = h * f.

Answer 14

True

Question 15

What happens to the kinetic energy of an electron when it is accelerated through a potential difference?

Answer 15

It is transferred into a photon

This is crucial for understanding photon emission in LEDs.

Question 16

What is the value of Planck’s constant used in calculations?

Answer 16

6.63 × 10^-34 Js

Question 17

What is the relationship between wavelength and energy in photons?

Answer 17

As wavelength increases, energy decreases

This is inversely proportional to frequency.

Question 18

Describe the key steps to measure the Planck constant using LEDs.

Answer 18

1. Connect an LED in the circuit 2. Adjust the resistor until current flows 3. Record voltage and wavelength 4. Plot voltage against 1/λ

This method provides a practical approach to finding Planck’s constant.

Question 19

What is the photoelectric effect?

Answer 19

The phenomenon where shining light on a metal causes the instantaneous ejection of electrons from its surface.

The emitted electrons are called photoelectrons.

Question 20

What type of electromagnetic waves typically cause the photoelectric effect in most metals?

Answer 20

Ultraviolet (U.V.) light.

This refers to the frequency range where free electrons can absorb energy and be ejected.

Question 21

What happens to a gold-leaf electroscope when exposed to ultraviolet light?

Answer 21

The gold leaf moves down as it loses its negative charge.

This occurs because the photoelectric effect causes free electrons to be ejected, resulting in a loss of negative charge.

Question 22

What is the threshold frequency in the context of the photoelectric effect?

Answer 22

The minimum frequency of radiation below which no photoelectrons are emitted from a metal surface.

If the frequency is below this threshold, electrons cannot be ejected regardless of intensity.

Question 23

How does the maximum kinetic energy of emitted photoelectrons relate to the frequency of incident light?

Answer 23

The maximum kinetic energy increases with the frequency of the incident light.

This relationship is described by Einstein’s photoelectric equation.

Question 24

What is the relationship between the number of photoelectrons emitted and the intensity of radiation?

Answer 24

The number of photoelectrons emitted per second is proportional to the intensity of the radiation, provided it is above the threshold frequency.

This means more photons hitting a surface per second leads to more emissions.

Question 25

According to wave theory, what is the predicted behavior of electrons when exposed to light?

Answer 25

Electrons should gradually gain energy from incoming waves and eventually be emitted, regardless of frequency. ## Footnote Wave theory fails to explain the observed threshold frequency and kinetic energy dependence.

Question 26

What does the photon model state regarding the photoelectric effect?

Answer 26

When light hits a metal surface, it bombards the surface with photons, and if a photon is absorbed by an electron, it gains energy equal to hf. ## Footnote 'hf' represents the energy of a photon, where 'h' is Planck's constant and 'f' is the frequency of the light.

Question 27

What is the work function energy?

Answer 27

The minimum energy required for an electron to escape from the metal surface. ## Footnote This value varies depending on the type of metal.

Question 28

What is Einstein's photoelectric equation?

Answer 28

hf = φ + KEmax ## Footnote Here, 'φ' is the work function energy and 'KEmax' is the maximum kinetic energy of the emitted electrons.

Question 29

True or False: The kinetic energy of photoelectrons depends on the intensity of the radiation.

Answer 29

False. ## Footnote The kinetic energy is determined solely by the frequency of the incident light.

Question 30

Fill in the blank: The maximum kinetic energy of a photoelectron is given by _____ = hf - φ.

Answer 30

KEmax

Question 31

What patterns do light produce, indicating its wave nature?

Answer 31

Interference and diffraction patterns - alternating bands of dark and light. ## Footnote These patterns occur due to waves interfering constructively (in phase) or destructively (out of phase).

Question 32

Who explained the photoelectric effect and how?

Answer 32

Einstein, by thinking of light as a series of particle-like photons that interact one-to-one with electrons. ## Footnote All the energy in the photon is given to one electron.

Question 33

What bold suggestion did Louis de Broglie make in his PhD thesis?

Answer 33

If 'wave-like' light shows particle properties, then 'particles' should show wave-like properties. ## Footnote This is known as the wave-particle duality theory.

Question 34

What does the de Broglie equation relate?

Answer 34

A wave property to a moving particle property ## Footnote The equation is h = λp, where h is Planck's constant.

Question 35

How is the de Broglie wave of a particle interpreted?

Answer 35

As a 'probability wave' - the likelihood of finding a particle is proportional to the square of the amplitude of the wave at that point.

Question 36

What significant experiment did Clinton Davisson and Lester Germer conduct in 1927?

Answer 36

They diffracted electrons, confirming that electrons show wave-like properties.

Question 37

What conditions lead to observable diffraction patterns in electron experiments?

Answer 37

Accelerated electrons in a vacuum tube interacting with spaces between carbon atoms in polycrystalline graphite.

Question 38

What happens to the diffraction pattern when the wavelength of the wave is greater?

Answer 38

The spread of the lines in the diffraction pattern increases.

Question 39

What is the relationship between electron speed and the diffraction pattern?

Answer 39

Increasing the electron speed causes the diffraction pattern circles to squash together towards the middle.

Question 40

What was the significance of electron diffraction experiments?

Answer 40

It provided the first direct evidence for de Broglie's theory.

Question 41

What is the typical size of the de Broglie wavelength for electrons accelerated in a vacuum tube?

Answer 41

About the same size as electromagnetic waves in the X-ray part of the spectrum.

Question 42

What is required for a particle to show wave-like properties such as diffraction?

Answer 42

The particle must interact with an object about the same size as its de Broglie wavelength.

Question 43

Fill in the blank: The de Broglie wavelength of a tennis ball is _______ compared to the nucleus of an atom.

Answer 43

10^-10 m smaller

Question 44

What is the formula for calculating the momentum of an electron?

Answer 44

p = mv, where m is mass and v is speed.

Question 45

What wavelength do electrons need to be diffracted by atoms in polycrystalline structures?

Answer 45

Around 1 × 10^-10 m.

Question 46

How does diffraction affect image resolution in electron microscopes compared to light microscopes?

Answer 46

Electron microscopes can resolve finer detail because they use shorter wavelengths than light microscopes.

Question 47

True or False: Quantum physics suggests that electrons and photons can be classified strictly as either waves or particles.

Answer 47

False

Question 48

What does the term 'juxtaposition of states' refer to in quantum physics?

Answer 48

The idea that electrons and photons exhibit both wave-like and particle-like properties.