Quantum Physics

Question 1

What is a photon?

Answer 1

It is defined as a quantum of electromagnetic energy.

Question 2

What is the formula for energy on a photon?

Answer 2

E = hf = hc/λ

Question 3

What is the photoelectric effect?

Answer 3

It refers to the emission of electrons from a metal surface when the surface is irradiated with eelctromagnetic radiation of a high enough frequency.

Question 4

What is a work function?

Answer 4

Work function is defined as the minimum energy required to eject an electron from the surface of a metal.

Question 5

What are some of the experimental observations of the photoelectric effect?

Answer 5

1. The emission of photoelectrons is almost instantaneous.
2. Electrons are emitted when the frequency of light is above the threshold frequency (no electrons are emitted regardless of light intensity below the threshold frequency)
3. The maximum KE is independent of the intensity of light but dependent on the frequency of light used.
4. The rate at which electrons are ejected and therefore the photocurrent produced is proportional to the intensity of the light

Question 6

How does the classical theory fail to predict the immediate emission of electrons?

Answer 6

Electrons should absorb energy over a period of time before it gains enough energy to escape the metal
A dim light after some delay would transfer enough energy whereas a very bright light would eject the electrons after a short while.

Question 7

How does the classical theory fail to predict the existence of a threshold frequency?

Answer 7

Energy of the wave is dependent on the square of its amplitude
Electrons will absorb enough energy to escape in due course.
There should not be any threshold frequency

Question 8

How does the classical theory fail to predict that the maximum kinetic energy depends on frequency?

Answer 8

The greater the light intensity, the larger the energy of the light wave striking the metal surface. Electrons should be ejected with greater kinetic energy.

Question 9

How does the classical theory successfully predict the observation that the rate at which electrons are ejected is proportional to the intensity?

Answer 9

Higher intensity of light should be able to simultaneously free more electrons from the metal at any instant which leads to a high current recorded.

Question 10

How does the Quantum Theory of Light explain the instanteneous emission of photoelectrons?

Answer 10

!! Photon-electron interaction is one-to-one !!
Energy of a single photon cannot be shared nad muist give up all of its energy to the electron.
This absorption then leads to a gain in energy for the electrons
There is no appreciable time delay for the electron to gain sufficient energy
hence photoemission is instantaneous.

Question 11

How does the Quantum Theory of Light explain the existence of a threshold frequency?

Answer 11

The electron is bounded to the metal and must acquire energy to escape . (i also dk how this explains the existence of a threshold frequency)
energy of photon = work function + max KE of electron
therefore max KE depends on radiation frequency (hf = Φ + 1/2mv^2)

Question 12

What is the threshold frequency?

Answer 12

It is the lowest frequency that will eject electrons from a particular surface.

Question 13

How does the Quantum Theory of Light explain the intensity as a measure of number of photons?

Answer 13

The intensity of the monochromatic light beam I is directly proportional to the number of photons passing through a unit cross-sectional area per unit time.
I = (N/t)x(hf/A) = Nhf/tA
Therefore the more intense the light, the larger the photon number per unit time hitting a particular surface.

Question 14

What is the formula for the work function?

Answer 14

Φ = hf0 (f0 is the threshold frequency)

Question 15

what is the formula for the de Broglie wavelength?

Answer 15

λ = h/p = h/mv

Question 16

how do you spell the funky wavelength thing? HAHA the λ = h/p yes

Answer 16

de Broglie wavelength

Question 17

What is the Heisenberg uncertainty principle?

Answer 17

ΔpΔx ≥ h

Question 18

What does it mean when an atom is in an excited state?

Answer 18

It is said to be excited when one or more of its electrons are not occupying the lowest possible states.

Question 19

What is an emission line spectrum? how does it look like yes

Answer 19

Its a series of dictinct coloured lines against a dark background.

Question 20

What is an absorption line spectrum? how does it look like?

Answer 20

It is a series of distinct dark lines against a continuous spectrum

Question 21

What are the formulas from the number of emission lines and the absorption lines?

Answer 21

emission lines: nC2 (when they de-excite, they can de-excite to many lower states)
absorption lines: n - 1 (when they excite, its usually from ground state)

Question 22

What are the characteristics of a X-ray spectra?

Answer 22

1. A broad continuous spectrum called Bremsstrahlung (how are we supposed to remember how to spell this? wn say agar agar know what u saying can alr but wtf)
2. Sharp peaks called characteristic lines
3. Sharp cut-off or minimum wavelength

Question 23

How do atoms excite?

Answer 23

When an accelerated incident electron collides with an electron from the innermost shell and kicks it out of the shell, there is a vacancy (!!) This vacancy in the K shell excites the atom.

Question 24

How is the continuous Bremsstrahlung spectrum formed?

Answer 24

errr When the electrons striking the atom interact with the crystal lattice, it experiences a force that causes them to be accelerated, decelerated or deflected. This kinetic energies are lost through the emission of Bremsstrahlung (braking radiation).
Since the magnitude of the deceleration is different for all and is not discrete, the wavelengths have a continuous distribution.

Question 25

Why is there a minimum/cutoff wavelength?

Answer 25

KE of incident electron = eV
hc/λ = eV
λmin = hc/eV

no electron possess more KE than this which can be lost as braking radiation.

Question 26

How does a X-ray spectra of a higher tube voltage look like?

Answer 26

there is high intensity as there will be greater number of electrons that can be removed and accelerated from the filament. Generally higher kinetic energy of the electrons undergoing deceleration should be able to produce more photons.

There is a lower cut-off wavelength too. The electrons with greater kinetic energy and thus emitting photons with higher energy when they completely decelerate. This leads to lower minimum wavelength.

There are peaks at the same wavelength. It is dependent on the energy level structure of the metal, which is dependent on element and is not affected by applied voltage.

Question 27

What is an alpha particle? (i.e what is it made of?)

Answer 27

2 protons and 2 neutrons and 0 electrons yes trust

Question 28

how do you spell the even more funky german word for “braking radiation” HAHAHAHAHA

Answer 28

Bremsstrahlung woah i typed this out from memory too good