Quantum Physics

Question 1

What is the photoelectric effect ?

Answer 1

Phenomenon in which electrons are liberated from a surface when electromagnetic radiation of sufficiently high frequency are incident upon it

Question 2

What is the evidence for photoelectric effect ?

Answer 2

1. KEmax dependent on frequency but independent on intensity
2. Instantaneous emission (negligible time delay between incidence of light on metal surface and emission of photoelectrons even at low light intensities)
3. Existence of threshold frequencies (state definition)

Question 3

What is the threshold frequency

Answer 3

Threshold frequency is the minimum frequency of the incident radiation to allow for emission of photoelectrons from the metal, regardless of the intensity of incident light

Question 4

What is KEmax affected by and why

Answer 4

KEmax is unaffected by intensity of light and applied voltage, only frequency of incident radiation. When the frequency of the photon increases, the photon energy increases (since Ephoton = hf) and hence more energy is absorbed by the photoelectrons, increasing Vs and KEmax

Question 5

What does intensity affect ?

Answer 5

If the frequency remains constant, the energy of each photon remains constant. A larger intensity means a larger number of incident photons per unit time and a PROPORTIONATELY larger number of electrons emitted per unit time, resulting in PROPORTIONATELY larger photoelectric current

Question 6

How is the X ray emission spectra formed ?

TAKE NOTE EMISSION VS EMISSION LINE SPECTRA / ABSORPTION LINE SPECTRA

Answer 6

Continuous X Ray spectrum origin (Bremsstrahlung)
- X Rays are produced when fast electrons are suddenly decelerated upon collision with the atoms on the metal target
- The frequencies of X rays have a continuous and large range as the deceleration of x rays occurs infinite number of ways and the frequencies of x rays are determined by the loss of KE of the decelerated electron
-Hence the energy lost by the electrons vary over a continuous range from one collision to another
[applied / accelerated voltage (energy of electrons that strike the target) affects min wavelength according to formula - note this formula can only be applied to min wavelengh for x rays]

Characteristic X Ray origin
- High energy electrons colliding with the metal target atom can knock electrons out of an inner shell of this metal target
- Another electron (of target metal) from a higher energy level drops down to fill the vacancy
- This emits an X Ray with a specific wavelength, which is determined by the discrete energy levels (which are characteristic of the target metal)
[independent of applied voltage and dependent on target metal]

THE EMITTER EMITS ELECTRONS FOR X RAYS AND PRODUCES PHOTONS AND PHOTONS FOR ELECTRIC EFFECT AND PRODUCES ELECTRONS

Question 7

What its the minimum wavelength (from the X ray formula)

Answer 7

It is the wavelength of an X Ray emitted when a bombarding electron loses all its kinetic energy from a SINGLE collision with the target metal ATOM

Question 8

What is the waveform observed in the EM spectrum during ionisation of hydrogen atom

Answer 8

Maximum amount of energy emitted in the electron transitions for hydrogen atom is 13.6eV which corresponds to UV radiation and the ionisation energy level is too low to observe X Ray or any other wavelengths with higher frequencies (and shorter wavelengths)

Question 9

Formation of emission line spectra

Answer 9

A series of discrete (separate) coloured lines of definite wavelength on a dark background, produced by an electron. Transition within an atom from higher to lower energy levels and emitting photons.
[produced by hot gas / when electrons (current) shine on cold gas which excites the gas, and when it deexcites, the gas emits photons in all directions, forming ELS)

Question 10

Formation of absorption line spectrum

Answer 10

It is a continuous coloured spectrum crossed by dark lines due to missing frequencies. It is produced when white light passes through a cool gas and the electrons in the gas absorbs photons of certain frequencies from the white light source and gets excited to a higher energy level and these photons are then quickly reemitted uniformly in all directions
[produced by shining hot light on cold gas]

Question 11

what is the ionisation energy

Answer 11

it is the minimum energy required to remove the outermost electron from the atom (grd state (outermost shell) to n=infinity where the energy at Einfinity=0)

Question 12

How does photoelectric affect provide the particulate nature of the EM radiation ?

Answer 12

2 ways

1. From the eqn KEmax=hf-work fn , if the energy of the photon (hf) < minimum energy required to remove the least strongly held electron (or the min energy required for emission) the emission cannot take place no matter how high the intensity of light
2. Even at low intensities, emission can take place as long as hf > work fn without a time delay between illumination of metal and emission of electron

Question 13

What is the stopping potential (Vs)

Answer 13

The minimum negative potential required to stop the fastest electron and hence all electrons from reaching the collector

Question 14

Why do photoelectrons have a range of KE, isn’t hf-work fn a constant ?

Answer 14

electrons below the surface lose some KE on the way to the surface if they collide with the metal lattice and they do not all experience the same loss of KE during such collisions before they are emitted

Question 15

what are photons

Answer 15

discrete packet of energy of electromagnetic radiation

Question 16

why do photoelectric current reach max value no matter how large V is made ?

Answer 16

given that intensity is constant, the number of photons incident per unit time on the metal surface is constant and the number of electrons emitted per unit time is constant. At a certain V, all the emitted photoelectrons per unit time has been collected and the current saturates

Question 17

why is KEmax not affected by intensity

Answer 17

By hf = work fn + KEmax, and the interaction between photons and electrons are one to one, when the same frequency of radiation is used, the photons will have the same energy and since the metal being shone on is the same, the work fn is the same and hence KEmax is the same. Thus the most energetic photons require the same Vs (stopping potential) that prevents them from reaching the collector
LINK KE MAX ALWAYS TO VS

Question 18

Why does the photoelectric effect have low efficiency

Answer 18

the rate of photoelectrons emitted < rate of photons incident

• very few incident photons have succeeded in injecting an electron
• the metal (emitter) may have reflected radiation and thus reflected photons away instead of being absorbed by electrons on the surface of the metal

Question 19

Why does the work function energy differ from the energy required to remove an outer electron from its atom ?

Answer 19

Ionization energy is the energy required to remove an outer electron from its atom, while work function is the minimum energy required to remove a delocalised electron from the solid surface of a metal. The delocalised electron was one of the valence electron in the metal and has already been freed from its atomic orbital and is free to move through the bulk of the metal and is less tightly bound and thus work fn requires less energy

Question 20

why are light of specific wavelengths produced in the lamp

Answer 20

The _gas__ in the lamp has DISCRETE SPECIFIC energy levels. As a current is passed through the (gas), electrons get excited and when they de-excite, they move from a higher to lower DISCRETE energy level, emitting photons of specific ENERGIES (hf) and wavelengths

Question 21

how do you show that a matter have dual nature

Answer 21

electron diffraction

Question 22

how do you show the wave and particle theory of photons

Answer 22

wave theory is shown through diffraction while particle nature is shown through photoelectric effect where one photon interacts with one electron NIOT PHOZTOPELEVTRON THAT IS AFTER LIBERATED

Question 23

transitions are always from higher to lower energy level

Answer 23

spectra transitions always mean dexcitation

Question 24

What is the difference between continuous x ray spectrum and characteristic x rays

Answer 24

continuous x rays are produced at lower intensities over a continuous wavelength with a typical minimum wavelength corresponding to the photon of the highest energy while characteristic x rays are produced at higher intensities at specific wavelength which are dependent on the material that is bombarded by electrons

Question 25

what is the saturation current

Answer 25

is is the maximum current and represents the maximum rate of emission of photoelectrons from the emitter as all photoelectrons reach the collector