Dual Nature of Radiation & Matter Flashcards
(22 cards)
What is electron emission?
What is work function?
The process of emission of electrons from the metal surface when a certain amount of energy is absorbed by the metal, is called ELECTRON EMISSION.
There are free electrons on the metal surface, present in the outermost (valence) shell that are loosely bound to the nucleus.
These electrons are emitted when a certain minimum amount of external energy is provided to the metal surface. This least value of energy is called WORK FUNCTION of metal (denoted by Φo)
What is the formula and the unit of work function?
Φo = hµo
[ where h is Plank’s constant = 6.6 × 10^(-34) and µo is threshold frequency ]
The unit of Φo is electron volt (eV). 1eV is defined as the energy needed to accelerate an electron through the potential difference of 1volt (V).
1eV = 1.6 × 10^(-19) J
What is work function dependent on?
Work function of a material depends mainly on the nature of metal and electronic configuration of metal (meaning the number of valence shell electrons - lesser the number, lesser the work function)
Which metals have the least and most work function?
Least Φo -> Cs = 2.1 eV
(due to large size of atom leading to less intensity of nuclear force on electron)
Most Φo -> Pt = 5.2 eV
What are the 3 types of electron emissions?
(i) Thermionic emission:
By suitably heating, sufficient thermal energy can be imparted to the free electrons to enable them to come out of the metal. Eg. thermionic converter
(ii) Field emission :
By applying a very strong electric field (of the order of 10^8 V/m) to a metal, electrons can be pulled out of the metal. Eg. spark plug
(iii) Photoelectric emission :
When light of suitable frequency illuminates a metal surface, electrons are emitted from the metal surface. These light - generated electrons are called photoelectrons. Eg. solar panel
What is threshold frequency?
What happens if frequency of light incident on a metal is higher than the threshold frequency?
The threshold frequency is defined as the minimum frequency of incident radiation below which photoelectric emission will not occur, irrespective of the intensity of incident radiation.
This minimum frequency depends on the nature of the material of the emitter plate.
If frequency of light incident on a metal is higher than the threshold frequency, the remaining energy after ejection of electron goes towards kinetic energy of the electron.
Write a short note on Hertz’ observations of photoelectric effect.
The phenomenon of photoelectric emission was discovered in 1887 by Heinrich Hertz (1857-1894), during his electromagnetic wave experiments.
In his experimental investigation on the production of electromagnetic waves by means of a spark discharge, Hertz observed that high voltage sparks across the detector loop were enhanced when the emitter plate was illuminated by ultraviolet light from an arc lamp. Light shining on the metal surface somehow facilitated the escape of free, charged particles which we now know as electrons.
When light falls on a metal surface, some electrons near the surface absorb enough energy from the incident radiation to overcome the attraction of the positive ions in the material of the surface. After gaining sufficient energy from the incident light, the electrons escape from the surface of the metal into the surrounding space.
Why were UV radiations used in Hallwachs’ and Lenard’s experiment and not visible/infrared radiations?
Frequency of radiations :
UV > Visible > Infrared
Hence, electrons from all metals can be ejected by UV radiations but electrons of only some metals can be ejected by other radiations (due to lower frequency of the radiations, they lie below the threshold frequency of some metals).
What was the setup for experimental study of photoelectroc effect (Hallwachs’ and Lenard’s experiment)?
The setup consisted of :
- Discharge tube - Glass tube with vacuum
- Vacuum - To prevent collision of ejected electrons with gaseous atoms
- Photosensitive plate (emitter) - Anode - To absorb visible light and emit electrons
- Metal plate (collector) - Cathode - To receive electrons emitted from the emitter, thus constituting flow of photocurrent
- Quartz window - To allow only UV radiations to enter
- Commutator - To reverse polarity of the plates
- Potential divider - To differ potential (voltage) of the plates
- Battery - To accelerate emitted electrons through a potential difference
- Voltmeter - To measure the potential difference between the emitter and the collector plates due to photoelectric current flow
- Ammeter - To measure the value of photoelectric current
What happens when the incident is of :
- High intensity
- High frequency
- More number of photons -> More electrons emitted
2. Higher energy photons -> Electrons ejected will be more energetic
What were the observations made through Hallwachs’ and Lenard’s experiment?
- Effect of intensity of light on photocurrent :
More photons per sec (intensity) means more electrons emitted per sec. Hence for a light having frequency greater than threshold frequency, if intensity is increased, photocurrent also increases.
Graph plotted b/w photoelectric current and intensity of light shows a straight line passing through the origin. - Effect of potential on photoelectric current :
When intensity & frequency are kept constant and potential is
(i) Zero - Some current flows as some electrons had enough KE after ejection to reach collector plate.
(ii) Positive - Attractive force pulls electrons that were not energetic enough to reach collector plate at zero potential so photocurrent continues increasing until saturation current is reached.
(iii) Negative - Photocurrent will begin decreasing in value with decrease in potential. At a particular potential (stopping potential or cutoff voltage), photocurrent becomes zero. - Effect of frequency of incident radiation on stopping potential :
Increased frequency -> Increased KE of electron
∴ Cutoff voltage will have more -ve value
Since intensity is constant, saturation current remains constant as it is independent of frequency of light.
State the laws of photoelectric effect.
(i) For a given photosensitive material and frequency of incident radiation (above the threshold frequency), the photoelectric current is directly proportional to the intensity of incident light.
(ii) For a given photosensitive material and frequency of incident radiation, saturation current is found to be proportional to the intensity of incident radiation whereas the stopping potential is independent of its intensity.
(iii) Above the threshold frequency, the stopping potential or equivalently the maximum kinetic energy of the emitted photoelectrons increases linearly with the frequency of the incident radiation, but is independent of its intensity
(iv) Photoelectric emission is an instantaneous process with no apparent time lag (∼ 10^(-9) s or less), even when the incident radiation is made exceedingly dim.
What are the properties of photon?
- Whenever a photon interacts with a metal, it acts as a particle.
- Photon travels with the speed of light and hence it’s rest mass is zero (does not exist at rest)
- Photon is a neutral particle and thus does not get affected by electronic/magnetic field.
- Energy carried by photon is given by h/λ or hμ/c (because c = μλ)
- Collision of photon with metal is elastic and thus energy and momentum remain conserved.
What is Einstein’s photon view?
- Photon is a neutral particle
- Photon is unaffected in the presence of electric and magnetic field
- Photon transfers momentum
- Number of photons is not conserved
(intensity ∝ no. of photons)
What is Planck’s formula?
hf = Φo + (K.E.)max
hf = hfo + (K.E.)max
∴ (K.E.)max = h ( f - fo )
Why is threshold frequency (fo) required to liberate a photoelectron?
(K.E.)max = h ( f - fo )
So if f < fo, (K.E.)max will be negative , which is not possible.
Hence, threshold frequency must be met for ejection of photoelectron.
What is another formula for (K.E.)max?
(K.E.)max = eVo
Verify graph between photocurrent and plate potential using Einstein’s photon view.
hf = hfo + eVo
hfo is constant
∴ f ∝ eVo/h
Verify graph between stopping potential and frequency using Einstein’s photon view.
(K.E.)max = hf - Φo eVo = hf - Φo Vo = hf/e - Φo/e Vo = (h/e)f + (-Φo/e)
Justify that interaction of radiation happens in particle form.
- One photon corresponds to one electron, which proves that energy is not distributed to the entire material
- Instantaneous emmision of electrons
Derive equation for de Broglie wavelength.
From Einstein’s photon view, E = mc^2
From Planck’s quantum theory, E = hf
∴ mc^2 = hf
mc^2 = hc/λ
∴ λ = h/mc
λ = h/p
(p = momentum = speed of photon × mass of photon)
Write the short version of laws of photoelectric effect.
- Photocurrent ∝ Intensity
- f > fo
- (K.E.)max depends on frequency, not intensity
- No time lag b/w incidence of radiation and emmision of electron
