Physics Flashcards
Define a blackbody
A blackbody is defined a body which absorbs radiations of all wavelengths incident on it. It neither radiates nor transmits any of the incident radiation, and therefore appears black. In actual practice there does not exist any black body, but lamp black may be regarded as a black body as it absorbs nearly 99% of all incident radiation.
In relation to black body radiation, give the Statement given by Stephan and Boltzmann on black body radiation along with the equation
Stefan-Boltzmann law, states that the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature.
Its equation is given as: E~T^4
In relation to black body radiation, give the important findings of Lummer and Pringsheim
(i) . At a given temperature, the energy is not uniformly distributed in the radiation spectrum of the blackbody
(ii) . At a given temperature, the intensity of radiation increases with increase in wavelength and at a particular wavelength, its value is maximum. With further increase in wavelength, the intensity decreases.
(iii) .With the increase in temperature, the λm decreases where λm is the wavelength at which the maximum emission of energy takes place.
(iv) . There is an increase in energy with increase in temperature.
State Wein’s Displacement Law
Weins Displacement Law states that the product of wavelength corresponding to maximum energy and absolute temperature is constant. i.e
λm . T = constant = 0.2892 cm K
It shows that with increase in temperature , λm decreases.
He also suggested that E~T^5.
Weins Displacement law is only applicable for shorter wavelengths of radiation.
State Rayleigh Jeans Law
According to Rayleigh Jeans Law, the distribution of energy is radiation spectrum is given by the following formula :
E(λ) dλ = (8πkT)/(λ^4) dλ
The Rayleigh Jeans law is applicable for longer wavelengths.
Using Planck’s Hypothesis and Radiation Law, prove Weins Law
Take guidance from the book, bhai.
Using Planck’s Hypothesis and Radiation Law, prove Rayleigh Jeans Law
Take guidance from the book, bhai.
In photoelectric effect, the number of electrons emitted and their kinetic energy depends upon which 4 factors
The 4 factors on which the number of electrons emitted in photoelectric effect and their kinetic energy depends upon are as follows:
(i) The potential difference between plates A and B
(ii) The intensity of incident radiations
(iii) The frequency of incident radiations
(iv) The photosensitive metals or plates used
State the 4 fundamental laws of Photoelectric Emission:
The 4 fundamental laws of Photoelectric Emission are :
(i) The number of electrons emitted per second i.e photoelectric current is proportional to the intensity of the incident light
(ii) For the ejection of photoelectrons from the surface of a given metal, a certain minimum frequency of incident light is required. If the frequency is less than the required frequency, no electron can be emitted from the metal surface howsoever intense the incident light may be. This minimum value of frequency is known as Threshold Frequency and the corresponding wavelength to the threshold frequency is called Threshold wavelength. This is the upper limit of the wavelength which means that its the minimum amount of wavelength required for photoelectric emission to take place.
(iii) The kinetic energy of the emitted photoelectrons depends upon the frequency of incident radiations and not on the intensity of the incident radiation. Hence the kinetic energy of the emitted photoelectrons is directly proportional to the frequency of the incident radiation.
(iv) The phenomenon of Photoelectric emission is different from thermionic emission because the rate at which electrons are emitted from the photodiode is independent of the temperature.
One more thing.
The stopping potential ( negative potential of B at which photoelectric current is zero ) is proportional to the frequency of the incident radiation but is independent of the radiation.
State Einstein’s Photoelectric Equation and also give its various postulates
According to Einsteins’s explanation, in the phenomenon of the photoelectric effect, one photon is completely absorbed by one electron of the metal ( photosensitive ), thus electron gains the quantum of energy and may be emitted from the metal surface. Hence the photo energy is utilized in two ways :
(i) A part of the energy is used to free the electron from the atom and move it to from the metal surface. The energy used in this task is known as photoelectric work function of the metal. It is denoted by W.
(ii) Rest of the energy is used to give the kinetic energy to the ejected photoelectrons. Thus the total energy of the incident photon is hν = W° + ½m∨²
In the photoelectric effect, state the formula of the work function
λ°= c / v° = ch / W°
In the photoelectric effect, give the formula of stopping potential :
V° = h/e (v-v°)
In the photoelectric effect, give Einstein’s photoelectric equation
½ m∨² = h(v-v°)
