Advertisements
Advertisements
Question
A 150 W lamp emits light of the mean wavelength of 5500 Å. If the efficiency is 12%, find out the number of photons emitted by the lamp in one second.
Advertisements
Solution
P = 150 W
λ = 5500 × 10−10 m
Efficiency = 12%
N = `("P"λ)/"hc"`
= `(150 xx 5500 xx 10^-10)/(6.626 xx 10^-34 xx 3 xx 10^8)`
= `(825 xx 10^-7)/(19.878 xx 10^-26)`
= 41.5 × 1019
Number of photons emitted is with 12% efficiency = `41.5 xx 10^19 xx 12/100`
N = 4.98 × 1019
APPEARS IN
RELATED QUESTIONS
Why should gases be insulators at ordinary pressures and start conducting at very low pressures?
How will the thermionic current vary if the filament current is increased?
Would you prefer a material with a high work-function or a low work-function to be used as a cathode in a diode?
Why does thermionic emission not take place in non-conductors?
The constant A in the Richardson−Dushman equation for tungsten is 60 × 104 A m−2K−2. The work function of tungsten is 4.5 eV. A tungsten cathode with a surface area 2.0 × 10−5 m2 is heated by a 24 W electric heater. In steady state, the heat radiated by the heater and the cathode equals the energy input by the heater and the temperature becomes constant. Assuming that the cathode radiates like a blackbody, calculate the saturation current due to thermions. Take Stefan's Constant = 6 × 10−8 W m−2 K−1. Assume that the thermions take only a small fraction of the heat supplied.
The work function of aluminum is 4⋅2 eV. If two photons each of energy 2⋅5 eV are incident on its surface, will the emission of electrons take place? Justify your answer.
The wave associated with a moving particle of mass 3 × 10–6 g has the same wavelength as an electron moving with a velocity 6 × 106 ms–1. The velocity of the particle is
Emission of electrons by the absorption of heat energy is called ____________ emission.
Name the factors on which photoelectric emission from a surface depends.
The work function of a metal is 2.31 eV. Photoelectric emission occurs when the light of frequency 6.4 × 1014 Hz is incident on the metal surface. Calculate
- the energy of the incident radiation,
- the maximum kinetic energy of the emitted electron and
- the stopping potential of the surface.
