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प्रश्न
Read the following paragraph and answer the questions.
| The figure shows the variation of photoelectric current measured in a photocell circuit as a function of the potential difference between the plates of the photocell when light beams A, B, C and D of different wavelengths are incident on the photocell. Examine the given figure and answer the following questions: |
- Which light beam has the highest frequency and why?
- Which light beam has the longest wavelength and why?
- Which light beam ejects photoelectrons with maximum momentum and why?
उत्तर
(i) We know that the stopping potential and frequency are related as
V = `((hf - phi))/q`
Where Φ is the work function defined as the amount of energy needed to bind the electrons in the metals, hf is the energy of photons, and q is the charge.
This relationship leads us to the conclusion that the greater negative the stopping potential, the higher the frequency. As a result, curve B has the highest frequency and the largest negative stopping potential in the graph.
(ii) We all know that the connection between wavelength and frequency is inverse. The wavelength reduces as the frequency rises and vice versa.
As can be seen from the graph, C has the lowest frequency out of all the possible values since its stopping potential is the least negative. Therefore, the wavelength is largest when the frequency is lowest. C has the longest wavelength as a result.
(iii) Highest momentum means highest kinetic energy which can be calculated with the help of velocity.
kinetic energy = `1/2mv^2`
Relation to the momentum, p2 = 2m(K.E.)
If kinetic energy is maximum, then momentum will be maximum.
We know that the stopping potential and frequency are related,
Vq = (hf - Φ)
The maximum kinetic energy of the electrons equals the stopping voltage when measured in electron volt. We can consider Φ as the kinetic energy.
Kinetic energy is maximum when the frequency is maximum and in the above part we have seen that it is maximum for curve B, so momentum is maximum for curve B.
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संबंधित प्रश्न
The work function for the following metals is given:
Na: 2.75 eV; K: 2.30 eV; Mo: 4.17 eV; Ni: 5.15 eV
Which of these metals will not give photoelectric emission for a radiation of wavelength 3300 Å from a He-Cd laser placed 1 m away from the photocell? What happens if the laser is brought nearer and placed 50 cm away?
Is it always true that for two sources of equal intensity, the number of photons emitted in a given time are equal?
The equation E = pc is valid
Light of wavelength λ falls on a metal with work-function hc/λ0. Photoelectric effect will take place only if
When the intensity of a light source in increased,
(a) the number of photons emitted by the source in unit time increases
(b) the total energy of the photons emitted per unit time increases
(c) more energetic photons are emitted
(d) faster photons are emitted
Calculate the number of photons emitted per second by a 10 W sodium vapour lamp. Assume that 60% of the consumed energy is converted into light. Wavelength of sodium light = 590 nm
(Use h = 6.63 × 10-34J-s = 4.14 × 10-15 eV-s, c = 3 × 108 m/s and me = 9.1 × 10-31kg)
The electric field associated with a light wave is given by `E = E_0 sin [(1.57 xx 10^7 "m"^-1)(x - ct)]`. Find the stopping potential when this light is used in an experiment on photoelectric effect with the emitter having work function 1.9 eV.
(Use h = 6.63 × 10-34J-s = 4.14 × 10-15 eV-s, c = 3 × 108 m/s and me = 9.1 × 10-31kg)
On the basis of the graphs shown in the figure, answer the following questions :
(a) Which physical parameter is kept constant for the three curves?
(b) Which is the highest frequency among v1, v2, and v3?
How would the stopping potential for a given photosensitive surface change if the intensity of incident radiation was decreased? Justify your answer.
- Assertion (A): For the radiation of a frequency greater than the threshold frequency, the photoelectric current is proportional to the intensity of the radiation.
- Reason (R): Greater the number of energy quanta available, the greater the number of electrons absorbing the energy quanta and the greater the number of electrons coming out of the metal.
