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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
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Consider a 20 W bulb emitting light of wavelength 5000 Å and shining on a metal surface kept at a distance 2 m. Assume that the metal surface has work function of 2 eV and that each atom on the metal surface can be treated as a circular disk of radius 1.5 Å.
- Estimate no. of photons emitted by the bulb per second. [Assume no other losses]
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- How many photons would atomic disk receive within time duration calculated in (iii) above?
- Can you explain how photoelectric effect was observed instantaneously?
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- 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.
