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A photon of energy 3.4 eV is incident on a metal having work function 2 eV. The maximum K.E. of photo-electrons is equal to

1.4 eV

1.7 eV

5.4 eV

6.8 eV

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Explanation

(a) $K_{\max} = (E - W_{0}) = (3.4 - 2) eV = 1.4 eV$

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The photoelectric threshold wavelength for a metal surface is 6600 Å. The work function for this is

1.87 V

1.87 eV

18.7 eV

0.18 eV

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Explanation

(b) $W_{0} = \frac{12375}{6600} = 1.87 eV$

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Photoelectric effect was successfully explained first by

Planck

Hallwash

Hertz

Einstein

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Explanation

Einstein explained the photoelectric effect successfully first.

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The spectrum of radiation $1.0 \times 10^{14}$ Hz is in the infrared region. The energy of one photon of this in joules will be

$6.62 \times 10^{- 48}$

$6.62 \times 10^{- 20}$

$\frac{6.62}{3} \times 10^{- 28}$

$3 \times 6.62 \times 10^{- 28}$

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Explanation

(b) $E = hv = 6.64 \times 10^{- 34} \times 1.0 \times 10^{14} = 6.62 \times 10^{- 20} J$

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A photo cell is receiving light from a source placed at a distance of 1 m. If the same source is to be placed at a distance of 2 m, then the ejected electron

Moves with one-fourth energy as that of the initial energy

Moves with one-fourth of momentum as that of the initial momentum

Will be half in number

Will be one-fourth in number

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Explanation

(d) Number of ejected electrons ∝ (Intensity) ∝ $\frac{1}{{(Distance)}^{2}}$
Therefore an increment of distance two times will reduce the number of ejected electrons to $\frac{1}{4} th$ of the previous one.

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In a photoelectric experiment for 4000 Å incident radiation, the potential difference to stop the ejection is 2 V. If the incident light is changed to 3000 Å, then the potential required to stop the ejection of electrons will be

2 V

Less than 2 V

Zero

Greater than 2 V

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Explanation

(d) According to Einstein's photoelectric equation

$E = W_{0} + K_{\max} \Rightarrow V_{0} = \frac{hc}{e} [\frac{1}{λ} - \frac{1}{λ_{0}}]$

Hence if $λ$ decreases $V_{0}$ increases.

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Photocell is a device to:

Store photons

Measure light intensity

Convert photon energy into mechanical energy

Store electrical energy for replacing storage batteries

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Explanation

(b) Number of electrons can be measured which are directly proportional to the intensity of radiation

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Stopping potential for photoelectrons

Does not depend on the frequency of the incident light

Does not depend upon the nature of the cathode material

Depends on both the frequency of the incident light and nature of the cathode material

Depends upon the intensity of the incident light

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Explanation

Stopping potential = $V_{0} = \frac{K_{m x .}}{e} = \frac{h ν_{0}}{e}$

It also depends on the nature of the material of the cathode.

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The maximum wavelength of radiation that can produce photoelectric effect in a certain metal is 200 nm. The maximum kinetic energy acquired by electron due to radiation of wavelength 100 nm will be

12.4 eV

6.2 eV

100 eV

200 eV

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Explanation

(b) $K_{\max} (eV) = 12375 [\frac{1}{λ (\overset{0}{A})} - \frac{1}{λ_{0} (\overset{0}{A})}] = 12375 [\frac{1}{1000} - \frac{1}{2000}] = 6.2 eV$

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The minimum energy required to remove an electron is called

Stopping potential

Kinetic energy

Work function

None of these

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Explanation

The minimum energy required to remove an electron is called work fuction.

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