1. State Einstein’s photoelectric equation. (1 Mark)

Answer:$$K_{\max} = h\nu – \phi_0$$

where:

• $K_{\max}$=Maximum kinetic energy of photoelectrons
• h = Planck’s constant
• ν = Frequency of incident light
• $\phi_0$ = Work function of the metal

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2. What is the work function? (1 Mark)

Answer:
The work function (ϕ0​) is the minimum energy required to remove an electron from the surface of a metal.

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3. Define threshold frequency. (1 Mark)

Answer:
The minimum frequency of incident radiation required to cause photoelectric emission is called the threshold frequency $$\nu_0=\frac{\phi_0}{h}$$

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4. Why does increasing intensity not increase the maximum kinetic energy of photoelectrons? (2 Marks)

Answer:

• The energy of each photon depends only on its frequency (E=hν).
• Increasing intensity increases only the number of photons, not the energy of each photon.
• Hence, the maximum kinetic energy of photoelectrons remains unchanged.

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5. Why is there no photoelectric emission below the threshold frequency? (2 Marks)

Answer:
When $\nu < \nu_0$$$h\nu < \phi_0$$

The photon energy is less than the work function, so electrons cannot escape from the metal surface regardless of the intensity of light.

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6. How does Einstein’s theory explain the effect of intensity? (2 Marks)

Answer:

• Intensity is proportional to the number of incident photons.
• More photons eject more electrons.
• Therefore, photoelectric current increases, while the maximum kinetic energy remains unchanged.

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7. Explain why photoelectric emission is instantaneous according to Einstein’s theory. (2 Marks)

Answer:
Each electron absorbs the energy of a single photon in one step. If the photon energy exceeds the work function, the electron is emitted immediately. Hence, there is no time lag.

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8. Write the relation between stopping potential and frequency. (2 Marks)

Answer:$$eV_0=h\nu-\phi_0$$

or$$V_0=\frac{h}{e}\nu-\frac{\phi_0}{e}$$

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9. Draw the graph of stopping potential versus frequency and write its significance. (3 Marks)

Answer:

• The graph is a straight line.
• Slope = $\frac{h}{e}$
• X-intercept = Threshold frequency $(\nu_0)$
• Y-intercept = $-\frac{\phi_0}{e}$

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10. State any four conclusions of Einstein’s photoelectric theory. (3 Marks)

Answer:

1. Light consists of photons.
2. Energy of each photon is $h\nu$hν.
3. One photon transfers its energy to one electron.
4. Photoelectric emission occurs only if $\nu \ge \nu_0$ν≥ν0​.
5. Maximum kinetic energy depends only on frequency.
6. Photoelectric current depends on intensity.

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Most Important Board Formulae

• $E=h\nu$
• $K_{\max}=h\nu-\phi_0$
• $\phi_0=h\nu_0$
• $\nu_0=\frac{\phi_0}{h}$
• $eV_0=h\nu-\phi_0$
• $V_0=\frac{h}{e}\nu-\frac{\phi_0}{e}$

These are the most frequently asked CBSE Class 12 board questions from Einstein’s Photoelectric Equation.