These are the most important CBSE board exam facts from this topic.

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1. Einstein’s Photon Theory (1905)

• Proposed by Albert Einstein in 1905.
• Light consists of tiny packets of energy called photons (quanta).
• Energy of one photon is

$$E = h\nu$$

where:

• h = Planck’s constant = 6.626 × 10⁻³⁴ J s
• ν = Frequency of light

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2. One Photon – One Electron Principle

One photon transfers its entire energy to one electron only.

One photon → One electron

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3. Work Function (φ₀)

• Minimum energy required to remove an electron from the metal surface.
• Unit: Joule (J) or electron volt (eV).

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4. Einstein’s Photoelectric Equation

$$\boxed{K_{\max}=h\nu-\phi_0}$$

OR$$\boxed{\frac12 mv_{\max}^2=h\nu-\phi_0}$$

This is the most important equation.

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5. Threshold Frequency

$$\boxed{\nu_0=\frac{\phi_0}{h}}$$

• Minimum frequency required to produce photoelectric emission.
• If

$$\nu<\nu_0$$

No photoelectric emission occurs.

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6. Threshold Wavelength

$$\boxed{\lambda_0=\frac{c}{\nu_0}}$$

Maximum wavelength capable of producing photoelectric emission.

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7. Stopping Potential

$$\boxed{eV_0=K_{\max}}$$eV0​=Kmax​​

Combining with Einstein’s equation,$$\boxed{eV_0=h\nu-\phi_0}$$

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

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8. V₀ vs ν Graph

• Straight line
• Slope

$$\boxed{\frac{h}{e}}$$eh​​

• Slope is same for all metals.
• Intercept on frequency axis

$$\nu_0$$ν0​

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9. Dependence on Frequency

Increasing frequency:

✅ Increases maximum kinetic energy

✅ Increases stopping potential

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10. Dependence on Intensity

Increasing intensity:

✅ Increases photoelectric current

❌ Does NOT increase maximum kinetic energy

❌ Does NOT change stopping potential

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11. Instantaneous Emission

Photoelectrons are emitted without measurable time delay.

Reason:

Each electron absorbs the energy of one photon instantly.

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12. Photoelectric Current

For$$\nu>\nu_0$$

Photoelectric current is$$\boxed{\text{Current} \propto \text{Intensity}}$$Current∝Intensity​

because higher intensity means more photons per second, producing more emitted electrons.

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13. Maximum Kinetic Energy

Depends only on

• Frequency of light

Does not depend on

• Intensity
• Time of exposure

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14. Threshold Frequency Depends On

Threshold frequency depends on

• Nature of metal
• Work function

It does not depend on intensity.

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15. Experimental Verification

• Einstein proposed the theory in 1905.
• Robert Andrews Millikan experimentally verified Einstein’s equation in 1916.
• Millikan measured Planck’s constant (h) using the photoelectric effect.

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Most Important Formula Sheet

Quantity	Formula
Photon Energy	$E=h\nu$E=hν
Einstein Equation	$K_{\max}=h\nu-\phi_0$Kmax​=hν−ϕ0​
Kinetic Energy	$K_{\max}=\frac12 mv_{\max}^2$Kmax​=21​mvmax2​
Stopping Potential	$eV_0=K_{\max}$eV0​=Kmax​
Combined Equation	$eV_0=h\nu-\phi_0$eV0​=hν−ϕ0​
Threshold Frequency	$\nu_0=\frac{\phi_0}{h}$ν0​=hϕ0​​
Threshold Wavelength	$\lambda_0=\frac{c}{\nu_0}$λ0​=ν0​c​

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CBSE One-Mark Questions (Frequently Asked)

1. Define work function.
2. Define threshold frequency.
3. State Einstein’s photoelectric equation.
4. What is the energy of a photon?
5. On what does maximum kinetic energy depend?
6. Why does photoelectric current increase with intensity?
7. Why is photoelectric emission instantaneous?
8. Who experimentally verified Einstein’s photoelectric equation?
9. Write the relation between stopping potential and maximum kinetic energy.
10. Why does increasing intensity not increase the maximum kinetic energy of photoelectrons?

These points cover almost all the theory-based questions typically asked from this topic in the CBSE Class 12 Physics board examination.