Photoelectric Effect in Cathode Materials Under 633 nm Laser Light
📘 Question
Different cathode materials are irradiated with laser light at a wavelength of 633 nm.
- Which of the cathode materials exhibits the photoelectric effect?
- The intensity of laser radiation increases continuously. Does this change the number of cathode materials that exhibit the photoelectric effect?
🔬 Concept: The Photoelectric Effect
The photoelectric effect occurs when photons with sufficient energy strike the surface of a material and liberate electrons. The minimum energy required is known as the work function (ϕ) of the material. The photon energy is determined by:
Where:
- h = Planck’s constant ≈ 6.626 × 10⁻³⁴ J·s
- c = Speed of light ≈ 3.00 × 10⁸ m/s
- λ = Wavelength of incident light (633 nm = 633 × 10⁻⁹ m)
🧮 Step 1: Calculate Photon Energy at 633 nm
✅ (a) Which Materials Exhibit the Photoelectric Effect?
Only materials with a work function less than 1.96 eV will emit electrons when exposed to 633 nm laser light. Materials like alkali metals (e.g., cesium, potassium) typically have low work functions and may exhibit photoemission. In contrast, metals such as copper or platinum have higher work functions and will not emit electrons under this light.
📈 (b) Effect of Increasing Laser Intensity
Increasing laser intensity means more photons strike the material per unit time. This results in:
- Higher photoelectric current if the material already emits electrons (i.e., ϕ < 1.96 eV)
- No change in emission behavior for materials with ϕ > 1.96 eV, because photon energy is unchanged
📘 Final Answers:
- (a) Only cathode materials with work functions less than 1.96 eV will exhibit the photoelectric effect when irradiated with 633 nm laser light.
- (b) Increasing laser intensity increases photoemission rate from eligible materials but does not cause new materials to emit electrons.
📊 Summary Table:
| Parameter | Value |
|---|---|
| Laser Wavelength | 633 nm |
| Photon Energy | 1.96 eV |
| Emission Condition | ϕ < 1.96 eV |
| Effect of Increasing Intensity | More electrons from emitting materials only |
🧠 Conclusion
In summary, the ability of a cathode to exhibit the photoelectric effect under 633 nm light depends entirely on its work function being below the photon energy of 1.96 eV. Intensity affects the quantity of emitted electrons—not the eligibility of materials for photoemission.