The GATE Grind

GATE 2022 PH – Question 22

Atomic and Molecular Physics · Zeeman, Paschen-Back and Stark effects · 1 mark · Multiple select

For normal Zeeman lines observed ∥ and ⊥ to the magnetic field applied to an atom, which of the following statements are true?

  1. Only π-lines are observed ∥ to the field
  2. σ-lines ⊥ to the field are plane polarized
  3. π-lines ⊥ to the field are plane polarized
  4. Only σ-lines are observed ∥ to the field

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (B) σ-lines ⊥ to the field are plane polarized; (C) π-lines ⊥ to the field are plane polarized; (D) Only σ-lines are observed ∥ to the field

Explanation

In the **normal Zeeman effect** a spectral line splits into three components:
- the $\pi$ component ($\Delta m=0$, undisplaced), and
- two $\sigma$ components ($\Delta m=\pm1$, displaced on either side).

**Observing along the field direction (longitudinal):** the $\pi$ component is not seen (the oscillating dipole radiates nothing along its own axis); only the two $\sigma$ lines appear, and they are **circularly polarised**. So D is true and A is false.

**Observing perpendicular to the field (transverse):** all three components appear.
- The $\pi$ line is **plane polarised** with its electric vector parallel to the field. ✓ (C)
- The $\sigma$ lines are **plane polarised** with their electric vector perpendicular to the field. ✓ (B)

Answer **B, C and D**.