when an atomic gas or vapour is excited at low pressure, by passing an electric current through it then

Options

A. emission spectrum is observed
B. absorption spectrum is observed
C. band spectrum is observed
D. both a and c

Explanation

To solve the question regarding what happens when an atomic gas or vapor is excited at low pressure by passing an electric current through it, we can break down the process step by step:

### Step-by-Step Solution:

1. **Understanding the Setup**:
- We have an atomic gas or vapor contained in a vessel at low pressure. When an electric current is passed through this gas, it gets excited.

**Hint**: Consider what happens to the gas when energy is supplied to it.

2. **Energy Absorption**:
- The atoms in the gas absorb energy from the electric current. This energy causes the electrons in the atoms to move from their ground state (lower energy level) to an excited state (higher energy level).

**Hint**: Think about how electrons behave when they gain energy.

3. **Transition of Electrons**:
- After a short time, the excited electrons cannot stay in the higher energy state indefinitely. They will eventually return to the lower energy state, releasing the absorbed energy in the form of electromagnetic radiation.

**Hint**: Recall the concept of energy levels in atoms and how electrons transition between them.

4. **Emission of Radiation**:
- The radiation emitted when the electrons fall back to their lower energy state corresponds to specific wavelengths of light. This results in the appearance of bright lines in the spectrum.

**Hint**: Consider what type of spectrum is formed when certain wavelengths are emitted.

5. **Observation of the Spectrum**:
- The emitted radiation creates an emission spectrum, which consists of bright lines on a dark background. This is characteristic of emission spectra, where specific wavelengths are emitted by the excited atoms.

**Hint**: Differentiate between emission and absorption spectra based on the background and the lines present.

6. **Conclusion**:
- Therefore, when an atomic gas or vapor is excited at low pressure by passing an electric current through it, an emission spectrum is observed.

**Hint**: Reflect on the definitions of emission and absorption spectra to finalize your answer.

### Final Answer:
- The correct answer is **Option A: Emission spectrum is observed**.