When a beam of white light is passed through sodium vapours and then through a spectrometer, spectrum so obtained has two dark lines present in the yellow region. This spectrum is called :

Options

A. A) band spectrum
B. B) continuous spectrum
C. C) absorption spectrum of sodium
D. D) emission spectrum of sodium

Answer

To solve the question regarding the spectrum obtained when a beam of white light is passed through sodium vapours, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Context**:
- When white light passes through sodium vapours, certain wavelengths of light are absorbed by the sodium atoms. This results in the appearance of dark lines in the spectrum.

2. **Identifying the Type of Spectrum**:
- The spectrum produced in this scenario is not a continuous spectrum but rather shows specific dark lines where certain wavelengths have been absorbed. This is characteristic of an absorption spectrum.

3. **Defining Absorption Spectrum**:
- An absorption spectrum is defined as the spectrum of light that has passed through a material (in this case, sodium vapours) and has specific wavelengths removed (absorbed) by the material.

4. **Recognizing the Specifics of Sodium Vapours**:
- Sodium vapours are known to absorb light in the yellow region of the spectrum, specifically at two wavelengths corresponding to the sodium D-lines (D1 and D2).

5. **Conclusion**:
- Based on the above understanding, the spectrum obtained after passing white light through sodium vapours is indeed an absorption spectrum. Therefore, the correct answer to the question is that the spectrum is called an **absorption spectrum**.

### Final Answer:
The spectrum so obtained is called an **absorption spectrum**.

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