Which of the following sources gives best monochromatic light?

Options

A. A candle
B. A bulb
C. A mercury tube
D. A laser

Answer

To determine which of the given sources provides the best monochromatic light, we need to analyze each option based on the definition of monochromatic light.

### Step-by-Step Solution:

1. **Understand Monochromatic Light**:
- Monochromatic light is light that has a single wavelength or frequency. This means it emits light of only one color.

2. **Evaluate Each Option**:
- **Option 1: Candle**:
- A candle emits light that is a mixture of different wavelengths, primarily in the yellow and red regions of the spectrum. Therefore, it does not produce monochromatic light.

- **Option 2: Bulb**:
- A standard incandescent bulb also emits a broad spectrum of light, combining multiple colors. Thus, it is not a source of monochromatic light.

- **Option 3: Mercury Tube**:
- A mercury vapor tube emits light due to the excitation of mercury gas, resulting in several spectral lines. This means it emits multiple colors and is not monochromatic.

- **Option 4: Laser**:
- A laser produces light through stimulated emission, which results in a very narrow wavelength range. This means it emits light of a single frequency or color, making it the best source of monochromatic light.

3. **Conclusion**:
- Based on the analysis, the best source of monochromatic light among the options provided is **Option 4: Laser**.

### Final Answer:
The best source of monochromatic light is **Option 4: Laser**.

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