Select best monochromatic source of light.

Options

A. Bulb
B. Candle
C. Tube light
D. Laser

Explanation

To determine the best monochromatic source of light, we need to understand what a monochromatic source is. A monochromatic source emits light of a single wavelength, meaning it produces light of only one color without any mixture of other wavelengths.

### Step-by-Step Solution:

1. **Define Monochromatic Light**:
- Monochromatic light is light that consists of a single wavelength. This means it has a single color and does not contain any other wavelengths.

2. **Evaluate Common Light Sources**:
- **Incandescent Bulb**: When operated at low voltage, it emits reddish light, but as the voltage increases, it emits white light, which contains multiple wavelengths. Therefore, it is not monochromatic.
- **Candle Flame**: A candle flame emits light of various colors due to the combustion process, which produces a range of wavelengths. Hence, it is not monochromatic.
- **Tube Light**: Similar to the incandescent bulb, a tube light emits different colors at various voltages and does not produce a single wavelength. Thus, it is not monochromatic.

3. **Identify the Best Monochromatic Source**:
- **Laser**: A laser is specifically designed to emit light of a single wavelength. It produces coherent light, which means the light waves are in phase and have the same frequency. This makes lasers the best source of monochromatic light.

4. **Conclusion**:
- Among the options evaluated, the laser is the best monochromatic source of light because it emits light at a single wavelength without the presence of other wavelengths.

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