Which of the below emits nearly monochromatic light?

Sodium vapour lamp:

It is low intensity so that the length of the lamp should be more. In order to get the desired length, it is made of a U-shaped tube.

This long U-tube consists of a small amount of neon gas and metallic sodium.

At the time of start, the neon gas vaporizes and develops enough heat to vaporize metallic sodium in the U-shaped tube.

Initially, the sodium is in the form of a solid, deposited on the walls of the inner tube.

When enough voltage is impressed across the electrodes, the discharge starts in the inert gas; it operates as a low-pressure neon lamp with pink colour.

The temperature of the lamp increases gradually, and the metallic sodium vaporizes and then ionizes thereby producing monochromatic yellow.

Important:

Advantages:

• Good efficiency
• Colour rendering is better than that of high-pressure sodium street lights
• Some lamps last far longer than the 24000-hour mark, sometimes 40 years

Disadvantages:

• Like many lamps, it contains traces of mercury which must be disposed of properly
• Human skin looks green under the light, it is poor for colour film/photography
• The warm-up time required to start the lamp

Applications:

Large areas like parks, street lighting, high ceiling buildings, and gyms