Why can laser light can never be white light?

Answer 1

You may be correct in inferring that you are unable to purchase a simplistic white laser pointer, however white laser lights do exist in the scientific community for research.

Before I go into detail, the reason why white lasers are fairly uncommon, especially in a commercially available laser pointer form is due to the construction of the regular laser pointer. You must understand that white light consists of a large variation of different wavelengths of light including the blue, red and green wavelengths all mixed together. The individual colors are restricted to a small range of wavelengths, such as blue, red or green.

Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses. Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam. In short, laser light is most easily made when restricted to a specific wavelength, hence giving it its characteristic color, such as a blue, red or green laser.

Now, typically, lasers are thought of as only emitting light with a narrow wavelength spectrum ("monochromatic" light). This is not true of all lasers, however: some emit light with a broad spectrum, while others emit light at multiple distinct wavelengths simultaneously. The laser type you may be interested in is the Ion Laser. An Ion Laser is a gas laser which uses an ionized gas as its lasing medium. Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium and mirrors forming a Fabry-Perot resonator. Unlike normal gas lasers, the energy level transitions that contribute to laser action come from ions. A mix of argon and krypton can result in a laser with output wavelength appearing as a brilliant white laser light.

Because of the large amount of energy required to excite the ionic transitions used in ion lasers, the required current is much greater, and as a result all but the smallest ion lasers are water cooled. A small air cooled ion laser might produce, for example, 130mW of light with a tube current of 10A @ 105V. This is a total power draw over 1 kW, which translates into a large amount of heat which must be dissipated. It is therefore easy to see why such a laser would not be easily commercially available as well as compact enough to fit into a laser pointer size, since it requires a cooling system and powerful energy source.

Answer 2

Laser light is monochromatic, meaning it consists of a single color or wavelength of light. White light, on the other hand, contains a mixture of all visible wavelengths. This fundamental difference in composition prevents laser light from being perceived as white light.

Answer 3

A laser is a device that emits only one wavelength of light at a time. White light is a combination of all wavelengths of light, so a laser can never be white.