2. Laser

Laser light is monochromatic

On the previous page, we noticed that at first instance the laser's light appears to be red and the halogen light is yellowish-white.

To understand this observation more precisely, the spectra of the laser light and the halogen light are measured with a spectograph (a spectrum shows the intensity of light by using wavelengths). The results of measurements are shown on the figure on the right. To get a better comparison of the spectral distributions, the curves are normalised to the same maximum value of 1.

We now observe that the spectrum of the halogen lamp is very broad-banded. The lamp emits radiation that is within the range of 400 nm up to infrared and the maximum intensity is beyond the measurement range, that is, outside of the visible range of 400 to 700 nm (in fact, it lies somewhere around 1500 nm). This gives the light a yellowish-white appearance. A halogen lamp is a thermal emitter; electric current causes the atoms of the filament to vibrate, and this energy is then emitted as light.

On the other hand, the spectrum of the laser light is very narrow. The light of the HeNe laser has a distinctive maximum at approximately 630 nm wavelength. As already observed, light having this wavelength appears red. Light that consists of one wavelength only is called monochromatic. Laser light is, therefore, (nearly) monochromatic.

Spectra of laser light (red) and halogen light (blue). HeNe laser light has a wavelength of 632,8 nm. A colour bar above the diagram shows the colours which are visible to the human eye and their corresponding wavelengths. Below 400 nm is the ultraviolet range, above 750 nm is infrared.

The width of the lines in the graphic above was measured with a spectograph at a resolution of 10 nm. In reality, the laser line is much more narrow-banded than shown here. The same is true for all other line spectra illustrated in this chapter.