Light and Lasers

To understand the operation and characteristics of lasers, it is necessary to know some details about the nature of light and its characteristics. This brief guide provides a quick and simple introduction to the subject.

Light can be described as an oscillatory phenomenon called an electromagnetic wave. Of the same nature as light are radio waves, microwaves or X-rays. Each of these phenomena manifests itself differently depending on its wavelength, that is, the space covered by a single oscillation or repetition. Radio waves, those used, for example, by television, have wavelengths between one meter and one centimeter. Light, which oscillates at a much higher speed, has wavelengths between 400 and 700 billionths of a meter (nanometers, nm).

Our eye perceives light of a single wavelength as monochromatic. The graph below shows the perceived color depending on the wavelength of the source. Between 380nm and 760nm is placed the visible spectrum, below 380nm we speak of ultraviolet light, above 700nm we speak of infrared. It should be mentioned that although ultraviolet and infrared are not visible to the human eye (for example, all bodies that release heat emit infrared), the threshold between visible and non-visible light is as subjective as color perception is.

We perceive a single color when our eye is hit by light with a single wavelength. In contrast, white light is a collection of the wavelengths of the visible spectrum.

Although objects around us do not emit light, we are able to distinguish color because each object absorbs certain wavelengths and reflects others. Ideally, a completely black object absorbs all the light that strikes it. A white object, in contrast, reflects all the radiation it receives. In fact, an object that we perceive as white may absorb some wavelengths that are not visible.

Most of the absorbed light is transformed into heat. This is why two differently colored objects exposed to full sunlight heat up differently.

Similarly, tissues absorb the light that strikes them differently. For this reason, lasers of different wavelengths are used for different tissue applications.

The section on medical applications describes how different tissues absorb different wavelengths.

Light has been described as a wave phenomenon. This nature is confirmed experimentally by diffraction and interference phenomena that are similarly observed in the propagation of sound waves and in liquids.

However, some behaviors of light can be identified as typical of particles, for example, the photoelectric effect (electron emission due to light excitation).

Theories related to quantum mechanics indicate that there is a minimal, non-divisible particle of light. The “quantum” of light is called a photon.

The image below shows how in the corpuscular model of light it is possible to visualize how energy transfer occurs between the photon and the atoms on the surface of the affected material.

When a photon strikes an atom, it can move its electrons to a higher orbit. In this condition, the atom has accumulated energy that it must give up to the outside world, and it does so by emitting a new photon with wavelength (color) precisely dependent on the distance between the orbitals.

With the oscillating model we can explain diffraction, such as what happens on a CD surface where the light is decomposed.

Laser (from English Light Amplification by Stimulated Emission of Radiation) is a monochromatic, coherent light source (light beam with the same phase, or same time course). It is a relatively recent device; the first practical demonstration was in 1960.

An ordinary light bulb emits light composed of different wavelengths (which we perceive as predominantly white), in different directions and not coherent. Conversely, a laser emits coherent, monochromatic light. Without a collimator, the light scatters at an angle dependent on the construction of the device.

There are different technologies for generating laser light. A large proportion of the laser devices used are diode devices, that is, they generate light through the passage of current within a junction of a semiconductor material in which two zones are cut out in which the passage of current causes photon emission. Simplifying the concept to the extreme, the following image schematizes the device:

An electric current is passed through (the direction of electrons is shown in the picture). The passage of current allows the spontaneous emission of photons (as occurs in LEDs). The photons thus generated can trigger the emission of new photons with the same wavelength in the surrounding atoms. The presence of the central cavity, sized to be a multiple of the wavelength of the emitted light, allows a resonant effect to be generated, in which the emission of photons occurs with the same phase.

In many cases, the properties of light emitted by lasers are exploited to concentrate light into a spot of minimal size. This allows a high amount of energy to be transferred to a small area. In addition, the properly powered laser allows extremely short pulses to be obtained for a number of special applications.