What in the world is laser? (Part 3 of 3)

PART 3

[The author would like to thank the geniuses behind the new Microsoft Paint for making the figures in this part possible.]        

How are laser beams produced?

            Laser beams are the product of stimulated emission of light. But what is stimulated emission of light? Before we can answer this question, we must first discuss how matter interacts with light.

Light absorption

Matter and light interact in two ways: matter can absorb light or it can emit light. When an atom absorbs a photon of light (a photon of light is a quantum of light energy), it takes the energy of that photon and one of its electrons become excited (although not sexually). An electron orbits the nucleus of an atom in one of the possible orbits. Larger orbits (orbits that take the electron farther from the atom) have greater energy. Normally, an electron will orbit the nucleus in the smallest possible orbit. This orbit is called the ground state. When an electron gets excited, it jumps from the ground state to an orbit with a higher energy. Such an orbit is called an “excited state”. An electron is excited when it is on one of the excited states. The excitation of an electron (represented by the symbol e^-) by a photon is illustrated by Figure 1 below.

Figure 1. A photon of light (red oval with wave inside) is absorbed by an atom, resulting in the excitation of one of its electrons (e^-, gray dot).

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What in the world is laser? (Part 2 of 3)

What in the world is laser?

PART 2

What do we mean by “highly coherent, usually monochromatic beam of electromagnetic waves”?

For the sake of brevity, from now on we will use the word ‘light’ to denote not only visible light (electromagnetic waves our eyes can see) but electromagnetic waves in general. That is, when we say ‘light’ we mean electromagnetic waves, including radio waves, gamma rays and ultraviolet.

Now, a monochromatic ray of light is one in which the waves have the same wavelength or frequency. (The word ‘monochromatic’ came from the Greek words mono, meaning one, and chromos, meaning color.) A ray of white light is not monochromatic since it consists of waves of different wavelengths. In other words, white light is made up of light of different colors. On the other hand, the light emitted by a colored light emitting diode (LED) is usually monochromatic. A blue LED emits only blue light.

The fact that laser beams are highly monochromatic finds use in many scientific and engineering applications, such as spectroscopy. In spectroscopy, laser beams with a very specific wavelength are sent through a sample to be analyzed.

The next important property of laser beams is their high coherence. (What is certain is that they more coherent than the CBCP or the Vatican.) In the language of wave physics, coherence is the property of having waves that oscillate in phase of each other. Coherence comes in two kinds, temporal coherence (coherence in time) and spatial coherence (coherence in space).

Temporal coherence (coherence in time) would be best explained by an analogy in dance: for two dancers to be able dance the tango well, their stepping must have the same tempo. In other words, dancers performing a ballroom dance must have temporally coherent foot movements.

Temporal coherence is very closely related to monochromaticity. In fact, temporal coherence is used to measure monochromaticity. Another important aspect of temporal coherence is uniform polarization. This gives laser beams their characteristic ‘glare’, which makes them dangerous to the eyes. Sometimes, the glare of laser beams is used by the police or the military to disorient a pursued individual or an enemy.

Spatial coherence, on the other hand, means that a ray of laser light can be focused to a very narrow beam, often called a “pencil beam”. In other words, laser light can be focused to a very small spot. This makes laser beams ideal for applications that require great precision, like reading digital information encoded in a CD, cutting intricate patterns into metal or wood, burning away tumors without destroying neighboring healthy cells, or correcting vision problems without further damaging the patient’s eyesight. In microscopy, lasers are used to obtain blur-free images of very small objects at various depths, and this is possible because laser beams can be very narrow. And do not forget the use of lasers in increasing the chance of a headshot.

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What in the world is laser? (Part 1 of 3)

[Disclaimer: The author does not own Figures 1-4, nor does he own Yoda, although he certainly wishes that he does.]           

We use them to point at things in a slide presentation; snipers use them to point at heads ready to be shot. They make cool technologies possible, from CDs and grocery store barcode scanners to metal cutters. We often hear that they can cause blindness, but we also know that they are used in surgical methods to correct many vision problems. In science fiction movies they shoot out of space ships, one can engage in duels using swords made of them, and there are ships which can destroy entire planets using them. Laser beams – they are both boon and bane. But what in the world are they?

Laser, what in the world is, hmm?

What in the world is laser?

            The word laser stands for Light Amplification through the Stimulated Emission of Radiation. Originally, ‘laser’ is a term for the light emission mechanism that produces a beam of laser light, or a laser beam.  Today, however, ‘laser’ is used to denote devices that use the said mechanism to amplify light via stimulated emission of radiation.

Now, a laser beam is a beam of highly coherent, often monochromatic electromagnetic waves. And these beams are produced when stimulation emission of electromagnetic waves amplifies light. If you think that these are quite a mouthful, then grieve not, for in this series we will explain what they mean.

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On Proof and Evidence

Words, words

Sometimes – actually oftentimes – we can get pretty sloppy and careless in our use of words.

Take the use of the words “proof” and “evidence”. Proof and evidence, like speed and velocity, or theory and guess, have colloquial definitions that often lead to confusion. In order to smooth the progress of communication and avoid misunderstanding, these words have been given technical definitions in science and philosophy. For example, speed is defined as the magnitude of velocity; the latter is a vector, the former is the scalar magnitude of that vector. Also, a scientific theory is not simply a guess; rather, it is a system of ideas constructed from a verified set of generalizations and observations. In the same way, scientists and philosophers use the words proof and evidence to designate two very different things. For example, we prove a mathematical theorem instead of “finding evidences” for its truth, while we accumulate the evidence for a particular scientific theory but we never “prove” a theory.

What’s the difference? The distinction is best illustrated by examples.

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