Electromagnetic radiation is energy travelling as waves or photons. This is not the place or the time to go into this discussion [1] but Einstein and Infeld said it well:

Light, or the visible light spectrum range from \( \in [400 - 700] \) nm in the electromagnetic radiation spectrum. XX insert picture of Elmag spectrum here XX. Above, with higher wavelengths, you will find infrared radiation (also known as IR), and under you will find the ultraviolet radiation (also known as UV). We call it visible light due to the fact that our eyes can only "pick up" these wavelengths. For this project we will mostly focus on light \( \lambda \in [470,520] \) nm region.

You have probably heard that nothing can travel faster than light? But not that many (non-physicist) remembers what velocity light actually travels with. In most cases it's enough to say that light travels in \( \sim 3.0 \cdot 10^{8}\) m/s in vacuum or \( \sim 6.7 {\cdot 10^{8}} \)mph.

When talking about light there is a couple of expressions that is good to know the meaning of

• Monochromatic
• Coherent
• Wavelength
• Optical Path

The given wavelength \(\lambda \) for a lightsource can be found using the following formula:

\begin{align} \lambda = d sin(\theta) \end{align}

Where \(d\) is the grid spacing and \(\theta\) is the angle.

Continuing this part we will refer to light as waves. As shown in the picture above, visible light ranges around ∈[400−700] nm. This is known as the wavelength of the light, and to label it we use the symbol \(\lambda\). To better understand the physics we will start by using the rainbow as an example. Most people have had the pleasure to see this magnificent phenomenon in their life. To understand what happens to the light, we need to introduce the term refraction index. In short terms, this means that light will behave differently in different mediums if hit with an angle different than zero from the optical path. For air, the refraction index is simply \(1\), but for water it is \(1.33\), both in vacuum. Using Snell’s law and the fact that the light travels from air to water we can see that the angle with which the light will be emitted can be found by:

\begin{align} n_1 sin\theta_1 = n_2 sin\theta_2 \end{align}

The light from a light-emitting diode, LED for short, can be viewed by the human eye as monochromatic. This means it appears to only light up as one color. From theory we know that a LED cannot be monochromatic, thus proving one of the biggest differences between a LED and a laser. More importantly, the light from a LED is not coherent, meaning that the light waves do not have the same frequency. A LED loses little energy to heat as it applies most of its energy to light up the diode. This is a very useful property.

To create a simple LED-circuit you only need a couple of components: a LED, a resistance, some wires, a circuit board and a voltage source. We used a blue LED lamp with the properties 20mA and 3.2V, a resistance of 1kΩ and a PHYWE power supply that allowed us to vary our voltage between 0-30V. The LED and the resistance were mounted to the circuit board using a soldering iron, before the wires were soldered to each side of the board (insert figure).

A laser is a device that emits monochromatic light amplificated by stimulated emission of electromagnetic radiation, hence the name (“Light Amplification by Stimulated Emission of Radiation”).

The first laser was built in 1960 by Theodore H. Maiman, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow. What makes a laser different from other light sources is the fact that it emits its light coherently. This means that the stream of light will stay narrow over a long distance and can also be focused in a tight spot, like for example for laser cutting or a laser pointer.

A laser consists mainly of five parts: the gain medium, the laser pumping energy, the high reflector, an output coupler and a laser beam. The gain medium is a material which allows light to amplify. This is usually located in an optical cavity. What this means is that there is a mirror on either side of the medium, in order to make the light bounce back and forth. This amplifies the light, as it passes through the gain medium each time. One of the mirrors is usually not 100% reflective, since there will a small output (the laser beam). For anything to happen within the optical cavity we need a power source that allows the gain medium to amplify the light. This energy is supplied through a process called pumping and is usually either light or an electrical current.

When we use the term biolaser we refer to a biological component that posesses the same qualities as a laser. What we mean by this, is that the gain medium is now the biological component.