And it became a light

You are Jack – retired USA soldier – living in the remote village in the middle of Nowhere. You saw so much pain in the entire life during ‘fraternal interventions’ that no amount of alcohol could kill memories about it. The only way is to stay sober.

You do not have your wife. Actually, not anymore. She left you while you were helping to remove the dictatorship in the wild East. The woman you still love left you for a random guy met in the worst club in the Town. They made it to the bed – many times. She got pregnant with him and fled taking away also your baby girl.

You are alone in the middle of Nowhere.

It is a dark cloudy winter night away from any traces of civilization. Stars are shining for the Other Earth in the Universe. You have just realized that the candle that was giving a small light in your kitchen was the last you had. Your eyes accommodate slowly to the new conditions of total darkness but even your trained eyes barely allow to detect objects’ edges. ‘It’s good to be divorced.’ – you think – ‘At least I know where my things should be.’.

You are heading upstairs to your too-big-for-one-person bed thinking that it could be a good time for crosswords but you are completely out of light sources.

Suddenly, you hear that the only friend in this world protecting your home from cats and Jehovah’s Witnesses starts barking. Loudly as never before. And stops even more rapidly after silent shot from suppressed gun. ‘Chapman’s revange’ – you are talking to yourself opening night closet looking for a toy from the good old times. Your ex-wife did not like to keep it nearby the bed changing its place a lot of times. But now you are the boss.

You are taking it with yourself going down to the ground floor. You hear him. His steps in the fresh deep snow were easily recognizable and then you see as he is coming in through the just broken window.

You are staring at him but you are not scared at all knowing that he is not able to see you and desperately looking for the light switch tripping over the wooden chair. You see him clearly from the good place to make a shot but you restrain from doing so for a few more seconds delighted by the view from your good-old days military toy. ‘They are not making such a good thermographic cameras these days’…

Let’s cut this story at this point.

Was the narrator right that there were no light sources in there? Yes… and no – depending on the definition of the light. We can think about the light as anything that our eyes are able to detect. Let’s call it the narrow definition. Why ‘narrow’? Because it is a very small piece of the available electromagnetic radiation spectrum.

Radiation spectrum is made from portions of energy, called photons (does this name ring any bells?). These photons can have (almost) whatever energy. Because of the way photons interact with matter the whole spectrum was conventionally divided into subspectra.

Looking at the image above you can clearly see that the radiation subspectrum that interacts with our built-in detector (our eyes) is very small. It is called visible spectrum between about 380 and 750 nanometers (in vaccum). Shorter wavelengths are percived as violet-blue colors (cold), about 550 nanometers we have green, with red on the opposite side the of visible spectrum (warm).

Photons, waves and energy

The nature can be tricky in many cases and physics to describe them uses lots of abstract concepts which used on daily basis start to be entirely normal. Photons do not only have their energy but also frequency. Saying that photons can be thought of as waves.

Waves are oscillations that happen at specific frequency and their propagation speed depends on the propagation medium. Medium can be air, lens of your camera or even vaccum (in case of photons). The medium affects only the wavelength – the minimum distance between points on wave oscillating in the same way (having the same phase of oscillation).

The energy of the photon is described by this very simple formula:

$E=hf=\frac{hc}{\lambda}$

where:

• $h$ is the universal Planck’s constant,
• $f$  – frequency of our photon,
• $c$ – propagation speed in the medium,
• $\lambda$ – wavelength in the current medium.

But scientists call also X rays as light! And infrared (IR) also!

Actually most of them really do not care about the visible spectrum. X-rays are commonly used to determine the internal structure of things because of its penetration range and usful wavelengths while UV and IR light are welcome for biological studies.

You need a proper detector

Our eyes are truly amazing detector, although very limited to a short range of frequncies. In fact we are in the sea of radiation coming from our phones, WiFi and more importantly from the cosmos which serves the most biologically harmful (UV and more energetic) photons. What we want for our camera is to have a detector which mimics the behaviour of our eyes to get the best possible color reproduction.

In cameras of today, there are used semiconductor devices which are matrices of photosensitive cells. When photons of certain energy reach the surface of the device they are converted into electric charge which is collected by the electronics, amplified and decoded into the image. That is the big picture.

It is good to point, however, that different semiconductor types are used to collect information about photons from other parts of the radiation spectrum as well and their principle of operation is very similar.

You are also the source of radiation!

Actually, every body with the temperature above the absolute 0 is the source of light. This fact is described by the Wien’s displacement law which connects the most probable emmited wavelength $\lambda_{max}$ with its temperature $T$:

$\lambda_{max} = \frac{b}{T}$

The higher the temperature the shorter is the most probable value of wavelength.  Since wavelength and frequency are inversely proportional values we see that increase of temperature gives more energetic photons. So higher temperatures give you colder colors (you are going towards blue light).

Why are we talking about the most probable wavelengths?

Thermal radiation emmision is a statistical process which means that the body is emitting a range of probable photon wavelengths instead of one specific wavelength. Its distribution is continuous with one extreme – that is the wavelength described by the Wien’s displacement law.

So what was the ‘wavelength’ of the intruder in the Jack’s house?

We can make a little approximate calculation. The constant of proportionality in the Wien’s law for the ideal black body model is about $b=2.9\cdot 10^{-3}\ \mathrm{m\cdot K}$ while we can assume that intruder’s temperature was $36.6^{\circ}\mathrm{C}$ ($310\ \mathrm{K}$) it gives us:

$\lambda_{max}=\frac{2.9\cdot 10^{-3}\ \mathrm{m\cdot K}}{310\ \mathrm{K}}\approx 10^{-5}\ \mathrm{m}$

According to the presented radiation spectrum this value lays in the mid-IR range. To ‘see’ in this range of radiation we need to use technology. The thermovision imagery systems collect incoming IR radiation and then they artifically convert it to the human visible range. That is the all magic.

The Summary

After reading this post you should be more openminded to the light term. Especially to the fact that what we are able to see is just a very small piece of radiation spectrum. Of course we can extend our abilities using technology to be as sly as Jack to see other kinds of waves. Actually this is what people of science do (this is, however, only the mean not the ultimate goal).

We will use Wien’s displacement law when we will be talking about the white balance.

Moreover, you should start to think of light as a bunch of targeted photons with a certain frequency that can interact with matter (for example by simply bouncing off from an object).

This is only a brief introduction. I did not want this post to be luscious with facts to keep it as simple as possible. We will be going deeper and deeper into physics and technology in the following posts, so do not worry 😉

I also hope that you liked the Intro! 🙂