Light, it’s something we rely on everyday, it helps us to see, to know where we’re going and to just live our lives. It’s required for respiration and phytoplankton need it to create oxygen. It’s a vital ingredient for life, but the light we see isn’t the whole picture. We see only a tiny portion of what we call light, but generally when we say light, we mean visible light.
Light is an electromagnetic (EM) wave, it consists of elementary particles known as photons, it behaves like a wave but is made of, well, stuff, so it’s neither a wave or particle, but both! But we don’t really know what that stuff is, so we says it’s an EM wave and so there is an EM spectrum or EMS as we will call it. The EMS consists of different energies, these energies determine what kind of EM wave we have. From UV and Gamma rays, to Infrared and visible light, to radio waves, they all fall on the EM spectrum.
The 1987 film Predator was a Sci-Fi action/horror film that saw an alien known as Predator come to Earth and terrorise a group of elite soldiers that were sent to retrieve hostages from a guerrilla territory in Central Americas Northern Triangle. Predator has the ability to turn completely invisible but slightly distorts the ground it passes over thereby allowing the protagonist Dutch (played by Arnold Schwarzenegger) and his team to catch glimpses of it. This cloaking technology is amazing but i’m interested in a different part of the movie which takes place a little further on. Let me set the scene:
Dutch is being pursued by Predator through the jungle and he is sliding downhill through the dense jungle until he reaches a cliff edge. He flies off the edge but luckily lands in the water below. He swims to the muddy bank where he lies for a minute to catch his breath. But then, he hears a loud splash and turns in fear to see the disturbed water and sees that something invisible is coming towards him. Frantically, he begins to claw his way up the bank, all the time getting more dark mud over himself. He finally crawls up against what looks like an uprooted tree where he sits and waits, maybe hoping predator doesn’t see him. Predator emerges from the water and beings looking around. We then go onboard with Predator and see what it sees, something obvious is that Predator can view the surroundings in thermal infrared, or in other words, can read the heat signatures of the surroundings. Predator looks around and sees something warm, he shoots it, but it turns out to be a bush rat of some sort. He then leaves, Dutch says “he couldn’t see me” quietly to himself as he wipes the mud on his arm. So somehow the mud has tricked Predator’s ingenious way of seeing the people through the dense jungle.
Could mud really trick Predator’s infrared vision? Well in order to answer that question, we need to understand a little about thermal infrared and light.
Properties of light
As I mentioned above, visible light is part of the EMS and this is all we can see as humans, the visible light part of the EMS. To put that into perspective:
So this is the whole EMS, you can see a low-energy end on the right and high-energy end on the left (the energy our Sun gives out). With visible light only occupying a small section in the middle-left of the whole spectrum. The energy of the light is related to the wavelength and frequency and when light passes through an object, it’s velocity changes, which means the wavelength changes. When light passes through an object, we say it refracts, to understand this, we need to define something called the refractive index of a material:
Refractive index (n) = C / Cm
Where C is the speed of light and Cm is the speed of light travelling through a material (m). So if n = 1, then the speed of light in the material is the same as light travelling in a vacuum (in space). An important rule is that n can never be less than 1, because that would mean Cm is greater than C and nothing can travel faster than light in a vacuum. So refractive indices are generally greater than 1.
So when light interacts with a material, you can have reflection or refraction, where refraction refers to the bending of light as it enters the material. Light will only refract when it enters a material with a different refractive index. You would have seen this if you’ve ever put a straw in a glass of water, the straw looks broken, this is because the light is refracting and distorting the image.
These are examples of light refraction, Snell’s law describes how light will behave when it reaches the interface of two materials with different refractive indices:
Snell’s Law → nl*sin(i) = nr*sin(r), where ni = density of material light is going through first, nr = material light is entering into, i = angle of incidence, r = angle of reflection.
Using this formula, you can calculate r, so you will know how much the light will refract, all you need to know is the angle of incidence and the refractive indices of both materials.
Difference between transparent and opaque
The reason grass is green is because grass absorbs all other wavelengths of visible light and reflects the green wavelength. We see colour because the material we are looking at is reflecting that colour. Something white is reflecting all visible light colours and something black is absorbing all of them. This makes sense, because a black car feels hotter than a white car on a hot day, the black car is absorbing more radiation (light) so it’s hotter!
But this reflection and absorption doesn’t just happen with visible light, it happens with all frequencies of light on the EMS. You just need to be able to see it. For example, every wondered why you can listen to the radio from inside your house? It’s because radio waves can pass through most solid materials, so solid materials for us are transparent when looking at radio waves. But if you shine a torch at a solid wall, the visible light will not pass through, this is because the walls of a house are opaque in visible light.
So how does this relate to Predator?
Predator’s thermal infrared camera
This is the secret to Predator’s efficiency when looking for people in the dense jungle. The camera mounted in its helmet can read heat signatures, so it is registering the thermal infrared part of the EMS which generally read the middle-infrared part of the EMS. Every single living thing on Earth emits heat, the temperature of the body will determine how bright it registers. Relatively cooler bodies appear blue and purple and warmer ones appear bright yellow and red:
So the middle-infrared is slightly lower energy than visible light and so our eyes literally cannot perceive it, it’s invisible to us.
So Predator identifies people by recognising warmer heat signatures than the backgrounds which are comparatively cooler. So if we go back to the original scene I described, what did Dutch look like when he was covered in mud? He looked like this:
Well, I think he looked like this, i’ve circled what looks like his head, but even if it’s not him, the point is that he doesn’t stand out from the background. So Predator couldn’t see the heat coming from his body. How did he do this? Well it seems he achieved this by unintentionally covering himself in dark mud.
So this begs the question, would it work?
Well it’s worth remembering that Predator reads the middle-wave infrared, which is a proxy for temperature. So if Dutch is covered in mud that is cooler than he, then you would expect he would be invisible. But the problem is, energy is transferred from hot to cold, meaning his body temperature (which is around 36-37 degreed celsius) would slowly heat up the mud and hence make him hotter than the surroundings and visible again!
He was able to mask his body temperature by covering himself in cold mud, but it would only be a temporary solution until his body temperature heats up enough and warms the mud. So if you encounter Predator, you could try this, but there is a better way.
Remember how I said the transparent and opaque rules don’t only apply to visible light. Well, there are certain materials which block infrared light. An example of a material that blocks the middle-infrared is something you come into contact with everyday, glass!
In this case, perspex is being used but achieves a similar effect, making Thunderf00t invisible! You can see his right hand because it is in front of the perspex and this gives you a comparison of what he would look like normally through the camera.
Glass reflects middle-infrared radiation so someone on the other side of a pane of glass would not be spotted by Predator. So where mud was a good idea for the short term, if you want escape Predator permanently, hide behind, well, any window and you’re safe!
References and further reading: