Have you ever caught a glimpse of your arm and then it just hit you- why does my blood look blue from this viewpoint if I know without doubt that it is red? Let us delve together into the curious aspects of how light and matter interact in order to untangle this puzzling optical illusion.
In order to fathom this whole process, we have to start from the very beginning- what is colour.
There is many fascinating physics revealed in the specifics of how light reacts to a substance. Light waves can travel through empty spaces, unlike water waves, and materials themselves, given that we can turn on a torch in a cave, for instance, and see the beam of light.
Light from a flashlight shining through
a narrow cave in the tow
The wavelengths of light that the object reflects are in fact the factors that lead to the creation of colour, determined by the arrangement of electrons in the atoms of that substance that will absorb and then re-emit photons of particular energies. On balance, when visible light strikes a substance, it usually releases some of its energy to the electrons present, which is known as excitation. Excited electrons will ultimately release this energy, and they can do so in a variety of ways. Fluorescence is one phenomena that results from one type of de-excitation. De-excitation frequently happens by the excited electron striking a nearby atom, which releases the energy as heat, or by the same frequency of light interacting with the electron again. Which energies, or in the case of light, which frequencies, are most likely to excite electrons depends on the structure of the atom. The frequency (or colour) of light that first struck the item will no longer be present in the spectrum of light, regardless of how that energy ultimately departs the electron. A (red) rose looks red because its structure absorbs all visible light frequencies apart from red.
How does light behave within material?
We have all thrown at least once a ball in sand and concrete during our childhood. If we were to analyse the ball’s behaviour, it is easily noticeable that the outcomes were not similar at all.
The key factor in scattering- a well-known process that explains many other day-to-day physics phenomena, when something in the way causes light to stray from its intended course. Blue light is more likely to be dispersed than red light due to the slight density variations in skin and fat tissue. As a result, blue light will not penetrate your skin as deeply before being deflected.
Because the ratio of red to blue light returning from a vein is a little bit lower than the ratio of red to blue light returning from neighbouring skin, veins will seem blue. This is because less blue light reached the vein to be absorbed there. Why do the veins seem blue then? In both places, more red light is reflected than blue.
The optical illusion
Now let us focus on two areas of the arm. Let there be the area with large veins under the skin (venous areas) and the one right next to them without veins (non-vein areas). Skin with less melanin is lighter and reflects most of the incoming light, making it appear white. Now, the (almost translucent) blood in the veins absorbs most of the light at all frequencies, but reflects a little in the red part of the spectrum. This is why blood looks red.
Both skin areas reflect more red light than blue light, but areas without veins reflect a higher ratio of red to blue light than areas with veins. This is because less blue light reaches the veins and is strongly absorbed by the veins. The brain interprets the vein area to appear slightly bluer than the surrounding skin. The same thing happens when you look at the image above (with the red lines). The blue rectangle is the same colour overall, but on the right side it looks a little darker because it is surrounded by dark red instead of bright white.
Let us take the photo of my hand, inserted above, as reference.
Note that both regions have higher red values, but the non-striped regions have a slightly higher ratio of red to blue.
Fun facts
a. A medical professional may shine red or infrared light into the skin to find a vein for injection.
b. Nightclubs may use blue light in restrooms to deter drug injections by making veins almost invisible through the skin.
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