LAB Physics - file 03
Light slows down
In my previous post, we examined the speed of light to see how fast light is! And in consequence, the speed of light is formidable compared to any human technology.
But in fact, light can be slowed down by the medium it’s traveling through.
Vacuum vs. Atmosphere
Light can travel at its maximum possible speed (approximately 299,792 km/s) but ONLY in a perfect Vacuum.
When light enters the Earth’s atmosphere, it has to struggle to navigate through Nitrogen, Oxygen, and other Molecules. This disturbance causes light to slow down, though ONLY a tiny portion…
So light is about 90 km/s (324,000 km/h) slower in the Earth’s air than in space!
How much does light slow down?
In physics, the Refractive Index is used to indicate how much a substance slows down light.
Speed Comparisons in different media
How fast does light go through various common substances?
So, as you can clearly see, the denser the material, the slower the light. Especially, speaking about diamonds, the secret of mysterious brilliancy might come from such density, which hardly allows light to pass through.
So I think that… we can imagine that light goes through a material like a quarterback in American Football struggles to run forward, dodging defenders’ (so electrons’) sacks to navigate the ball going as much straight as possible toward the goal line!
Or light going through a dense material is like you struggling to run through a waist-deep pool in water exercise.
Then, do individual Photons slow down?
The basic unit of light that carries Electromagnetic energy is called a Photon. A photon is a fundamental and massless particle, and exhibits the property of Wave-Particle Duality, acting as both a Wave and a Particle. More energetic photons show a higher Frequency, seen as blue light, while less energetic photons show a lower frequency, seen as red light. This property explains the Photoelectric Effect.
But don't be confused that a photon is NOT a charged particle, unlike an electron. Electrons have a negative charge, while photons are neutral (zero charge) as well as massless. And photons act as the carriers of electromagnetic force, whereas electrons are fundamental matter particles.
Surprisingly, individual photons always travel at the speed of light (“c”) regardless of how dense the medium light is going through. When we say light “slows down” in glass, it correctly means that the light wave is interacting with the electrons in the atoms of glass. The atoms of glass absorb and re-emit the energy, or their electric fields interfere with the light wave. This interaction creates a tiny delay.
So, while the electric wave takes longer to go through the glass, the actual energy packets Photons are still zipping between atoms at full speed! It's a little difficult to make intuitive sense about this gap between the electric wave and photons…
So here I would like to introduce you to the Cherenkov Radiation, which explains this phenomenon. The Cherenkov Radiation just occurs when photons actually travel faster than light itself does in a specific medium.
When light delays behind photons — Mysterious, fantastic blue glow
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| Fantastic blue glow of the Cherenkov Radiation (source: Idaho National Laboratory) |
You might easily recall the fantastic blue glow in the pool of an underwater nuclear reactor. This is a typical phenomenon of Cherenkov Radiation. When charged particles like electrons travel through a transparent medium faster than the actual speed of light in that medium, such a blue glow emits. But note that photons can NOT exceed the speed of light in a vacuum, but constantly as fast as that (“c”).
In general, a particle's electric field interferes with and polarizes atoms in its path in a medium, and then the atoms of the substance re-emit photons.
The material that slows down the speed of light, such as pure water (H2O), glass, diamond, etc., is called a Dielectric.
When the charged particle runs faster than the actual speed of light in the dielectric medium, these waves add up constructively and form an electromagnetic shockwave.
For example, during the process of Radioactive Decay or Nuclear Fission, the decaying Atomic Nuclei (nucleuses) emit Beta Particles, which are extremely high-energy, superluminal electrons or Positrons. These beta particles outpace light in water and excite water molecules. And in turn, the water molecules release photons. This sequence creates the blue glow of photons in the pool of the nuclear reactor. It's a Cherenkov Radiation!
This phenomenon is analogous to a Sonic Boom of a sound wave. When a supersonic aircraft flies through the air faster than the speed of sound, the sound waves are intensely compressed and merged into a powerful shockwave.
Likewise, when photons travel through a specific medium comparatively faster than the actual speed of light, a light shockwave is created, releasing photons with intense, higher frequency light. And also, the light shockwave forms a cone shape, which is analogous to the Mach Cone of sonic boom.
The photons are emitted more intensely and carry more electromagnetic energy. For human eyes, these photons are seen as blue or violet light due to their high frequency and shorter wavelength.
The actual speed of light is NOT constant by medium, while a photon travels constantly the 100% of speed of light (“c”) as its property. This discrepancy, as well as the interference with molecules of water bears the fantastic, beautiful blue glow in a nuclear reactor pool. But at the same time, this indicates extremely high energetic nuclear activity in it.
Light is overtaken by photons. Then, is there anything else exceeding the speed of light? Is the speed of light in a vacuum really the fastest and the upper limit in the Universe?
We will examine it in my next post. Coming soon, and stick around!
Further Reading (sponsored by Amazon):
In this fully revised 2nd edition of the classic textbook “Light Science: Physics and Visual Arts,” the authors Thomas Rossing (Stanford University) and Christopher Chiaverina present the science of light – that is, the science behind what and how we see!
Table of Contents
● Ramamurti Shankar (2020). Fundamentals of Physics II: Electromagnetism, Optics, and Quantum Mechanics (The Open Yale Courses Series; Expanded edition). 680 pages. Yale University Press.
“Fundamentals of Physics II: Electromagnetism, Optics, and Quantum Mechanics” is a beloved introductory physics textbook, including exercises and an answer key, accessibly explains electromagnetism, optics, and quantum mechanics!
Table of Contents
In this “Light: A Very Short Introduction,” the author Ian Walmsley (experimental physics, the University of Oxford) discusses early attempts to explain light, and the development of apparently opposing particulate and wave theories by scientists such as Isaac Newton and Christiaan Huygens!
