Vishrut Kinikar
It is well-known among students of science that the blue color of the sky exists as a result of blue wavelength being scattered disproportionately more than other wavelengths. Although this oversimplified explanation may suffice, further analysis reveals a more intricate story. It is this further analysis which led British physicist John William Strutt, 3rd Baron Rayleigh, in the 19th century to formulate his namesake principle of scattering, the Rayleigh Effect.
Rayleigh Effect
Rayleigh scattering occurs under the main condition that the size of the particle that the light collides with is much smaller than the wavelength of the light. This, coupled with a refractive index close to 1 (the medium causes negligible refraction to the light), allows Rayleigh scattering to occur. One of the most vital properties of the Rayleigh effect is that the collision between the photon and the particle is always elastic, meaning the kinetic energy of the system (photon + particle) is always conserved. Rayleigh later proceeded to derive another property of his effect, which is that the scattered light intensity is inversely proportional to the wavelength raised to the fourth power. Produced below is the mathematical relation:
I ∝ 1/λ4
Therefore, the Rayleigh Effect elucidates the sky’s blue color in the following manner: Due to the particles in Earth’s atmosphere being smaller than the wavelength of sunlight and having a refractive index close to 1, sunlight is able to undergo elastic collisions with atmospheric particles. The wavelength scattered the most is the sunlight with the smallest wavelengths (blue and violet) due to the aforementioned inverse fourth power rule. This explanation for the blue color of the sky was then extended to explain the blue color of seas. The blue color of water bodies was purported to be a reflection of the sky by the scientific community and Lord Rayleigh himself for decades. However, despite its initial acceptance, it was challenged multiple times – most notably by the Indian physicist Chandrasekhara Venkata Raman.
Raman Effect
C.V. Raman, when returning from England to India in 1921, was traveling by sea and he began to wonder why the Mediterranean Sea was of a blue color. Given his avowed skepticism of the currently held theory, he used this question to perform experiments and derive conclusions. Using a handheld prism he had aboard the SS Narkunda, Raman filtered out the skylight bouncing off the sea and observed that the sea still appeared vividly blue. He thus concluded that the sea performs its own scattering analogous to the sky, as opposed to the sea reflecting the sky. Upon reaching Kolkata in 1928, C.V. Raman performed a thorough and comprehensive experiment with the use of a mercury arc lamp, a liquid, and a spectrograph. The mercury arc lamp would transmit a filtered light beam through a liquid, and light scattered at a ninety degree angle from the liquid was collected and passed through a spectrograph in order to break up the light into its constituent wavelengths. The resulting light was shown to have the same wavelength as the incident light, which corresponded with Rayleigh scattering. However, a minuscule fraction of the light had slightly shifted wavelengths. In his quest to explain the color of the sea, Raman stumbled on a new unprecedented phenomenon. These slightly shifted wavelengths were the result of inelastic collisions of the light with the molecules in the liquid, which left the earlier theory of Rayleigh rattled. Raman repeated this experiment with a total of 60 distinguished liquids and all of those trials yielded the same result, i.e. a tiny fraction of light scattering inelastically. Raman, then, along with his colleague K.S. Krishnan, published these results in Nature under the name of “A New Type of Secondary Radiation”. C.V. Raman was later awarded the Nobel Prize in physics in 1930 for his discovery. Diving deeper into this inelastic scattering predicted by C.V. Raman, Irish physicist George Gabriel Stokes took note of two outcomes. The first one being that the scattered photon loses energy to a molecule in a ground vibrational state, leading to an increased wavelength. This outcome is the more common possibility and is known as the Stokes shift. The second possibility, albeit less common, is where a photon gains energy due to a collision with a molecule in an excited vibrational state, leading to a decreased wavelength. This phenomenon is known as the anti-Stokes shift. The discovery of Raman led to unprecedented advancements in not only optics, but also in chemistry where an offshoot field known as Raman spectroscopy (based on Raman scattering) was utilized with the purpose of understanding the structure of molecules during their collisions with photons. What began as a simple and rather innocent question gave rise to a new field of science altogether, encapsulating the significance of scientific inquiry. Perhaps this is why C.V. Raman once famously remarked, “Ask the right questions, and nature will open the doors to her secrets.”
Image Sources:
Phys.org
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