Light travels more slowly in water than in air due to the difference in density between the two mediums. Water molecules slow down the propagation of light compared to less dense air molecules.
The speed at which light travels directly depends on the refractive index of the medium it passes through. This index, essentially, is like a number that indicates how much light slows down in a material compared to a vacuum. The higher it is, the more light struggles to move quickly. In a vacuum, the index is exactly 1, which means light travels at its maximum speed: about 300,000 km/s. When light passes through air, the index is just slightly above 1, so it slows down only very slightly. But in water, with an index close to 1.33, light is significantly more hindered and travels at only around 225,000 km/s. This slowdown mainly comes from the interaction between light and water molecules, which delays its progression.
When light passes through water, it constantly encounters water molecules that disrupt its progression. Specifically, the molecules temporarily absorb the light and then immediately re-emit it—this interaction occurs extremely quickly but still adds slight pauses in the movement of the light beam. Each time this phenomenon occurs, the trajectory of the light slows down slightly. Even though each interaction is very brief, their accumulation overall makes its journey slower. This photon-molecule ping-pong doesn't really change the light at the exit, but it is enough to explain why light travels more slowly underwater than in air, where molecule-photon interactions are significantly rarer.
When we talk about optical density, we refer to how much a medium slows down light based on interactions with its molecules. Specifically, the denser a material is optically, the more photons (the tiny particles of light) will be hindered in their movement. Contrary to what one might imagine, this does not necessarily relate to physical density—for example, some transparent solids that are less dense than water can slow down light more. It all depends on how the molecules of the medium interact with the light passing through, affecting its path and therefore its speed. In water, for instance, these interactions are more numerous than they are in air, which explains why light slows down more in this medium.
The wavelength of light is a bit like its identity card: it influences its speed when it passes from one medium to another. Some wavelengths, like those of red or blue, are not slowed down in the same way. Water absorbs certain colors more efficiently, especially towards the red, reducing their intensity and affecting their propagation. When a wavelength is absorbed, its energy disperses into the molecules and does not continue its path as easily. That's why underwater, everything quickly becomes blue-green: these are the least absorbed colors, which diffuse better and further. This difference in absorption greatly affects how light travels in water compared to air, explaining why the speed is lower there.
In air, molecules are spaced out, not very numerous, and they interact very little with light. As a result, light travels at nearly its maximum speed. In water, on the other hand, molecules are very close together and more numerous. This means that photons constantly encounter them, are briefly absorbed, and then re-emitted. This significantly slows them down. To give an idea: light travels at about 300,000 km/s in air, while in water, it's more like 225,000 km/s. This slowdown is not a detail; it directly explains everyday phenomena like the famous illusion of the "broken" stick when you dip it in water.
The relative slowness of light in optical fibers (slower than in air or vacuum) is deliberately used to transmit information over long distances with very little loss.
The intense blue color of certain tropical waters is explained by a low absorption of blue wavelengths, allowing this color to penetrate deeper, giving the water that magnificent turquoise appearance.
The refractive index of diamond is one of the highest among natural materials (about 2.42). This means that light slows down significantly within it, which explains why diamonds sparkle so much!
Rainbows appear because water disperses white light into different colors. Each color, having a slightly different refractive index, slows down at varying rates, thus separating the light into a colored spectrum.
The impression of distortion also comes from refraction. When the light reflected by the submerged object passes through the water-air interface, it changes angle, creating the illusion of a distortion of the object's actual shape.
Sure! Here’s the translation: "Yes, slightly. Temperature affects the density of water and its molecular structure. A thermal change thus leads to a slight variation in the refractive index, although this change remains minimal under typical natural conditions."
Underwater, light no longer passes directly from air to our eye, but rather from water to our cornea, drastically altering the refractive index to which our eye is accustomed. Diving goggles restore an optimal air-eye interface, allowing for clear vision.
In water, the different colors (wavelengths) of light do not travel at exactly the same speed. This is why light dispersion occurs, which is responsible, for example, for rainbows visible through water droplets.
It is due to the phenomenon of refraction. When light passes from water to air (or vice versa), its rays are bent. This change in angle tricks our visual perception and makes submerged objects appear deeper than they actually are.
Yes, even though in a vacuum light always travels at a constant speed of about 299,792 km/s, as soon as it enters another medium like water or glass, its effective speed decreases. This reduction depends on the medium and is characterized by the refractive index specific to each material.
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