The Behavior of Light in Different Media

In the realm of physics, light’s behavior has always been a subject of fascination and extensive study. This article, based on the video titled “Why Does Light Slow Down in a Medium…Or Does It?” by Arvin Ash, delves into the intriguing phenomenon of light slowing down when it passes through different media.


The Constant Speed of Light

Einstein’s theory of special relativity, proposed in 1905, is based on two simple assumptions: the laws of physics are the same for all observers in any non-accelerating reference frame, and the speed of light in a vacuum is constant for everyone, regardless of their motion. The latter does not imply that the speed of light is the same everywhere; the keyword here is “in a vacuum.” In a vacuum, like the emptiness of space, the speed of light is constant. However, in other media, such as water or glass, the speed of light changes. This change in speed is what causes us to see the bending effect when a straw is placed in a glass of water, or the spectrum of colors when light passes through a prism (Ash, 2023).

The Paradox of Photons

While the perceived speed of light changes depending on the medium it’s in, the speed of individual photons that make up light does not. They remain at ‘c’ – the maximum allowed speed, whether the photon is in space, water, or glass. This might seem paradoxical, but it is a fundamental aspect of how light behaves.

The Role of Maxwell’s Equations

One way to explain the constancy of the speed of light is to look at Maxwell’s equations. Maxwell first demonstrated that light is an electromagnetic wave, and its speed is embedded in his equations. According to him, it was simply the inverse square root of the permeability and permittivity in space. The permeability is the ability of a medium to resist magnetic fields; permittivity is the ability of a medium to resist electric fields. It’s logical that the speed of light is inversely proportional to these values because the less resistance an electromagnetic wave faces, the faster it travels (Ash, 2023).

The Quantum Perspective

While the classical picture of light behavior is fascinating, the quantum mechanical description provides a more accurate depiction. In quantum mechanics, the slower group velocity of light is due to the superposition of all the different paths that the undisturbed light can take and its interactions with all the atoms in the medium. In quantum mechanics, each photon is a wave with a non-zero probability of taking every path through the glass or any transparent substrate it passes through (Ash, 2023).

Refractive Index and Speed of Light

The speed of light in different media is determined by the medium’s refractive index. The refractive index is a measure of how much the speed of light is reduced inside a medium compared to its speed in a vacuum. The higher the refractive index, the slower the speed of light in that medium. For instance, light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0. However, it slows down to 225,000 kilometers per second in water (refractive index of 1.3) and 200,000 kilometers per second in glass (refractive index of 1.5) (Olympus LS, n.d.).

Light Speed and Optical Density

The speed of light is also influenced by the optical density of the medium. Optical density is a measure of how much a medium slows down light. The greater the optical density, the slower the light travels. This is why light travels slower in more dense substances. For example, the order of the speed of light in different media is Air > Water > Oil > Glass. As the optical density increases, the speed of light decreases (The Physics Classroom, n.d.).

Interaction with Charged Particles Expanded

The speed of light, while constant in a vacuum, is subject to change when it traverses through different media. This variation is primarily due to the inherent nature of light as an electromagnetic wave. Electromagnetic waves, including light, have an intrinsic ability to interact with charged particles present in various media.

When light enters a medium other than a vacuum, it encounters a multitude of atoms along its path. These atoms have charged particles – electrons and protons. The electromagnetic wave of light causes these charged particles to oscillate. As these particles oscillate, they in turn generate their own electromagnetic waves. The resulting waves then interfere with the original light wave, causing it to slow down.

This process of interaction and subsequent slowing down is not a one-time event but occurs continuously as long as the light wave is within the medium. Each interaction with an atom causes a minute delay, and when summed over countless atoms, it results in a noticeable decrease in the overall speed of light within that medium.

Therefore, the speed of light is not a fixed value but a variable that depends on the medium it’s traveling through. The denser the medium, the more atoms there are for light to interact with, and the more it slows down. This is why light travels slower in water or glass compared to air or a vacuum.

The video


The behavior of light as it travels through different media is a subject that combines principles from classical and quantum physics. The speed of light, while constant in a vacuum, changes as it passes through different media due to factors such as the refractive index, optical density, and interactions with charged particles. This understanding helps us explain everyday phenomena like the bending of light in water but also deepens our knowledge of the fundamental nature of the universe.


  1. Ash, A. (2023). Why Does Light Slow Down in a Medium…Or Does It? [Video]. YouTube.
  2. Olympus LS. (n.d.). What is Speed of Light? Olympus LS. Retrieved from
  3. The Physics Classroom. (n.d.). Optical Density and Light Speed. Retrieved from

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