Light's Medium Journey: Understanding Photon Travel

Light is a type of energy known as electromagnetic radiation. Unlike sound waves, light does not need a medium to travel through. It can travel through a vacuum, which is a completely airless space. Light travels as a wave, but it does not need any matter or material to carry its energy along. This is because light is self-propagating. The speed of light is constant and experimentally determined to be independent of the movement of the source or detector or the direction in which it travels. Light can also travel within some materials, like glass and water, but in these cases, some light is absorbed and lost as heat.

Light travels through a vacuum

Light is a transverse wave, and it contrasts with sound, which is a longitudinal wave. Sound requires a physical medium, such as air or water, to travel. If you are moving with respect to the air, the speed of sound will vary. Light, however, exhibits constant speed in all circumstances. This is because light does not need a medium to travel through.

The speed of light is experimentally determined to be constant and independent of the movement of the source or detector. This is why light can travel through a vacuum.

Radio waves, microwaves, visible light, and X-rays are all examples of electromagnetic waves that can travel through a vacuum.

Light travels through air

Light is a wave in the electromagnetic field, which is fundamentally a quantum field. It does not need a physical medium to travel through, unlike earthquake or sound waves. In the past, people thought that light travelled through a medium called the luminiferous aether, but a series of experiments in the late 19th century, along with the introduction of special relativity, disproved the existence of this aether.

Light travels fastest in a complete vacuum at 299,792,458 metres per second (approximately 300,000 kilometres per second). This is a universal constant, often denoted as "c" in equations. The speed of light slows down when it passes through an absorbing medium, such as water or glass. For example, the speed of light in air is about 90 kilometres per second slower than in a vacuum.

The finite speed of light has noticeable effects when communicating with distant objects. For instance, there is a brief delay in communications between Earth and spacecraft. The communications delay between Earth and Mars can vary between five and twenty minutes. Similarly, light from distant stars takes a long time to reach Earth, allowing humans to study the history of the universe.

Light travels through water

Light can travel through water, but its speed is reduced. Light travels fastest through a complete vacuum. When light travels from air into water, it slows down and changes direction slightly. This change of direction is called refraction.

Refraction is the bending of light as it passes from one transparent substance into another. When light enters a more dense substance, it 'bends' more towards what is called the normal line. All materials have what is known as an index of refraction, which is linked to how fast light can travel through the material.

When light hits water, some of it is reflected off the surface. The rest of the light passes through the water but it bends as it enters. This is why objects seen through water appear distorted and fuzzy.

Light travels through glass

Light can travel through certain mediums, such as glass. While light is often thought of as a particle, it is also a wave in the electromagnetic field. This means that light can pass through glass because there is space between the atoms that make up the glass.

When light passes through glass, it interacts with the electrons in the atoms of the glass. The electromagnetic wave of light causes the electron clouds in the atoms to vibrate, and as they vibrate, they regenerate the wave. This process takes time, which is why light slows down when passing through glass.

Different colours of light have different frequencies. Glass is made up of atoms arranged in a way that allows them to sustain the vibration of light waves at visible light frequencies, so light can pass through. However, if the atoms were arranged differently, the light would be absorbed instead.

In classical electrodynamics, light waves are thought to be absorbed by the atoms and/or electrons within the glass, which then emit new light waves. In this model, it appears as if light is travelling through the glass, but it is actually being absorbed and re-emitted with mostly the same properties.

In quantum electrodynamics, light is thought of as a particle (a photon) that can be absorbed and re-emitted by the atoms in the glass. In this model, the photon is briefly a polariton—a joint excitation of the electromagnetic field and the charge oscillations it produces in the glass.

Thus, light can travel through glass because glass is made up of atoms with space between them, and the properties of glass allow it to sustain the vibration of light waves at visible light frequencies.

Light travels through some exotic media, such as Bose-Einstein condensate

Light can travel through some exotic media, such as Bose-Einstein condensate (BEC). BEC is a state of matter typically formed when a gas of bosons is cooled to extremely low temperatures, close to absolute zero (-273.15 °C). At these temperatures, bosons occupy the lowest quantum state, and microscopic quantum-mechanical phenomena become apparent at a macroscopic scale.

The creation of BEC was first predicted in 1924-1925 by Albert Einstein, based on the work of Satyendra Nath Bose. However, it was only in 1995 that Eric Cornell and Carl Wieman successfully created a BEC using rubidium atoms. Wolfgang Ketterle also produced a BEC using sodium atoms later that year. For their achievements, Cornell, Wieman, and Ketterle shared the 2001 Nobel Prize in Physics.

BEC has some fascinating properties and applications. For example, light can be slowed down significantly when passing through a BEC. In one experiment, a light beam was slowed from 186,282 miles per second to just 38 miles per hour, a reduction of 20 million times! This property of BEC has potential applications in optical sources for entangled and correlated light states, which could be useful in quantum computing.

Another intriguing aspect of BEC is its ability to create exotic supersolid phases. In these phases, a rigid material can flow without resistance, something that has only been achieved in atomic gases with magnetic interactions. BEC made from polar molecules, which have stronger interactions, could lead to new insights and applications in this area.

BEC also has potential applications in quantum sensing, atomic clocks, and space exploration. Additionally, BEC made from photons, or "super-photons," could be used to create highly entangled many-body states with potential applications in quantum information and communication.

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