Unraveling Light's Journey: Understanding Its Travel Secrets

The speed of light is a universal constant, travelling at 299,792,458 metres per second (approximately 186,282 miles per second). This speed is so reliable that it is used to define standard measurements such as the metre, mile, foot and inch.

The first recorded measurements of the speed of light were taken by Danish astronomer Ole Rømer in 1676, using light from Jupiter's moon Io. However, it was not until the 20th century that scientists discovered that light travels as both a wave and a particle.

Light travels in a straight line and, unlike other waves, can pass through a vacuum. It has no mass but can be absorbed, reflected or refracted when it comes into contact with a medium.

Light travels in a straight line

To demonstrate that light travels in a straight line, we can perform a simple activity. Take three CDs and align them together in a straight line. Place a candle at the other end, ensuring that the candle's height is the same as that of the CDs. Now, if we displace the centre CD, we will not be able to see the candle flame because the light gets blocked. If light could travel in a curve, we would still be able to see the candle flame. However, since light travels in a straight line, we are unable to see the flame when the CD is moved. This proves that light travels in a straight line.

Another way to observe light travelling in a straight line is to notice the sharp edges of shadows. Shadows are formed when light is blocked by an object. The light rays that fall on the object are not reflected or refracted, and the space behind the object remains void of light. This void is the shadow. The fact that shadows have sharp edges indicates that light travels in a straight line.

Although light travels in a straight line, it can be bent under certain conditions. For example, light can be bent when it passes through a medium with a different refractive index, such as water or glass. Additionally, light can be affected by gravity, such as in the case of gravitational lensing.

Furthermore, while light generally travels in a straight line, it can also exhibit a phenomenon called diffraction, where it spreads out as it travels. Diffraction causes different parts of the light beam to bend away from the forward direction, resulting in an overall spread of the beam. However, the effect of diffraction is usually negligible in real-life situations, and light is often considered to travel in a straight line.

Light travels at different wavelengths

In a vacuum, light of all colours travels at the same speed, which is approximately 299,792 kilometres per second. However, when light enters a medium such as glass, its speed changes depending on the properties of the medium and the light.

The refractive index of a medium is a dimensionless number that describes how much light is slowed down when it passes through that medium. It is calculated as the ratio of the speed of light in a vacuum to the speed of light in that medium. Dispersion is a phenomenon where different colours of light experience slightly different refractive indices when they pass through a medium, causing each colour to travel at a different speed. Dispersion occurs because the refractive index is not constant for all wavelengths of light. Instead, it is a function of the wavelength.

Shorter wavelengths, like blue, are refracted more than longer wavelengths, like red. This difference in refraction causes the colours to spread out and form a spectrum. This phenomenon is not only fascinating but also useful. It helps us develop technologies such as spectrometers, which are tools used to analyse the composition of materials by examining the spectrum of light they emit or absorb.

Light is just one part of a vast spectrum of electromagnetic waves, including radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. Electromagnetic waves have both electric and magnetic components that oscillate perpendicular to each other and to the direction of the wave's travel. This oscillation allows them to propagate through space.

Visible light, which our eyes can detect, occupies a small segment within this spectrum and is further divided into the colours red through violet. Each of these colours corresponds to a specific range of wavelengths. Red light has the longest wavelength and lowest frequency among visible light, while violet light has the shortest wavelength and highest frequency.

Light can be slowed down

Light travels at different wavelengths and exhibits properties of both a particle and a wave. In a vacuum, light travels at a constant speed of 299,792,458 metres per second, denoted by the letter c. This speed is a fundamental constant of the universe, and nothing can travel faster.

However, when light travels through a medium other than a vacuum, it can be slowed down. This is because light scatters off the molecules that make up different materials. The photons are absorbed by electrons and then re-emitted. In some materials, such as water, light will slow down more than the electrons will. This means that an electron in water can travel faster than light in water, but the speed of light in a vacuum (c) is never exceeded.

In 2001, scientists at the Rowland Institute for Science in Cambridge and Harvard University managed to slow light to 38 miles per hour. They did this by shooting a laser through extremely cold sodium atoms, which worked like "optical molasses" to slow the light down.

While it is not possible to speed up or reduce the speed of light in a vacuum, scientists have been successful in manipulating the time it takes for light to travel through various mediums. For example, at extremely low temperatures, atoms become so densely packed that they behave like one super atom, acting in unison. This is known as a Bose-Einstein condensate, a distinct state of matter. In 1999, Lene Vestergaard Hau, a professor at Harvard University, aimed a laser beam through a cloud of nearly motionless sodium atoms. The light crawled at a speed of 38 miles per hour.

There are also practical applications for slowing down light, such as improved communications technology, switches, and night-vision devices.

Light is affected by gravity

In general relativity, gravity affects anything with energy. While light doesn't have rest-mass, it still has energy and is thus affected by gravity. Gravity can be thought of as a distortion in space-time, and in this context, it doesn't matter what the secondary object is; as long as it exists, gravity affects it.

According to general relativity, massive objects are influenced by gravity and do not move in a straight line but along a curve in a flat spacetime. Light also travels along these curves, despite having no mass. This path appears bent and leads to the phenomenon of gravitational lensing.

Another way to think about this is that light follows a "path of least resistance" along the curved space created by gravity. Light always travels in a straight line, but mass can bend that line.

The impact of gravity on light can be observed when light passes by massive objects in space, such as neutron stars and black holes.

Light can be stopped

In 1999, Lene Hau, a Harvard professor of physics, slowed light down 20 million-fold to 38 miles an hour. Hau and her team passed a beam of light through a small cloud of atoms cooled to temperatures a billion times colder than those in interstellar space. The atom cloud was suspended magnetically in a chamber pumped down to a vacuum 100 trillion times lower than the pressure of air.

In 2001, Hau and her team managed to stop light completely for one-thousandth of a second. They did this by using sodium atoms and two laser beams to make a new kind of medium that entangles light and slows it down. When a second laser beam, directed at right angles to the cloud of atoms, was cut off, the light was stopped. When the laser was turned on again, the light was released and continued on its way.

In 2013, researchers at the Technische Universität Darmstadt stopped light for about a minute. They used a glass-like crystal containing a low concentration of ions of the element praseodymium, and two laser beams. One laser beam changed the optical properties of the crystal, and the other was the beam to be stopped. When the first laser beam was switched off at the same moment that the second beam entered the crystal, the decelerated beam came to a stop.

Although light can be stopped, it is not possible to stop time. If time were to stop, light would also stop moving, and you would not be able to see anything.

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