What is the Speed of Light?
The speed at which the light wave propagates through different materials is known as the speed of light. So, the speed of light will vary depending on the medium it propagates. In particular, the speed of light in vacuum is about 299,792,458 metres per second. When comparing the speed of light in vacuum with the speed of light in air, the speed of light in air is 1.0003 times slower than the speed of light in a vacuum. Usually, the speed of light is considered constant in nature. The constant value of the speed of light is about 3*108 m/s. This article describes the speed of light formula, equation and facts of the speed of light, and all other details are given here.
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The above image shows the values of speed of light per second, speed of light in vacuum, speed of light in km
The common terms used to measure the lights include frequency, wavelength, phase and intensity.
Frequency: The number of waves that crosses a particular point in the unit time is known as frequency. The frequency of the light refers to the colour of the light. It ranges from 430 trillion hertz to 750 trillion hertz.
Wavelength: The distance between two successive crests/ troughs of a light wave is known as wavelength. Here if the wavelength of light increases, then the frequency of light get decreases.
Intensity: The rate of emission of light from the source per unit area is called the intensity of light. The intensity of light is usually measured in Watts.
Polarization: Usually, the light wave will vibrate in all directions perpendicular to the propagation of light. If the light waves get vibrate in certain directions, then the light is called polarized light.
Properties of Light
Light has seven basic properties. They are listed below.
Reflection of Light
Refraction of Light
Dispersion of Light
Diffraction of light
Interference of light
Polarization of light
Scattering of light
The speed of light is always considered constant in nature. But it may vary for the property of electromagnetic waves. The light can serve as a single velocity throughout the universe. They don’t have any upper bound to the propagation speed of signals and for the speed of all material particles.
Relativity Equation
According to Einstein’s relativity equation, the energy of the signal (E) is equal to the product of mass and square of the speed of light (c).
E = mc2
Here, the energy of an object in rest mass is considered as m. And the speed of the object is considered as v. so, Einstein’s relativity equation is given by γm0c2.
Where, =\[\gamma = \frac{1}{\sqrt{1-\frac{\nu ^{2}}{c^{2}}}}\] represents the Lorentz factor.
Here,
m0 represents the moving particle of rest mass.
v represents the velocity.
c represents the speed of the light in a vacuum (3*108 m/s).
In the above equation, if v is zero then γ is equal to one. So, Einstein’s relativity equation, E = mc2. If the Lorentz factor γ reaches infinity, then v approaches c and it takes an infinite amount of energy to accelerate an object with mass to the speed of light. In general, no signal or energy can travel faster than the speed of light. If any object or particle that travels faster than c relative to an inertial frame of reference, then the particle or object would experience backward time travel with respect to another frame. Such a violation of causality has never been recorded and it would lead to paradoxes such as the tachyonic antitelephone.
Velocity of Light
The velocity of the light value can be calculated with the frequency and wavelength of the electromagnetic waves.
Speed of light formula
c = 𝛌f.
Here,
c represents the speed of light, the exact value of speed of light is 299 792 458 m s-1
In approximate,
The speed of light per second is equal to 3.0 x 108 m/s. The speed of light in km is equal to 300,000 km/sec.
𝛌 represents the wavelength of the electromagnetic wave
f represents the frequency of the light wave, which passes through.
As the speed of light is constant. If the wavelength of the wave increases, then the frequency of the wave will decrease accordingly.
Propagation of Light
According to classic physics, the lights are the type of electromagnetic waves. The classical behaviour of the electromagnetic waves are described in Maxwell’s equation. Here, the speed of light c in a vacuum is related to the distributed capacitance and inductance of the vacuum. They can also be represented with the electric constant ε0 and the magnetic constant μ0.
c = \[\frac{1}{\sqrt{\varepsilon _{0}\mu _{0}}}\]
In modern quantum physics, the theory of quantum electrodynamics (QED) describes the electromagnetic field. According to quantum electrodynamics, as photons are massless particles, they can travel at the speed of light in a vacuum and follow special relativity.
Speed of Light In Medium
Usually, the light cannot travel at the speed c in a medium. Here, the different types of light waves will travel at different speeds. The speed of the individual wave in a plane is considered as phase velocity vp. The largest part of the pulse travels at the group velocity Vg. The front velocity of the wave is represented as Vf.
Here, the phase velocity of the light wave is important to determine the direction the wave travels from one material to another. The phase velocity is also represented in terms of a refractive index. The refractive index of a material is defined as the ratio of light c and the phase velocity vp. The refraction index of a material also depends on the frequency, polarization, intensity or direction of propagation of the light. The refractive index of air is 1.0003 and the index of refraction denser media like water, glass and diamond are around 1.3, 1.5 and 2.4, respectively. The effective speed of light in exotic materials like Bose-Einstein condensates are only a few metres per second. Because of the occurrence of the absorption and re-radiation delay of light between atoms.
The refractive index of the transparent materials is always greater than 1, whose phase velocity is less than the speed of light c. In some cases, the refraction index will become smaller than 1 for some frequencies. The refraction index may also become negative for some exotic materials
This article described the equation and facts of speed of light. Also gave complete information about the relativity equation, propagation of light with formula and velocity of light value in detail.
FAQs on Speed of Light
1. What is the fundamental definition of the speed of light in Physics?
The speed of light, denoted by the symbol 'c', is the speed at which all massless particles and associated fields, including electromagnetic radiation such as light, travel in a vacuum. It is a fundamental physical constant and represents the maximum speed at which energy, matter, and information can travel through spacetime.
2. What is the value of the speed of light in different units like m/s, km/s, and km/h?
The speed of light has a precise, defined value in a vacuum. Here are its values in commonly used units:
In metres per second (m/s): 299,792,458 m/s
In kilometres per second (km/s): Approximately 299,792.458 km/s
In kilometres per hour (km/h): Approximately 1,079,252,848.8 km/h (over one billion km/h)
3. Why is the speed of light often approximated as 3 x 10⁸ m/s in calculations?
The value 3 x 10⁸ m/s (or 300,000,000 m/s) is used as an approximation because it is very close to the exact value of 299,792,458 m/s and simplifies calculations significantly. For most school-level and many university-level physics problems, the difference between the exact and approximated value is negligible and does not impact the outcome of the calculation in a meaningful way.
4. Does the speed of light change when it travels through different mediums like air or water?
Yes, the speed of light decreases when it passes from a vacuum into a transparent medium. The constant 'c' refers only to its speed in a vacuum. In a medium like air, water, or glass, light travels slower. This change is determined by the refractive index (n) of the medium, according to the formula v = c/n, where 'v' is the speed of light in the medium. For example, light travels only about 0.03% slower in air, but roughly 25% slower in water.
5. What is the formula that connects the speed of light with its frequency and wavelength?
The relationship between the speed of light (c), its frequency (f or ν), and its wavelength (λ) is given by the fundamental wave equation: c = fλ. This formula shows that for electromagnetic waves, frequency and wavelength are inversely proportional. If the frequency of the wave increases, its wavelength must decrease to maintain the constant speed 'c' in a vacuum.
6. Why can't objects with mass travel at the speed of light?
According to Einstein's theory of special relativity, as an object with mass accelerates and approaches the speed of light, its relativistic mass increases. To reach the exact speed of light, the object's mass would become infinite. Accelerating an infinite mass would require an infinite amount of energy, which is physically impossible. Therefore, only massless particles, such as photons (the particles of light), can travel at speed 'c'.
7. Why is the exact value of the speed of light 299,792,458 m/s and not some other number?
This value is exact by definition, not by measurement. Since 1983, the international scientific community has defined the metre based on the speed of light. One metre is officially defined as the distance light travels in a vacuum in 1/299,792,458 of a second. This decision made the speed of light a fixed, universal constant, which in turn provides a stable and precise standard for the unit of length.
8. How is the concept of the speed of light used to define a 'light-year'?
A light-year is a unit of astronomical distance, not time. It represents the total distance that a beam of light can travel in a vacuum in one Julian year (365.25 days). Since the speed of light 'c' is the fastest possible speed, a light-year provides a convenient way to measure the vast distances between stars and galaxies. For example, the nearest star system, Alpha Centauri, is about 4.37 light-years away from Earth.
