Is Light Pure Energy?

What is Light?

Light is a form of electromagnetic radiation that is visible to the human eye (source: https://byjus.com/question-answer/what-is-the-definition-of-light/). It enables us to see objects around us by reflecting off surfaces and entering our eyes. Light has properties of both waves and particles, exhibiting phenomena like diffraction and interference which are wave-like, as well as behaving as discrete energy packets called photons.

Light is characterized by its wavelength (or color), frequency, and speed. Wavelength determines the color of visible light, with violet light having the shortest wavelength and red light having the longest. Frequency refers to the number of wave cycles per second. The speed of light is a fundamental constant, measured at 299,792 km/s in a vacuum (source: https://www.meritnation.com/ask-answer/question/what-is-the-actual-definition-of-light/physics/471177).

Wave-Particle Duality of Light

According to quantum mechanics, light exhibits properties of both waves and particles. This wave-particle duality of light was first proposed by Louis de Broglie in 1924. It was strongly supported by Einstein’s work on the photoelectric effect, which demonstrated that light carries energy in discrete quantized packets called photons. However, light also demonstrates wave properties like diffraction and interference, as seen in the double slit experiment.

The double slit experiment beautifully demonstrates the wave-particle duality. When light passes through two parallel slits, a wave interference pattern is observed on the screen behind the slits. This interference pattern can only be explained if light behaves as a wave. However, when light intensity is reduced such that individual photons pass through one at a time, a particle impact pattern slowly emerges on the screen. This can only be explained if light travels as discrete particles or photons. Thus, the same experiment demonstrates both the wave and particle nature of light. As quoted by Einstein, “It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do.”

Sources:

https://bigthink.com/13-8/quantum-nature-of-light/


https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_(Averill)/06%3A_The_Structure_of_Atoms/6.04%3A_Wave_-_Particle_Duality

Light as Electromagnetic Radiation

Light is a form of electromagnetic radiation that is visible to the human eye. Electromagnetic radiation consists of oscillating electric and magnetic fields that propagate through space carrying energy. The electromagnetic spectrum encompasses all wavelengths of electromagnetic radiation, ranging from radio waves to gamma rays. Visible light occupies a small segment of the electromagnetic spectrum between infrared light and ultraviolet light, with wavelengths from approximately 400 to 700 nanometers (Britannica, n.d.).

The wavelength of light is related to its frequency and energy. Wavelength refers to the distance between repeating peaks of an electromagnetic wave. Frequency refers to the number of wave oscillations that pass a point per second. As the wavelength decreases, the frequency increases. Shorter wavelengths correspond to higher frequencies and higher photon energies. Longer wavelengths equate to lower frequencies and lower photon energies. For instance, gamma rays have very short wavelengths and extremely high frequencies and energies, while radio waves have very long wavelengths, low frequencies, and low energies (Europa.eu, n.d.).

In summary, visible light that humans see is part of the electromagnetic spectrum. It has specific wavelengths, frequencies, and photon energies associated with it. Understanding light as electromagnetic radiation is key to grasping many of its wave-like and particle-like properties.

Photons

Light is composed of discrete packets of energy called photons. Photons are massless light quanta that exhibit properties of both waves and particles (Britannica, 2023). Photons carry electromagnetic energy and momentum, with the energy being directly proportional to the photon’s frequency. The higher the frequency, the more energy the photon carries (Livescience, 2022).

Photons are emitted when electrons transition between energy levels in atoms and molecules. For example, when an electron falls from a higher to lower energy level, the excess energy is emitted as a photon. The color or wavelength of the emitted photon corresponds to its energy. Photons also mediate electromagnetic interactions between charged particles (Universetoday, 2010).

Overall, photons are the fundamental “particles” of light that carry discrete amounts of electromagnetic energy. Their dual wave-particle nature is a key part of quantum mechanics and our modern understanding of light.

Pure Energy?

Light is commonly referred to as “pure energy”, but this is not entirely accurate from a physics perspective. While light is massless, it does carry momentum and energy according to Einstein’s famous equation E=mc^2. Light exhibits properties of both waves and particles, as postulated by the wave-particle duality theory. The discrete quanta of light energy are called photons. Photons have zero mass but carry momentum p=h/λ, where h is Planck’s constant and λ is the photon’s wavelength. So while photons themselves have no mass, they still carry momentum and kinetic energy. This momentum allows light to exert pressure and transfer energy when interacting with matter. So in summary, light cannot be considered “pure energy” since it has measurable momentum and wave-like properties. However, the fact that light is massless but still carries energy is what leads to the common description of it being pure energy.

Speed of Light

One of the most well-known facts about light is that it travels at a constant speed through a vacuum, commonly denoted as c. The speed of light in a vacuum is exactly 299,792,458 meters per second (CODATA Value: speed of light in vacuum). This speed is considered a universal physical constant and does not change based on the motion of the source or observer (Speed of light).

The constancy of the speed of light was established in experiments by Albert A. Michelson and Edward Morley in 1887. No matter how fast or in what direction the light source or observer is moving, the speed of light in a vacuum stays the same. This challenges earlier intuitive ideas about light, and led to key developments in physics like Einstein’s theory of relativity.

Visible Light Spectrum

Visible light is the portion of the electromagnetic spectrum that is visible to the human eye. The wavelength range for visible light is approximately 380 to 750 nanometers (nm) (Visible Light Spectrum Wavelengths and Colors, https://sciencenotes.org/visible-light-spectrum-wavelengths-and-colors/). Other parts of the electromagnetic spectrum that we cannot see include gamma rays, x-rays, ultraviolet radiation, infrared radiation, microwaves, and radio waves. Their wavelengths are outside the visible range.

The visible spectrum consists of a range of wavelengths corresponding to different colors – red light has the longest wavelength while violet has the shortest. The wavelength of light determines its color. Red light has wavelengths between 620-750 nm, orange light between 590-620 nm, yellow between 570-590 nm, green between 495-570 nm, blue between 450-495 nm, indigo between 440-450 nm, and violet between 380-440 nm (The Visible Light Spectrum – Wavelengths of Colors, https://multiverse.ssl.berkeley.edu/Portals/0/Documents/FiveStarsCurriculumDocs/WavelengthsOfColors.pdf).

Light Interactions

Light interacts with matter in a number of ways including reflection, refraction, diffraction, and absorption. Reflection occurs when light bounces off a surface. The angle of incidence equals the angle of reflection according to the law of reflection. Refraction happens when light passes from one medium to another, causing it to change speed and bend due to the difference in refractive indices. Some examples of refraction include looking through water or glass. Diffraction refers to light bending around an obstacle or opening. It spreads out light waves and produces effects like rainbows. Finally, absorption takes place when light is taken in by matter. The light’s energy is converted to other forms like heat. Different materials absorb various wavelengths of light.

According to the book Chapter 5 Light and Matter: Reading Messages from the …, interactions between light and matter allow us to see objects, produce images, and more. Light striking matter is absorbed, reflected, scattered, emitted, or transmitted, producing the effects we observe.

Light in Quantum Mechanics

Quantum mechanics helped explain some perplexing observations about light through the early 20th century. In 1905, Albert Einstein resolved the photoelectric effect by proposing that light interacts as discrete packets or quanta of energy, later called photons. The photon model accounted for light’s particle-like behavior.

Yet light also demonstrated wavelike interference and diffraction patterns. So light exhibited both particle and wave properties, a seeming paradox known as wave-particle duality. In 1924, Louis de Broglie hypothesized that electrons also have an associated wavelength. Their wave-particle duality was soon confirmed. Thus quantum mechanics ascribes both types of properties to all matter and energy.

The photoelectric effect lent support to the photon model of light. Einstein explained that photons striking a metal surface provide energy to eject electrons. Below a certain frequency threshold, photons lack enough individual energy to dislodge electrons, no matter their brightness or number. Thus light’s energy depends on its frequency, not just its amplitude as with classical waves.

Other quintessential quantum effects seen with light include entanglement, superposition, and quantum tunneling. Quantum optics continues yielding new discoveries about the fundamentals of light.

Applications of Light

Light has many practical applications in fields like communication, imaging, spectroscopy, and medicine. For communication, fiber optic cables use light pulses to transmit data over long distances. In imaging, technologies like cameras, microscopes, and telescopes all rely on the collection and focusing of light. Spectroscopy analyzes the interaction between matter and radiated energy to determine chemical composition – it is widely used in astronomy to study distant stars and planets.

In medicine, lasers allow for non-invasive surgeries that vaporize tissues with high precision. Phototherapy uses light to treat jaundice in newborns and skin diseases like psoriasis. Light is also used in medical imaging techniques like endoscopes which use fiber optics to look inside the body. X-rays and CT scans involve the detection of transmitted radiation through the body. Pulse oximeters measure oxygen saturation by shining light through the skin. Overall, light is invaluable for observing and interacting with our world in diverse ways.

Sources:
https://photonterrace.net/en/life/
https://byjusexamprep.com/upsc-exam/what-are-the-applications-of-light
https://shiken.ai/physics/applications-of-waves