Light Energy: Category, Properties And Important Uses

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What is light energy?

Light energy definition:

Light is the only energy form that is visible to the human eye. Light energy can be defined in two ways:

Light is composed of massless energy packets known as photons. Photons are energy packets that carry a fixed amount of light energy depending on the wavelength.

Light energy refers to the range of electromagnetic energy that consists of gamma rays, x-rays, visible lights, etc.
The visible range of the electromagnetic spectrum is generally known as light.

The nature of light :

In the 17th century there were two ideas regarding the nature of light.

Particle Nature of Light

Isaac Newton believed that light was made of tiny discrete particles called corpuscles. According to him, these tiny particles were emitted by hot objects such as the sun or fire and travelled in a straight line with a finite velocity and possessed impetus. This came to be known as Newton’s Corpuscular theory of light.

Wave Nature of light

Christiaan Huygens claimed to disprove Newton’s Corpuscular theory by proposing the Wave theory of light. According to him light was made up of waves vibrating up and down perpendicular to its direction of propagation. This came to be known as ‘Huygens’ Principle’

In the early 19th century, an English physicist Thomas Young conducted an experiment that showed light from a point source after passing through two slits form an interference pattern on a screen placed at an appropriate distance. This came to be known as Young’s double-slit experiment, which advocated the wave nature of light supporting the Huygens’ Principle.

James Clerk Maxwell laid the foundation of modern electromagnetism that described light as a transverse wave composed of oscillating magnetic and electric fields at 90° to each other. The formulation of light as transverse waves contradicted Huygens, who believed light wave to be longitudinal.

Albert Einstein revived the particle theory by bringing the concept of photons. Einstein’s experiment, famously known as the photoelectric effect showed that light comprises discrete bundles or quanta of light energy, called photons

The phenomenon of interference and diffraction could only be explained by considering light to be a wave. In comparison, the explanation of the photoelectric effect was possible only by light’s particle nature.
This huge dilemma regarding light’s nature was solved with the foundation of quantum mechanics that established wave-particle duality on the nature of both light and matter 

Properties of light:

Interactions of light:

Light waves interact with matter in different ways:

Reflection of Light

– When a light wave bounces off the surface of a material into its previous medium of propagation, the process is termed as reflection. For example, the image formed on a calm pond/lake.

reflection by lake
Reflection
Image Source: Basile Morin, Water reflection of the mountains of Vang Vieng with crepuscular rays, CC BY-SA 4.0

Absorption of light

When a material absorbs the energy of a light wave that falls on it, the process is termed as absorption. For example, glow-in-the-dark plastics, that absorbs light and re-emits in the form of phosphorescence.

Transmission

When a light wave travels/passes through a material, the process is termed as transmission. For example, light passing through a glass windowpane.

Interference

Interference refers to the phenomenon in which two light waves superpose to produce a resultant wave that can have lower, higher, or the same amplitude. Constructive and destructive interference occurs when the interacting waves are coherent with each other, either because they share the same source or because they have the same or comparable frequency.

interference of waves
Interference of waves
Image source: Dr. Schorsch 12:32, 19 Apr 2005 (UTC) (Dr. Schorsch, Interferenz, CC BY-SA 3.0

Refraction

Refraction is an important behavior demonstrated by light waves. Refraction takes place when light waves deflect from their original path as they enter a new medium. Light exhibits different speeds in different transmitting materials. The change in speed and degree of deviation depends on the angle of incoming light.

Diffraction

Diffraction is defined as the bending of light waves around the corners of an aperture into its geometrical shadow region. The diffracting obstacle or aperture becomes a secondary source of the propagating light wave. One of the most common examples of diffraction is the formation of rainbow patterns on a CD or DVD. The closely spaced tracks on a DVD or CD serve as diffraction gratings, forming patterns when light falls on it.

diffraction of light
Diffraction of light
image source: Lazord00d, Argon laser beam and diffraction mirror, CC BY-SA 3.0

Dispersion

Dispersion of light refers to the phenomenon of splitting of white light into its constituent spectrum of colors (.i.e. VIBGYOR) when passed through a glass prism or similar objects. For example, the formation of rainbow due to diffraction of sunlight by prism-like rain drops.

Types of light

  • Light as a whole refers to electromagnetic radiation of every wavelength.
  • Electromagnetic radiation can be classified in terms of wavelengths as
  • Radio wave ~ [105 – 10-1 m]
  • Microwave ~ [10-1 – 10-3 m]
  • Infrared wave ~ [10-3 – 0.7 x 10-6m]
  • The visible region (we perceive as light) ~ [0.7 x 10-6 – 0.4 x 10-6 m]
  • Ultraviolet waves ~ [0.4 x 10-6 – 10-8 m]
  • X-rays ~ [10-8 – 10-11 m]
  • Gamma rays ~ [10-11 – 10-13 m]
  • The functioning of electromagnetic radiations is based on its wavelength.

Frequency and Wavelength of Light

Wavelength Scale

frea 1
Image source: Inductiveload, NASA, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons

Frequency of Light

Radio Waves :

Radio wave is an electromagnetic wave having a frequency between 20 kHz to around 300 GHz and are known for their use in communication technologies, such as mobile phones, television, and radio. These devices accept radio waves and transform them into mechanical vibrations to produce sound waves.

Microwave :

Microwave is electromagnetic radiation having a frequency between 300 MHz and 300 GHz. Microwaves have a variety of applications, including radar, communication, and cooking.

Infrared Waves :

Infrared wave is electromagnetic radiation having a frequency between 300 GHz and 400 THz.
Infrared waves find its application in heating food and television remotes, fiber optic cables, thermal imaging cameras, etc.

Visible light :

Visible light is electromagnetic radiation having a frequency between 4 × 1014 to 8 × 1014 hertz (Hz). The reason behind the human eye seeing only a specific range of frequencies of light is that those certain frequencies stimulate the retina in the human eye.

Ultraviolet rays :

Ultraviolet light is electromagnetic radiation having a frequency between 8 × 1014 and 3 × 1016 hertz (Hz). Ultraviolet radiation is used for nullifying microbes, sterilizing medical equipment, treating skin issues, etc.

X-Rays :

X-rays are electromagnetic radiations having frequencies between 3×1019 and 3×1016 Hz. X-rays are used to nullify cancer cells, in X-Ray machines, etc.

Gamma Rays:

Gamma-rays are electromagnetic radiations having frequencies more than 1019 hertz (Hz). Gamma rays are used to nullify microbes, sterilize medical equipment, and food.

Examples of light energy

Light Sources

Light sources can be classified into two basic types: Luminescence and Incandescence.

Incandescence:

Incandescence encompasses the vibration of all the atoms present. When atoms are heated to a very high optimum temperature, the resultant thermal vibrations are released as electromagnetic radiations. Incandescent light or “black body radiation” is created when light arises from a heated solid. Based on the temperature of the material, the photons released differ in their colours and energies. At low temperatures, the materials produce infrared radiations.

In black body radiation, with an increase in temperature the peak gets shifted towards shorter wavelengths, as it moves towards the ultraviolet range of the spectrum, it generates a red then white, and lastly a bluish-white color.
Incandescent light is the most commonly used light. It consists of the sun, light bulbs, and fire.
Fires embroil chemical reactions that release heat, causing materials to touch high temperatures and eventually leads the gases and materials to incandescence. On the other hand, light bulbs produce heat due to the passage of electric current through a cable. Incandescent light bulbs emit around 90% of their energy as infrared radiations and the rest as visible light.

Luminescence

Luminescence involves only electrons and generally takes place at lower temperatures, compared to incandescent light.
Luminescent light is formed when an electron emits a part of its energy as electromagnetic radiation. When an electron jumps down to a lower energy level, a certain amount of light energy is released in the form of lights of a specific colour. Generally, to maintain continuous luminescence, the electrons need a constant push to reach higher energy levels so that the process continues.
For example, Neon lights produce light through electroluminescence, which involves a high voltage {push}, which excites the gas particles and eventually results in light emission.

How does light Travel?

Light practically travels as a wave. Although according to geometrical optics, light is modeled to travel in rays. The transmission of light from a source to a point can happen in three ways:

  • It can travel directly through a vacuum or an empty space. For example, light traveling from the Sun to Earth.
  • It can travel through various mediums, like air, glass, etc.
  • It can travel after being reflected, such as by a mirror or a still lake.

Light Energy vs Electron Energy

Electron energyLight Energy
• Electrons have rest mass energy, i.e., the energy corresponding to its mass when at rest. The rest energy of an electron can be calculated by using Einstein’s equation E=MC2.

• When the electron changes its energy levels by moving from a higher energy state to a lower energy state, it emits photons.

• Light energy is in the form of tiny massless energy packets called photons. The amount of energy in a photon depends upon the wavelength of light. E = hc/λ

• When photons with an adequate amount of light energy fall on a material, electrons absorb the energy and escape the material.

Uses of Light Energy.

Light has its applications in every aspect of life. Without light energy, it would have been impossible for us to survive.
Here are some essential applications of light energy in our life:

  • Light permits vision. A specific range of wavelengths of light provides the perfect amount of energy required to stimulate the chemical reactions in our retina to support vision.
  • Light energy allows plants to produce food through the process of photosynthesis.
  • Light energy is used as a source of power in satellite and space technologies.
  • Solar energy is used for various domestic and industrial activities.
  • Light energy (electromagnetic radiation) is used in the telecommunication industry.
  • Light energy is also used for multiple medical treatments.

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