Visible light is the portion of the electromagnetic spectrum that is visible to the human eye. It is a very small slice of the entire electromagnetic spectrum, which ranges from radio waves with long wavelengths to gamma rays with very short wavelengths. Visible light lies between the infrared (longer wavelengths) and ultraviolet (shorter wavelengths).
Properties of Visible Light
Visible light has wavelengths between about 380 nanometers (violet) to about 740 nanometers (red). Within this range, there are all the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet. The color of visible light depends on its wavelength.
|Color||Wavelength range (nm)|
Visible light travels in a straight line and at very high speeds. In a vacuum, visible light travels at a speed of about 186,000 miles per second or 300,000 kilometers per second. This speed is commonly known as the speed of light and denoted by the letter c.
What is Light Made of?
Visible light, along with other types of electromagnetic radiation, is made up of particles called photons. Photons are massless particles that have discrete amounts of energy. The energy of a photon depends on its frequency/wavelength and is given by the formula:
E = hf
Where E is the energy of the photon, h is Planck’s constant, and f is the frequency of the photon.
Photons oscillate up and down perpendicular to their direction of motion. The frequency/wavelength of this oscillation determines the color of the photon. Higher frequency photons have higher energy and correspond to violet/blue light. Lower frequency photons have lower energy and correspond to red light.
Remarkably, in addition to having particle properties, light also acts as a wave. So light exhibits properties of both particles and waves. This strange behavior is known as wave-particle duality and is a fundamental concept of quantum mechanics.
As a wave, light can be described by a wavelength, frequency, and amplitude. It can undergo phenomena such as interference, diffraction, refraction, and polarization. Maxwell’s equations describe light as waves of oscillating electric and magnetic fields.
So in summary, visible light can be described both as discrete particles (photons) and as a continuous electromagnetic wave. This wave-particle duality of light is deeply rooted in the quantum mechanical nature of the world we live in.
Where Do Visible Light Photons Come From?
Visible light photons originate from oscillations of electric charges. At the atomic level, photons are emitted when electrons transition between different energy levels around the nucleus. For example, when an electron falls from a higher to lower energy orbital, the excess energy is emitted as a photon.
On larger scales, photons are emitted from things being heated up. For example, heating an object makes its atoms and molecules vibrate faster, causing oscillations of charge that emit electromagnetic radiation across the spectrum, including visible light. This is why heating something makes it glow.
Our primary source of visible light is the Sun. In the core of the Sun, nuclear fusion reactions emit photons in the ultraviolet and x-ray part of the spectrum. As these photons travel outward towards the cooler surface, they lose energy and get “downgraded” to visible light photons which can then radiate from the photosphere into space.
Interactions of Visible Light with Matter
When visible light encounters matter, several types of interactions are possible. These include:
- Reflection – Light bounces off a surface. The color we see is the light reflected.
- Refraction – Light bends as it travels from one medium to another, like from air to water. This can make objects seem distorted.
- Absorption – Light may be absorbed by atoms and molecules, converting its energy to internal energy. Different materials absorb different colors.
- Transmission – Light passes through a material without being absorbed. Clear materials like glass transmit visible light.
- Scattering – Light gets diverted in many directions by particles in a material. The blue sky is caused by scattering of sunlight off nitrogen and oxygen molecules.
These interactions of visible light with matter enable many useful technologies and give rise to the colors, sights, and images we see in the world around us.
The Visible Light Spectrum
The visible light spectrum can be represented on a graph with wavelength or frequency on the x-axis and intensity or amplitude on the y-axis. The spectrum forms a continuous band with violet/blue at the short wavelength end and red at the long wavelength end.
Within this band, each color has its own narrow wavelength range. The ranges blend together with no clear boundaries between adjacent colors. Here is a diagram of the visible light spectrum:
Our eyes contain photoreceptors called cone cells that are sensitive to red, green and blue light. By interpreting the signals from these three types of cones, our visual system is able to perceive the full range of colors in the visible spectrum.
Applications of Visible Light
Visible light has many important practical applications, including:
- Illumination – Artificial lighting using incandescent bulbs, LEDs, etc. enable us to see indoors and at night.
- Vision – Our eyes evolved to detect visible light, allowing us to see the world around us.
- Displays – Screens on phones, TVs, computers etc. emit visible light to display images and information.
- Optical fibers – Used to transmit information via pulses of light over long distances.
- Photography – Visible light is captured and recorded to make photographs and movies.
- Lasers – Coherent, monochromatic visible light beams enable laser applications.
- Spectroscopy – Studying how matter interacts with visible light reveals chemical composition.
So in summary, visible light is an incredibly useful and versatile portion of the electromagnetic spectrum, with applications across science, technology and everyday life.
In conclusion, visible light consists of tiny particles called photons that have specific amounts of energy corresponding to the wavelengths of light we see. It exhibits properties of both particles and waves. Visible light originates from oscillations of electric charge, primarily within atoms and molecules. Our main source of visible light is the Sun. We perceive the colors of the world around us based on the interactions of visible light with matter. This narrow band of the electromagnetic spectrum plays an invaluable role enabling human vision, lighting, displays, communications, photography and many other applications that are central to modern society.