S2 2.1 The Electromagnetic Spectrum

​The first person in the world to figure out that light was an electromagnetic wave was James Clerk Maxwell.

Electromagnetic waves are transverse waves caused by changing electric and magnetic fields.

Since these waves consist of fields in space and not an interaction of matter, they don't require a medium to propagate and can travel through the vacuum of space.

​The full range of frequencies of electromagnetic waves is called the electromagnetic spectrum.

The electromagnetic spectrum contains the spectrum of visible light and several forms of radiation that aren't visible.

The visible spectrum consists of different frequencies of light corresponding to different colors.

​Electrons in an atom emit light when they move from a higher energy level to a lower energy level.

The frequency of the emitted light is directly proportional to the energy given off by the electrons.

The higher the energy given off by the electron transition, the higher the frequency of the emitted light.

This energy is expressed by the equation E = hf, where h represents Planck's constant, which is 6.626×10^-34 Joule seconds (Js).

​A blackbody absorbs all the radiation incident upon it, heats up, and then re-radiates energy.

The spectrum of blackbody radiation depends only on the body's temperature, and not on the type of radiation incident upon it.

When blackbodies emit radiation, they glow to a certain extent, such as burning coals, incandescent light bulbs, and the sun.

Fluorescence is the emission of light caused by the excitation of atoms in a substance.

In fluorescence, a substance absorbs light at a particular wavelength and nearly immediately emits light of longer wavelength.

The emission spectrum produces discrete frequencies for specific glowing gases, such as those from a mercury vapor lamp.

LEDs are electronic devices that give off light in specific bands of energy, which is a very efficient way to generate light.

In a LED, the recombination of electrons and electron holes in a semiconductor produces light in a process called electroluminescence

The wavelength of the light depends on the energy band gap of the semiconductors used.

When white light passes through a prism, it is split apart into its separate wavelengths.

Each wavelength is seen as a slightly different color, and each has a specific frequency.

From lowest frequency to highest, the colors are red, orange, yellow, green, blue, indigo, violet (or Roy G Biv).

The human eye can see electromagnetic wavelengths from about 380 nm (violet) to 750 nm (red).

Infrared waves blend with the red light on one end of the visible light spectrum, and have wavelengths of roughly 0.750 to 300 μm.

Infrared waves with wavelengths closer to the red light are called near infrared, while hose with wavelengths closer to microwaves are called far infrared.

Night–vision glasses can detect human presence in the dark, because you can see infrared light emitted by skin and by other warm objects.

Ultraviolet (or UV) radiation has wavelengths roughly from 10 to 400 nm.

The sun emits UV waves in three recognized energy ranges: UVA, UVB, and UVC.

UVA have the longest wavelengths, UVB have the next longest, and UVC have the shortest and highest energy wavelengths.

The atmosphere blocks most of the sun's UV radiation, including all UVC.

About 95 percent of the UV radiation that reaches Earth is UVA.