The speed of light in a vacuum is a fundamental constant of nature, but when light travels through different mediums, its speed changes due to interactions with the particles of the medium. The speed can be measured using various techniques. One common method involves measuring the time light takes to travel a known distance. In mediums other than a vacuum, the speed of light is affected by the medium's refractive index—a measure of how much the light slows down compared to its speed in a vacuum. It’s calculated by dividing the speed of light in a vacuum by its speed in the given medium.

Electromagnetic waves are produced naturally and can also be generated artificially. Natural sources include the sun and other astronomical bodies, emitting a range of electromagnetic radiation. Artificially, electromagnetic waves can be produced through various methods, depending on the desired type. Radio waves, for instance, are typically generated by accelerating charges in antennas, while microwaves can be produced by electronic circuits such as Gunn diodes. Infrared waves are emitted by heated objects, visible light by luminous sources, and X-rays can be artificially generated by striking a target material with high-energy electrons.

The ability of electromagnetic waves to pass through or be absorbed by materials depends on their frequency and wavelength and the nature of the material. Radio waves, with their long wavelengths, can pass through many obstacles because their energy levels are not sufficient to excite the electrons within the materials, allowing them to penetrate deeper. In contrast, visible light has shorter wavelengths and higher energy levels; it can be absorbed or reflected by objects, leading to the objects being seen. The interaction between electromagnetic waves and materials is central to various applications, including communication and imaging technologies.

Indeed, health risks can be associated with exposure to certain types of electromagnetic waves. For example, exposure to ultraviolet (UV) light can lead to skin and eye damage, including sunburn, cataracts, and increased risk of skin cancer due to the ionising nature of UV radiation, which can damage cellular DNA. Similarly, excessive exposure to X-rays or gamma rays can increase cancer risks. It is essential to manage and mitigate exposure to these higher-energy electromagnetic waves to reduce potential health impacts, utilising protective equipment and adhering to exposure guidelines.

Electromagnetic waves are unique in that they can transport energy without requiring a medium, thanks to their composition of oscillating electric and magnetic fields. These fields are orthogonal to each other and generate each other as they propagate. The electric field varies in time, inducing a changing magnetic field, and vice versa. This self-perpetuating system allows electromagnetic waves to carry energy even in the absence of a medium. Energy is stored in these oscillating fields, enabling the wave to transfer energy from one location to another across a vacuum.