Ultraviolet light belongs to invisible light, and the essence of light is electromagnetic waves. Electromagnetic waves also have wave-particle duality,
that is, energy is analyzed from the perspective of particles (called photons), and its propagation is analyzed from the perspective of waves
(it can be simply imagined as each particle propagates in space according to a certain regular trajectory).
The shorter the wavelength of the electromagnetic wave, the greater the energy of the photon, the greater its permeability, and the greater its harmfulness.
We are familiar with these characteristics as X-rays, which can penetrate the human body and be medically filmed. At the same time,
everyone knows that it is harmful to the human body. The longer the wavelength of the electromagnetic wave, the lower the photon energy,
and the stronger its penetration (referring to the ability to bypass objects, which is different from the permeability).
The most direct example is that visible light is less harmful than ultraviolet light. In visible light, the longer-wavelength
such as red, orange, yellow, and green lights are used as warning lights, and microwaves and radio waves, which are widely used in wireless communications,
have the longest wavelength in the entire electromagnetic spectrum.
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