In the context of light, adding or subtracting wavelengths refers to the phenomenon of shifting the color or frequency of light. There are two primary ways this can occur: through the processes of absorption and scattering.
Absorption: When light passes through a medium, such as a transparent material or a gas, it can interact with the atoms or molecules within that medium. These atoms or molecules have specific energy levels, and they can absorb certain wavelengths of light while allowing others to pass through. When a particular wavelength of light is absorbed, it is effectively subtracted from the original spectrum of light. The remaining transmitted light will have a modified spectrum with fewer or different wavelengths.
Scattering: Scattering refers to the process of light interacting with particles or structures in its path. Depending on the size and nature of the particles, scattering can occur in different ways. Rayleigh scattering, for example, is responsible for the blue color of the sky during the day. In this process, shorter wavelengths (blue and violet) are scattered more strongly than longer wavelengths (red and yellow). As a result, the scattered light appears bluish. By altering the scattering properties, such as changing the size of particles or the structure through which light passes, the wavelengths that are scattered or absorbed can be modified, thus adding or subtracting specific wavelengths.
It's important to note that these processes occur naturally and are not typically controlled by humans in everyday situations. However, scientists and engineers can manipulate the wavelengths of light in controlled environments using techniques such as filters, prisms, diffraction gratings, and other optical devices to selectively transmit or block specific wavelengths.