What is defined as the distance from one crest to the next in radiation?

The correct answer is the wavelength, as it specifically refers to the distance between successive crests (or peaks) of a wave in the context of radiation. Wavelength is a fundamental property of waves, including electromagnetic radiation, and is typically measured in meters. It plays a crucial role in determining the characteristics of the wave, including its energy and frequency.

Frequency, on the other hand, refers to how many crests pass a given point in a certain amount of time, and is inversely related to wavelength. Amplitude represents the height of the wave crest and relates to the intensity of the wave, not the distance between crests. Energy level corresponds to the amount of energy carried by the radiation, which is also related to frequency and wavelength but does not define the spacing between crests. Understanding these distinctions is essential in the study of wave behavior in various fields such as physics and medical imaging.