Days when the Sun’s light hits both hemispheres equally are called <u>equinoxes</u>.
These are the moments when the Sun is exactly above the equator. These days are called equinoxes because the night and day are nearly the same lengths – 12 hours – all over the world ("equinox," comes from Latin, meaning "equal night).
There is no fundamental difference between visible light and invisible light such as radio waves and X rays. They are all electromagnetic waves that differ in only one way: their wavelength. ... Ultraviolet light, X rays, and gamma rays all have shorter wavelengths than visible light.