The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. The optical spectrum is a composite, or mixture, of the various colors including red, orange, yellow, green, blue, indigo, and violet.
There are no exact bounds to the optical spectrum; a light-adapted eye typically has a maximum sensitivity of ~555 nm (lime). Commonly the response of the eye is considered to cover 380 nm to 780 nm although a range of 400 nm to 700 nm range is more common. The eye may, however, have some visual response at even wider wavelength ranges.
Wavelengths in the range visible to the eye define the range of the "optical window". Most wavelengths are easily transmitted through the Earth's atmosphere. However, some of the high frequency bluish wavelengths get trapped in the atmosphere, which is why the sky is blue in the daytime.
Sir Isaac Newton 1666 "white light" is made up of a spectrum
It was Sir Isaac Newton in 1666 who first used the word spectrum to refer to the celebrated Phenomenon of Colours in which he demonstrated that "white light" was actually made up of a spectrum of colors. He refracted "white light" by projecting a slit of sunlight into a glass prism. You can simulate his discovery with a slit or spot of "white light" projected onto a triangular prism. It will refract the differing wavelengths at different angles/speeds, resulting in a projected spectrum of the light's constitutive colors. This is because the glass of which the prism is made is a dispersive medium. It's triangular shape allows the longer (red) wavelengths to pass through first, then the green, then the blue. Blue is the higher frequency color, so it stays in the prism longer, and bends and exits the prism at the steepest angle than the red or green in the prism. The resulting image is a saturated spectrum or rainbow of colored lights projected on the wall. However, with the advent of diffraction gratings the easiest is to purchase a $12 spectroscope at a kids science store.