Optical astronomy encompasses a wide variety of observations via telescopes that are sensitive in the range of visible light. It includes imaging, where a picture of some sort is made of the object; photometry, where the amount of light coming from an object is measured, spectroscopy, where the distribution of that light with respect to its wavelength is measured, and polarimetry where the polarisation state of that light is measured. 50 cm refracting telescope at Nice Observatory. ... The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. ... In astronomy, photometry is the measurement of the flux or intensity of an astronomical objects electromagnetic radiation. ... Spectroscopy is the study of spectra, that is, the dependence of physical quantities on frequency. ... The wavelength is the distance between repeating units of a wave pattern. ... This article treats polarization in electrodynamics. ...

An example of spectroscopy is the study of spectral lines to understand of what kind of matter light is going through. Spectroscopy is the study of spectra, that is, the dependence of physical quantities on frequency. ... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ... Matter is commonly referred to as the substance of which physical objects are composed. ...

Optical astronomy also includes looking up at night.