A spectroscope is a scientific instrument used to measure various properties of light waves. Spectroscopes can measure the intensity of light, its wavelength, or its polarization. These instruments are based on the principle of light diffraction. When light passes through a diffracting substance or object, such as water or a prism, the waves of light are bent, or diffracted. Some wavelengths of light are diffracted more than others, so that diffraction of light waves enables scientists to isolate different wavelengths of light, and measure their properties.

The spectroscope was originally invented by Joseph von Fraunhofer, a German optician, in 1814. Early types of spectroscopes were usually based on the use of a prism, or a tool called a diffraction grating, to diffract light. The light being measured was focused by means of a lens, and the resulting ray of light was dispersed by means of the diffracting prism or grating. As different wavelengths of light were diffracted by differing degrees, this enabled measurement of the light waves. Joseph von Fraunhofer later pioneered the use of spectroscopic equipment, such as a simple hand spectroscope to measure the wavelength of light arriving on Earth from other planets.

Spectroscopes are often used by chemists to identify the molecular or atomic structure of a substance. When matter is heated up to a particular temperature, it is said to become incandescent, which means that the matter has been heated to a point at which it is emitting heat in the form of light waves. The characteristics of the light waves that are generated can be used to identify the molecules or atoms that make up the substance, almost like a signature. For example, salt in the form of sodium, when heated to incandescence, emits a characteristic yellow band of light that can be identified using a visible light spectroscope. Organic molecules can be identified using an infrared spectroscope, which is an important tool for chemists who work in food analysis.

When photographic film came into use, new kinds of light measuring devices were invented, called spectrographs. Spectrographs are built using similar principles to spectroscopes, but the use of a camera in the device allows measurements that are more accurate. In modern spectrometry, spectrographs are the most commonly used tools, largely replacing the original spectroscope, and with advances in digital photography, modern spectrographs provide extremely accurate measurements. Astronomers often use these devices to determine the chemical makeup of celestial bodies.