Oscilloscope, an instrument that produces a visual image of an electrical signal. The image is produced on a luminous screen similar to the picture screen in a television receiver. The screen forms one end of a cathode-ray tube. By observing the visual image of an electrical signal, scientists or technicians can determine at a glance such things as the strength of the signal, the wavelength (or frequency) of the signal, whether any undesired phenomena are affecting the signal, and whether the source producing the signal is functioning properly.

The electrical signals displayed on the oscilloscope are not necessarily of electrical origin. A large number of physical phenomena—such as light, radio waves, and sounds—can be converted to electrical signals suitable for display on an oscilloscope. Sounds, for example, can be converted into electrical signals by a microphone.

If desired, the images displayed on the oscilloscope screen can be photographed. An oscilloscope equipped with a camera is sometimes called a cathode-ray oscillograph.

The German physicist Karl Ferdinand Braun invented the oscilloscope in 1897.