Which instrument is a triangular or wedge-shaped piece of plastic or glass that displaces light?

The correct answer is a prism, which is a transparent optical element that has a specific geometric shape, typically triangular or wedge-shaped. When light passes through a prism, it is refracted, or bent, due to the change in medium from air to the material of the prism. This bending of light allows the prism to separate various colors from white light, demonstrating phenomena like dispersion, which is crucial in various optical applications, including vision correction and experimentation in optics.

The other instruments listed do not serve the same purpose as a prism. A pinhole is primarily used to control the amount of light entering an optical system but does not displace light in the manner a prism does. A slit lamp is an essential tool in ophthalmology for examining the anterior segment of the eye and does not manipulate light in the same fundamental way. An optical coherence tomographer is a diagnostic device that uses light waves to take cross-sectional pictures of the retina but, again, does not function as a light-displacing medium like a prism.