By imaging with the appropriate wavelength of electromagnetic energy, materials considered completely opaque become transparent, revealing hidden wonders. Objects the human eye perceives as dark look light, and light objects look dark; indeed the whole notion of dark and light becomes very subjective, as it depends on the wavelength response of the imaging technology used and not what the human eye sees. However, imaging with light waves is not always possible or desirable because of basic imaging limitations imposed by the optical properties of matter between the object and the imaging system. As we turn the imaginary knob on our head and sweep the response of our eyes through the electromagnetic spectrum, we may never find a waveband of light capable of imaging through a particular obstacle. For instance, we cannot see through even the purest ocean water for more than about 100 m at any wavelength of light, yet the ocean floor is full of interesting things worth imaging. Imaging with light is not always desirable because sometimes the light we need to see through intervening matter may be harmful. Imaging of a fetus can be done with x rays - in fact doctors used to x ray pregnant mothers as a means of diagnosing prenatal conditions - but this practice is now discouraged due to the potentially harmful effects of x rays on the developing child.
Acoustic or sonic imaging is an alternate way to see through intervening matter. This is literally "seeing with sound," made possible because sound waves are very similar to electromagnetic waves in their properties, yet they travel freely through fresh or salt water, tissue, and a variety of other materials that are opaque at many or all wavelengths of electromagnetic energy. Acoustic pictures are different from pictures made from light in that they are essentially maps of density variations. Sound waves will reflect from boundaries between different materials and carry back information about changes in density and object shape to a sensor. There are several different ways to image with acoustic waves, and these methods are all analogous to techniques used to image with light.