Scientists are said to have created a transparent mouse. How is that supposed to work?

Human beings have always longed to be invisible. Fables and myths are full of potions, incantations and cloaks of invisibility. H. G. Wells attempted to grasp the idea of invisibility a little more scientifically in his novel The Invisible Man. In it, the hero invents a substance that makes all the cells of his body transparent.

Total invisibility is interesting for literary fiction; partial invisibility would be amazing for science. Biologists are eager to find out what's going on inside the organisms they study. They'd like to see neurons firing, hearts beating, muscle fibres contracting, or intestinal peristalsis working. But to see inside a living creature, they have to cut it open. A transparent animal could be studied completely non-invasively under normal conditions.

The opacity of the skin is due to its optical properties. Light falling on an object is partly absorbed and partly reflected by it. The reflected rays are then perceived as the colour of the object. A red ball is therefore not red — it just reflects red rays and absorbs rays of other colours. If the arrangement of molecules in the object is such that the ray can pass through without significant change, we perceive it as transparent. If the ray inside the object is reflecting in all directions, it cannot reach the other side and we perceive the object as opaque.

A complex object like skin is made up of many substances. There's water, fats, proteins, hemoglobin, and each molecule has a different refractive index. When light passes into a different environment, it changes its direction of propagation. It is different when it passes through water, and different when it passes through fat. As a result, the skin looks opaque and we can't see our internal organs.