Invisible Glass? – Matching Refractive Indicies
Small sheet of glass or Pyrex Rod, Transparent Beaker, Transparent Cooking Oil, Water
Begin with the pane of glass standing upright in the beaker, whilst completely immersed in the oil. It should appear completely invisible – get observers to confirm this by walking around the beaker and looking down at it from the above. Then pull then pane out of the oil and re-immerse it showing how it appears to disappear when it is in contact with the oil. Now place the same pane in the beaker and cover it with water showing that it is now visible but its image is distorted.
This effect is cause by the matching refractive indices of glass and oil. When we observe things, what we see is the light scattered by them so changes in this scattering tell our eyes when we are looking at different materials. The refractive index is a specific property of the material caused by its electrons, how they interact with each other and how strongly they are bound to parent atoms. What we see this as is that light appears to travel faster in a low refractive index material (air) than in a high refractive index material (glass or plastic). When you place a pane of glass in water, the refractive indices are not the same and light ‘bends’ as it travels from one to the other, when it speed changes. As out eyes only ‘think’ of light as travelling in straight lines we see a distorted image instead of what is there. However, if two different materials have the same refractive index, then there is no distortion because the light does not ‘bend’ as it travels from one to the other, so there is no scattered light for us to see and, ergo, it appears invisible.