With the recent announcement that BICEP2 had discovered direct evidence of primordial gravitational waves, I learned that they were detected by B-mode polarization in the cosmic microwave background. There are many possible errors in observation that could have occurred, but the biggest concern in an effect called gravitational lensing. Other than true primordial gravitational waves, this is the only other thing that could cause the B-mode polarization pattern. Gravitational lensing is a type of gravitational distortion due to the bending of light around an object. If a distant light source being observed has a large object in between our line of sight, the light will deflect around that object, and can appear as more than just one light source depending on the position and complexity of the object. Since a gravitational lens has a focal line instead of a focal point, the light will appear as a ring, known as Einstein's ring, or several arcs of light. To an observer, this appears as two or more images of the object, known as a gravitational mirage.
A gravitational or cosmic mirage is formed in a very similar process that a regular mirage is. A mirage is formed when light rays are refracted to create a false image of the object. A common example of this is the sunrise. A surface on the Earth causes an inverted image of the sun to be created, called an inferior mirage. Another false image of the sun is created by atmospheric refraction. Every morning, the sun appears to rise before the actual sun reaches the horizon. This is because the atmosphere acts to refract the sunlight, forming a false image of the sun higher in the sky than it actually is. A simplified example of refraction can be found in this link. However, a spherical surface would need to be used to simulate the refraction in a curved atmosphere to describe the false image of the sun at sunrise and sunset.
Mood rings are said to change color based on the mood or emotional state of the person wearing them. While they can't directly tell what a person is feeling, the colors do change based on temperature. This is because mood rings are made of liquid crystals, whose molecules change position when there is a change in temperature. This changes the molecular structure, including which wavelengths of light are absorbed and reflected, thus portraying a different color. The molecular structure of liquid crystals can also be effected by electric fields. The use of liquid crystals under the influence of an electric field has effects that alter polarization. An example of a kind of set up that could model this is described here under applications of liquid crystals.
Fraunhofer lines are the dark absorption lines that appear in the continuous solar spectrum. Here is an example of how a simple spectroscope can be created to view the Fraunhofer lines using a holographic diffraction grating and an adjustable slit.