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Mia Ihm Lecture

Mia Ihm’s talk consisted of a short presentation on gravitational lensing with an interactive demonstration on general relativity. It began with a short lesson on optics utilizing Fermat’s principle and Snell’s law. Fermat’s principle is also known as the principle of least time; it states that the path of a ray of light is the path that can be traversed in the least amount of time. This principle naturally leads into Snell’s law, which relates the entry and exit angles of a ray of light traveling through two different mediums. Ray diagrams of reflection and refraction were shown with mirrors and both concave and convex lenses. The methods of ray tracing were also used to construct both real and virtual images. Real images are where light actually converges and virtual images are where light only appears to converge. The relationship between distances and focal length was explored with examples of how to calculate each and determine the nature of the image.

The talk then focused on gravity and included a brief history of Newtonian physics and how it laid the framework for relativistic physics. Einstein’s theory of general relativity states that gravity is simply the curvature of space-time. This means objects with gravity such as dark matter can act as gravitational lenses by changing the direction of rays of light. The two principle types of gravitational lensing are strong and weak lensing. Strong lensing is when an objects apparent location is completely changed by the presence of gravity, often forming “Einstein Rings” which are circles of distorted images. Weak lensing happens when the gravitational lens is not strong enough to produce multiple images, so the images are slightly distorted in appearance. String theorists are looking for cosmic strings in star fields, which would appear like a mirror image that has been lensed along an imaginary filament.

In closure, Mia setup a demonstration using a bedsheet as a 2 dimensional space-time and supported it along a circular perimeter by a table frame. She then placed a large object in the center as her "star” or "planet” and noticeably warped the fabric of space-time (a bedsheet in this case). Students then rolled balls around the star and noticed that they followed Fermat’s principle, like light, and were bent into other directions by gravity.