# Gravitational Wine Glass Lens

Gravitational lensing bent light from this galaxy into a 90-degree arc.
Image courtesy NASA/ESA/J. Rigby/K. Sharon.

A large gravitational force can bend light similarly to how a lens also bends light. The effect of light bending as a result of gravity is called gravitational lensing, predicted in Einstein’s general theory of relativity.  Gravitational lensing has allowed astronomers to locate large amounts of dark matter in the universe.  Fortunately, you don’t need massive amounts of dark matter to understand this phenomenon – you can simulate gravitational lensing at home. All you need is a few household materials and a bit of creativity.

What You Need

• Wine Glass
• Graph Paper
• Printouts of a star/galaxy map

What to Do

First place a wine glass on top of your graph paper.

As you look through the base of the wine glass, slowly slide the glass across the paper. Watch for the “straight” graph lines to bend as you move the glass.  The graph lines that are closer to the center of the base will bend more than the lines that are near the edge of the base.

Now that you know what you are looking for, replace the graph paper with a print out of a star/galaxy map. Move the glass around, and watch the stars/galaxies bend and curve.

What's Going On?

The bottom of the wine glass acts as a circular lens and bends the image of the paper underneath. The roundedness of the images is due to the rounded curve of the base, where the middle is thicker than the outside. Because light travels at a different speed through glass compared to air, it bends whenever it encounters glass, and thicker glass bends light for a longer period of time. Similarly, gravitational lensing acts like a circular lens bending the light of stars that are behind it. More massive objects exert a stronger force, causing a more pronounced bend in light. Astronomers search the sky to try to find locations where galaxies and stars are bent into such a curved/circular image.

What Changes . . .

• When you use a wine glass that has a more curved base?
• If the glass base weren’t curved but flat?
• When you hold the glass farther from the paper instead of placing it on top of the paper?