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Learning Introductory Physics with Activities

Paul J. Emigh, Rebecka Tumblin, Kathryn Hadley, Danielle Skinner

Section 19.3 Plane Mirrors

The ray model for light can be used to understand how images are created in plane mirrors.
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Images are produced by intersecting light rays and can be seen all over the natural world.
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A green arrow, labelled as "object", sits a distance d sub o away from a plane mirror on the left side. Two rays extend from the tip of the arrow and hit the mirror: the first hits the mirror perpendicularly, and is reflected backwards along the same line. The second ray hits the mirror at some angle and is reflected back at that angle. Both reflected rays are extended as dashed lines behind the mirror and intersect. Another green arrow, labelled as "image", sits behind the mirror with the tip of the arrow pointing at the intersection point.
Figure 19.3.1. Diagram of a plane mirror. An object sits at a distance \(d_o\) from the mirror, and an image is produced at a distance \(d_i\) behind the mirror.
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In this situation, you are seeing a virtual image. These types of images are produced when light rays do not physically travel to the image location. When you are looking at yourself in a mirror, there is no physical copy of yourself behind the mirror. The image that you see is virtual. When you are looking at ray diagrams of mirrors and lenses, you are typically interested in figuring out where the image is located.
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Definition 19.3.3. Virtual Images.

A Virtual Image is created when light rays do not travel through the image location. These images will have a negative image distance.
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Exercises Plane Mirror Activities

1. Plane Mirror Simulation.

Spend some time playing with this simulation. Click on the "Mirror" option. At the top of the screen, select the flat mirror option.
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Make some observations as you move the object away from and towards the mirror.
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2. Mirror Illusion.

Watch this video about a supposed “mirror illusion”.
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Answer the following questions:
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  1. What is the illusion here?
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  2. Draw a ray diagram to explain how you can still see the object in the mirror.
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  3. How would you explain this to someone that doesn’t know anything about the ray model for light and reflections?
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References References

[1]
"HOW does the MIRROR know this? 😳😨 #Shorts." YouTube, uploaded by Cam Casey, 7 April 2023, https://www.youtube.com/watch?v=gNgSlGn59xA&ab_channel=CamCasey
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