Video: Image Formation by Plane Mirrors

The reflection of an object is seen in a mirror by an observer whose eye is shown in the diagram. At which of the points A, B, C, D, and E is the object’s image seen?

02:17

Video Transcript

The reflection of an object is seen in a mirror by an observer whose eye is shown in the diagram. At which of the points A, B, C, D, and E is the object’s image seen?

Okay, in this scenario, we have an object shown here with light coming off of that object reflecting off of a plane or flat mirror and then reflecting into the eye of an observer, shown here. Behind to this plane mirror to the right of it as our diagram shows, there are these five points, A, B, C, D, and E. The question asks, at which of these five points is the image of the object seen by the observer? Now to answer this question, we need to know a bit about mirrors. In particular, we need to know that any time we see something in a mirror, it’s reflected light that our eye is seeing.

This diagram shows a great example of that. It’s not that the observer is seeing something in the mirror, but rather light that reflects off the mirror that comes from another source. But here’s something interesting. The observer looking at this light would not say that the object that’s creating a light is up here where it actually is located. The reason for this is that the observer is only seeing the reflected ray of light. That means to figure out where the image of this object would appear to this particular observer, we would need to trace back this reflected ray. And in fact, we would need to trace it back behind the mirror the same distance that this incident ray from the object itself traveled before it reached the mirror.

So here’s what we’ll do. Starting at the point where this ray of light reaches and then reflects off of the mirror, we’ll trace the reflected ray back behind the mirror using a dashed black line. And we’ll continue following along this line until it reaches one of our points. We see that the point is point A which is where the image of this object is formed according to the observer’s observation. It’s interesting to know that this image that the observer’s seeing doesn’t exist in real space. After all, this mirror might be hanging on a wall so that there’s no way an object could be behind it.

For that reason the images that we see in mirrors are called virtual images. They don’t really exist where we think they do. But they’re just reflections of real objects in real space. But anyway, our question was interested in which of the five points, A, B, C, or D [A, B, C, D, or E], the object’s image is seen. And we’ve seen that it’s seen at point A. That’s where the virtual image of this object is formed.

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