Describe how an image is formed by a plane mirror.Distinguish between real and virtual images.Find the location and characterize the orientation of an image created by a plane mirror.

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You only have to look as far as the nearest bathroom to find an example of an image formed by a mirror. Images in a plane mirror are the same size as the object, are located behind the mirror, and are oriented in the same direction as the object (i.e., “upright”).

To understand how this happens, consider (Figure). Two rays emerge from point P, strike the mirror, and reflect into the observer’s eye. Note that we use the law of reflection to construct the reflected rays. If the reflected rays are extended backward behind the mirror (see dashed lines in (Figure)), they seem to originate from point Q. This is where the image of point P is located. If we repeat this process for point

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) is the distance from the mirror to the object (or, more generally, from the center of the optical element that creates its image). Similarly, the image distance (denoted
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Summary

A plane mirror always forms a virtual image (behind the mirror).The image and object are the same distance from a flat mirror, the image size is the same as the object size, and the image is upright.
What are the differences between real and virtual images? How can you tell (by looking) whether an image formed by a single lens or mirror is real or virtual?


Virtual image cannot be projected on a screen. You cannot distinguish a real image from a virtual image simply by judging from the image perceived with your eye.


Yes, you can photograph a virtual image. For example, if you photograph your reflection from a plane mirror, you get a photograph of a virtual image. The camera focuses the light that enters its lens to form an image; whether the source of the light is a real object or a reflection from mirror (i.e., a virtual image) does not matter.


No, you can see the real image the same way you can see the virtual image. The retina of your eye effectively serves as a screen.


Devise an arrangement of mirrors allowing you to see the back of your head. What is the minimum number of mirrors needed for this task?


If you wish to see your entire body in a flat mirror (from head to toe), how tall should the mirror be? Does its size depend upon your distance away from the mirror? Provide a sketch.


The mirror should be half your size and its top edge should be at the level of your eyes. The size does not depend on your distance from the mirror.


Consider a pair of flat mirrors that are positioned so that they form an angle of 120. An object is placed on the bisector between the mirrors. Construct a ray diagram as in (Figure) to show how many images are formed.


Consider a pair of flat mirrors that are positioned so that they form an angle of 60. An object is placed on the bisector between the mirrors. Construct a ray diagram as in (Figure) to show how many images are formed.

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By using more than one flat mirror, construct a ray diagram showing how to create an inverted image.


Glossary

plane mirrorplane (flat) reflecting surfaceimage distancedistance of the image from the central axis of the optical element that produces the imagemagnificationratio of image size to object sizeobject distancedistance of the object from the central axis of the optical element that produces its imagereal imageimage that can be projected onto a screen because the rays physically go through the imagevirtual imageimage that cannot be projected on a screen because the rays do not physically go through the image, they only appear to originate from the image