PhysicsLAB Resource Lesson
Properties of Plane Mirrors

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For flat mirrors the Law of Plane Mirrors states that "the image is always the same distance behind the mirror as the object is in front of the mirror." The image, I,  and the object, O, always line up along the same normal. The image is upright, but left-right reversed.
 
 
Notice that the rays entering the eye are diverged from the mirror's surface. Since the eye has to "dot back" the rays to form an image, this image is virtual - a cool image, trapped within the mirror. Take a moment to examine a demonstration of the formation of the virtual image of a lit candle.
 
As mentioned earlier, the Law of Plane Mirrors states that di = -do. This reflection, like ALL reflections, obeys the Law of Reflection: the angle of incidence equals the angle of reflection. Note that these angles are measured from the normal to either the incident or reflected rays.
 
 
The following diagram illustrates that the minimum length of a plane mirror required for someone to view their entire image equals half their height. Note that the top of the mirror should be placed at eye level.
 
 
The previous diagram also emphasizes the fact that the size of the image equals the size of the object; that is, the magnification of a plane mirror equals one. Recall that magnification is calculated using the formula:
 
 
Rough surfaces are defined as those on which successive elevations vary by more than 1/8 th of the incident radiation's wavelength. These surfaces scatter the light and produce what is called a diffuse reflection. Note that all reflections obey the law of reflection, but because of the "tilted" nature of rough surfaces, incident parallel rays are not reflected parallel to each other. Surfaces from which incident parallel rays of light remain parallel after being reflected are called polished surfaces and this type of reflection is called a specular reflection. Surfaces which are smooth for long wavelengths can therefore be rough for shorter wavelengths - hence the use of  wire mesh antennas for radio astronomy and polished mirrors for visible astronomy. 
 
 
When more than one plane mirror is used, multiple images are formed. Take a moment to view the following two demonstrations which show the production of infinite images when two plane mirrors are placed parallel to each other or when three plane mirrors are placed in an equilateral triangle. To practice, please reference the following diagrams.
 
 
 
Locate the images: P1, P2, P12, P21, P121, and P212. Label all images of P1 in one color and all images of P2 in a second color.
 

 
Locate P1 and P2. Draw in the rays which are used to sight each of these images. Use one color for both rays for P1 and a second color for the rays for P2. Finally, locate P12 and P21.
 
 
 

 
Locate P2 and P21. Then ONLY draw in the rays which are used to sight P21. Use one color for the ray that enters the top of the eye, a second color for the one that enters the bottom of the eye.
 
 
 




 
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