plane mirrors
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PRODUCERS :SEHAJPAL SINGH & MANJOT SINGHBUDHA DAL PUBLIC SCHOOL, Patiala,IndiaClass Xth (2010-11)
In this section we deal with plane mirrors. These are mirrors that are flat. In most cases the mirrors we see are pieces of glass with a silvered coating on the back, while in some cases the silvered coating may be on the front of the glass. Such a mirror is referred to as a front-surface mirror. Other objects behave like mirrors including smooth surfaces of :
What are the rules by which mirrors operate
when light strikes them? When an ordinary object is placed in front of a plane mirror, it is sending light rays out from itself in all directions, either by emission or by reflection. This is shown here:
Therefore the light rays that strike the mirror will be doing so from a wide variety of angles. What is the pattern for light's reflection and how does it depend on angle?In Reflection we picture a line perpendicular to the mirror at the point of reflection. This is called the Normal. (Remember when we used the normal to define the angles in Snell's Law.) The Law of Reflection states that light reflecting from a plane mirror forms an angle to the NORMAL equal to the angle between the normal and the incident light.
When an object is placed in front of a mirror, two different observers appear to see an image of the object in different places. The light, however, travels from the single object to the mirror and then to the respective observers. The manner in which it reflects must meet the conditions stated on the previously.
Notice how the light reflects from the smooth surface of the mirror, creating congruent or equal angles to the normal at each place it strikes the mirror. The two observers only see light when it has been reflected from the mirror in the proper manner .
What is the nature of the light at the image location? In fact, there is no light there. This is
only the apparent source of the light so it is referred to as a virtual image. Virtual images are ones that are not formed by putting the light rays together, but only appear to exist as the apparent source of the light rays.
GEOMETRY:What is the geometry of the plane mirror's image? In the next series of drawings we can follow the geometric proof of where the image is located.Use the vertical angle theorem to establish other angles congruent to the ones you had seen in the last drawing.
And use the complementary angle theorem to establish two more sets of congruent angles.Now, using the logic of ASA, the two triangles formed, one on the object side of the mirror, the other one behind the mirror, can be proven congruent Thus the triangle "behind" the mirror is the same size and shape as the triangle in front.
The object and its virtual image can be joined by a line which is perpendicular to the mirror, forming altitudes for the two triangles. Now because the two perpendicular lines are altitudes for triangles that are exactly the same shape and size, their lengths must also be the same. This leads us to the following conclusion: The image formed by a plane mirror will be as far behind the mirror as the object is in front of it. A line joining the two will be perpendicular to the mirror.
The relationship between the number of images and the angle between the mirrors is
No. of images = 360° angle between mirrors
1.
We can change the number of images produced by two mirrors by changing the angle between them .The relationship between the number of images and the angle between the mirrors is
No. of images = 360° 1 angle between mirrors
We will proof it by doing an ACTIVITY…
WHAT WAS REALLY HAPPENING WHILE DOING THE ACTIVITY ???
Multiple reflections are difficult to understand and visualize. If you have one flat mirror, lightfrom the object will reflect off the mirror once and leave the system. When you have more than one mirror, you can create multiple reflections. These multiple reflections can lead to multiple images.
I1
I2
OBJECT
I1
I2
Consider the two mirrors (solid lines) shown at right at an angle of 120 degrees to each other. The object is a small ball. The images, I1 and I2, form exactly on the opposite side of each mirror, the same distance from the mirror, just as you would expect for reflection from a single mirror. The two mirrors each yield an image.You will also notice that I1 and I2 lie equal distances from the dotted line (representing a “reflection” of the mirror surface itself).
WHAT WAS REALLY HAPPENING WHILE DOING THE ACTIVITY???
When the angle is decreased, more reflections will occur. Let’s look at two mirrors that are perpendicular to each other. One image will form from a single reflection from each mirror. You might recall from “Right is Right, or is it Left?” that perpendicular mirrors yield two reflections. These two reflections create the third image, I3. You can see these reflections by looking into the mirror as shown. .
