What is the plane of a concave or convex mirror? Is it along its principal axis or its area? I am convinced that the plane should be along the area of mirror but i am not sure about it. I tried to find it in books but didn't got any answer. Some books have questions like a point object is moving in a circle in a perpendicular plane to the mirror so find radius of image. Such questions are confusing to solve if we don't know where would be the plane of mirror.
Geometric Optics – What is the Plane of a Concave or Convex Mirror?
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Related Solutions
Any discussion of concave/convex mirrors needs to begin with a statement of the particular version of the mirror equation to be used, along with the convention for setting and interpreting the signs of focal lengths, and object/image positions.
For example, from http://scienceworld.wolfram.com/physics/MirrorFormula.html:
Unmentioned in this is the convention that virtual images and objects are found behind the mirror and have negative values of $d$.
In the particular example you present, the image formed by the primary mirror is a real image. If you put infinity for the object distance and a positive focal length, you find a positive image distance.
But when you insert a convex mirror, with a negative focal length, into the optical path, you must also consider the position of the real image (now an object) relative to the convex mirror. The object is behind the convex mirror; it is a virtual object, and its distance from the convex mirror is negative.
With appropriate positioning of the convex mirror, the formula will produce a positive value for the image distance. There will be real image formed in front of the convex mirror.
You've just designed a Cassegrain telescope...
to see the real inverted image "behind" the mirror is kind of an optical illusion. The picture is still in front of the mirror, but your eyes can not find a frame of reference, so it "sees" the picture at the usual place you see it in plane mirror, a virtual picture would not be inverted. to help your ey to see the picture, at the place where it really is, place som kind of mauve frame instead of the screen, sometimes it even helps if you put just your finger beside the place the screen was before and you see the real picture in the air beside your finger.
Best Answer
There is no such particular definition for plane of mirror, in fact this term is very ambiguous since spherical mirrors aren't planar. "Plane of mirror" - these words are just used to express questions (like plane of paper or plane of the plane mirror). Physics doesn't defines that this particular plane is called the "plane of mirror".
In your question-
The plane of motion of the point object is perpendicular to the Principle Axis of the mirror. Generally, we take principle axis of the mirror as our reference for all calculations and observations.