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The mathematical solution for the formation of infinite images in parallel mirrors is incorrect?

Physics Asked by WebDevNoob on February 23, 2021

Textbooks and websites say that parallel mirrors form a $0^circ$ angle between them and that is why the number of images formed is $(360^circ/0)$ or infinite.

  But I challenge this. There are two reasons of why this can not be true:

$ 1)~~$ A $0^circ$ angle is formed when two lines/rays lie exactly over each other. In simple words, they intersect at every point. This contradictory to the fact that the mirrors are parallel.

$ 2)~~$ Even if we accept the $0^circ$ angle, $(360^circ/0)=$ undefined; not infinite There is a simple difference between being undefined and being infinite.

I am a class $9$ student, so please explain in simple terms…

3 Answers

This will be an explanation of the phenomena without the use of the intersecting angle of parallel mirrors.

Consider a set of parallel mirrors with an object in between them, the object creates a virtual image behind each of the mirrors respectively, let's call them $I_1$ and $I_2$. Now consider that $I_1$ and $I_2$ themselves have rays coming outwards towards the mirror which they are $textbf{not}$ behind (i.e. towards the other mirror), and as result they each make 2 other images, lets call them $I_3$ and $I_4$. Notice that for 2 perfectly parallel mirrors, because they share a normal axis, all of the images we have placed so far are going to be on the same axis, that means we can draw a line through all of them.

Now we see a pattern forming; $I_3$ and $I_4$ will create images $I_5$ and $I_6$, etc. And this will go on growing forever, we can say the number of images approaches infinity.

Separate note: When mathematicians use "undefined" vs. "infinity", there is a subtle difference; since both are equally not numbers that you can be "equal to" in the strict sense, however they mean different things. For example imagine the following graphs: https://www.desmos.com/calculator/vnnzwnezzw . Both functions are not defined at 0, however we can see that the blue graph approaches infinity from both sides, whereas the green graph goes to positive infinity on one side, and negative infinity on the other, so it doesn't really approach anything. So we can say that the blue graph approaches infinity as x approaches 0, but for the green graph we say it is undefined as x approaches 0. (This concept is called limits, it forms the basis of Calculus)

Answered by xXx_69_SWAG_69_xXx on February 23, 2021

For completeness to the answer by xXx_69_SWAG_69_xXx , here is a photo of real life parallel mirrors,

enter image description here

First, a few words about the “infinity effect”. As the picture above implies, to see an infinite number of images of an object one has to look at it in the mirror off-center: the cameraman in the photograph is standing behind and to the left of the woman.

The rest of the article may interest you.

Answered by anna v on February 23, 2021

  1. A 0⁰ angle is formed when two lines/rays lie exactly over each other. In simple words, they intersect at every point. This contradictory to the fact that the mirrors are parallel.

If you have line A and line B which are at angle phi, any pair of lines which are parallel to A and B are also at the same angle. So just because two coincident lines are at 0 angle to each other does not imply they are the only lines at that angle to each other.

Note that when you have two normal mirrors in the real world, they won't meet at a point, but there will be gap between their reflective surfaces (the 'silver' on the back of the glass is what reflects the light, so even if the glass is touching the reflective surfaces do not). If it's ok for those non-touching surfaces to be at an angle to each other, and not ok for other non-touching surfaces to be at an angle because the angle has decreased to zero, then I really can't see how to explain any further.

  1. Even if we accept the 0⁰ angle, (360⁰/0)= undefined; not infinite There is a simple difference between being undefined and being infinite.

Yes, as a simple expression the value is undefined. Instead, consider changing the angle between the mirrors until it is zero. So mathematically we care about the limit of k/x as x tends to zero; this value increases indefinitely as x becomes infinitesimally small, so we can informally say the value is infinite.

Answered by Pete Kirkham on February 23, 2021

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