EVERYTHING HERE IS FOR CONCAVE MIRROR
Everywhere I see the derivation of the mirror equation is given by placing an object before the focus and then proving similarity of the triangles to get to the form
$\frac 1u + \frac 1v = \frac 1f$ .......(1)
( $v$ is toward the reflecting side of mirror)
It absolutely true and i completely agree with it , it should be valid for all the cases where you keep an object before the focus , my question is that how is it valid for the cases where object is kept between focus and pole ?? The triangles whose similarity we had used to get to the equations are not even formed here .....how then are we using the equation here ??
Moreover the '$v$' that we used here is actually the side length of a triangle , it cannot be negative this clearly shows that the equation is invalid for object kept between the focus and centre ... Because we get a negative '$v$'
After thinking for some time and calculations i have come to an answer for the requirement Of a sign convention
If we keep an object between the focus and the pole and extend the rays backward to form a virtual image , we can prove (again by similarity of different triangles ) that for object placed between the focus and pole
$\frac 1u - \frac 1v = \frac 1f$ ........(2)
( $u$ and $v$ are the object and image DISTANCE respectively from pole) ($v$ is towards the opposite of mirror)
Now we can see that for an object kept before the focus we can use eq(1) and for object between focus and pole we can use eq(2) to get image distance respectively
Where is the sign convention now?
What if we want to generalise the formula i.e use a single formula for both cases .. Can we do that?
Lets see First case For cases of objects before focus we can clearly use equation 1
Second case For objects between focus and pole we can use equation 1 to get v
What if we use equation 1 for the second case It us logically absurd but lets see what happens ..
Acc. To equation 1
$v=\frac {uf}{(u-f)}$ ( v is towards reflecting side)
And acc to equation 2 $v=\frac{uf}{(f-u)} = -\frac{uv}{(u-f)}$ ($v$ is opposite to reflecting side)
So we see even though it is absurd to use equation 1 for the second case logically but due to some type of similarity between the two equations the result we get from using equation (1) for the second case is negative of the result from equation (2) (which is correct equation for the second case) for the second case
So if we ise equation (1) in second case we get a negative answer (I)Negative answer means that we are using wrong equation and we have to use eq(2) (II)By using eq(2) we get the same answer of $|v|$ but this time opposite side
So we can generalise equation (1) by using a method that negative answer of '$v$' means image is at distance $|v|$ opposite to the mirror
This method is sign convention
This is the reason for involvement if a sign convention system in mirrors.
Clearly it is a method to generalise all equations to a single mirror equation.
I just want reviews and want to know that isnt it right?