I tried to prove that distance of image is same as the distance of object in a plane mirror. The image attached is the ray diagram. First of all, I have assumed object and image to be parallel to mirror. First I proved angle TSA = angle TSa Then I have made similar triangle TSA and triangle TSa with AA similarity. ( where angle ATS = angle aTS ----each 90) Therefore, TS/TS = TA/Ta => TA=Ta ( TS/TS = 1 ) => AS=aS ( corresponding parts of similar triangle are equal in ratio) Using this, I have made triangle ABS and abs congruent. where, As=aS angle ASB = angle aSB angle ABS = angle aBS => AB=aB ( corresponding parts of congruent triangles are equal) Therefor, size of image is same as that of object. Also, Bs=bS (corresponding parts of congruent triangles) That is, distance of image from mirror is same as that of object from mirror. Now, I want to know how to prove that angle of image is same as that of object with principal axis? Because in my above method I already assumed both to be 90 degree. In other words, how to prove that image is erect too?