1. Introduction: The Question That Looks Easy but Trips Everyone
You know the mirror formula. You know the lens formula. You can even write them from memory:
Mirror: 1/v + 1/u = 1/f
Lens: 1/v − 1/u = 1/f
So when a problem says "a concave mirror has focal length 15 cm and the object is placed 20 cm in front of it — find the image distance," you should be able to solve it in 30 seconds. Right?
And yet, this is where most students lose marks. Not because the algebra is hard. The algebra is one substitution and one fraction. The reason students lose marks is that they put the wrong sign on a number. Or they forget which formula to use. Or they get an answer and cannot tell if it makes sense.
This lesson is not about memorising formulas. You already know the formulas. This lesson is about the translation step — turning a word problem into the right set of signed numbers, plugging them into the right formula, and checking that the answer makes physical sense.
Stop scrolling. Try it in your head before reading on: in the mirror formula, is the object distance u positive or negative? What about for a lens?