While deriving mirror and lens formula, we substitute for obj and img distance with proper sign following the carteian sign convention i.e. we have taken sign convention into consideration while deriving the formulae. But while doing problems, we again substitute obj distance as some negative number. WHY?
33Oh you can do that..
Write the vector along incident ray apply law of reflection or refraction at point of reflection or refraction...
Get the vector along refracted or reflected ray and get the image...
But don't You think applying snells law at every surface will be little unpractical... literally you have to derive the refraction formula or lens maker formula twice or thrice in every question...
33The answer to the question in written in japanese... read if U want the answer :P
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11How does poor light know whether we're considering it as wave or particle? It always shows interference
3while deriving the lens and mirror formula we've consider the particle nature of light. if we take the wave nature of light can these formulas can be proved. will interference of waves happen.if interference occurs how the formula is to be derived
1i think its because of mathematical interference :P........ actually what happens is that while we apply sign conventions while deriving the formulas we get all the terms in negative terms.... that implies the formula can also be written in all positive terms which contradicts the sign convention (but not maths)..... so we have to apply again the sign convention to get the proper result ! :P
33Hey this is a common doubt 4 everyone...
Don't ask me i had same doubt but You have to understand the feeling what is being done.. really see each calculation step and derivation in detail.... what is being done.....
I can't have words to say that... you have to feel it :P
11But my question is why do we apply sign convention twice, thus effectively neutralizing it?
3I have the same doubt from my tenth standard.But priyam we are using sign convention for the same theme in which we have derived the lens and mirror formula. whats the reason behind this
11Then why do we study cartesian conventions when we don't really use it in solving numericals. Extra things to study
33Yes i think so..
we do in practicals and verify it also isn't it ....
33Yes in deriving lens and mirror formula we use sign convention...
because we only take one specific case for deriving it but for it to be true for all cases we use sign convention..
While applying it we use it again....... because it give right answer... :P
11Well, in NCERT textbook they've taken obj distance as -u in the derivation itself. And they've applied -ve values for u while solving problems. In this way are'nt we sort of neutralising the sign convention, thus making it useless?
62at the end of it, we do use sign conventions...
depending on the light direction or whichever...
and in our usage, we will have to take the sign convention in that form...
62As far as i think that the derivation of lens and mirror formula come from geometry and not by taking sign conventiion...
If that was the case, then we would have taken u and v as opposite signs!
11But we put obj distance as -u in the formula derivation itself, don't we?
So why apply sign convention twice? That's my question.
