Why do we have to put the appropriate signs when using any formulas about optics?
I am having difficulties with understanding the concept of lens formula and mirror formulas. Like why do u have to put whether its negative or positive sign when using the formulas? As, the formulas itself are derivated by using the appropriate signs..innit? For example.. for mirror, 1/v+1/u = 1/f, we found this by assuming that f u and v three are negative, so why do we have to do sign conventions again when calculating with this formula? The same goes for refraction from a curved surface formula.. like there we just assumed that u is negative, v is positive and f is pos, so thats how we get mu1/r -mu2/u = mu2/r - mu2/v, so why do we again have to do sign conventions again when solving another question???
9 Replies
@Gyro Gearloose
Note for OP
+solved @user to close the thread when your doubt is solved. Mention the user who helped you solve the doubt. This will be added to their stats.think about signs just like when you learnt addition and subtraction using a number line. here, too, a change in sign means a change in the direction you are walking towards (agar bachpan me number line se addition and subtraction waise padha ho toh :D)
additon is 2 + 2 so you are facing right
subtraction 2 - 2 so you are facing the left
now what about 2 - (-2) how do you internalise this?
https://www.khanacademy.org/science/in-in-class10th-physics/in-in-10th-physics-light-reflection-refraction
Check this out, he explains the basics really well.
if we dont apply sign conventions in the derivation, the formula will be only for the problem we're solving it for. thats why we have to first apply sign conventions and generalize the formula and again apply the sign convention for any other problem to get the answer.
and since the same formula for mirror is both concave and convex mirror and same formula for lens is for both concave and convex lens, and if we dont apply sign convention, how will we get to know that a particular value for lets say focal length is for convex or concave? so thats why we need to.
Derivation/Proof is not in syllabus. dont worry they wont ask it .
if youre well versed with the formulae and sign conventions, youre good to go.
if youre asking about lens, then the centre of the curved surface of convex lens lies on the right side , hence its focal length is +ve whereas the the same for concave lens lies on the left side and hence by convention, has -ve focal length
if you want to go in detail , then watch some youtube videos
Yeah, but still I'm curious 😅
Im sorry, That's not what I meant but thanks, I'll check later 😅
Hmmm, It sounds understandable, still I'm not getting it, can you elaborate please... Like why generalize the formula? We already put the appropriate signs in the formula when making it. Let's say, 1 question of lens has u = negative. So, if we put the negative u value into the formula (which already has negative u from where we derived it) wouldn't it be positive ( 2 negative equals positive)?
Im so confused 😅
yes but while deriving it we just used 1 case to derive it, in qs there are a lot of different cases, we cant always derive and get the formula right? thats why we generalise the formula and then apply sign convention for different questions
https://www.youtube.com/watch?v=roFZmWAzQfc
watch this, might help
Woah, that actually clears all the doubts, I couldn't ask properly on google to get it, thank you so much, so we use sign conventions once to just get the general formula, so that we can remember it, and then we use it the 2nd time to go back to the original formula which will be used for the specific question, got it!! Thanks
+solved @trin
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