IIT JEE , AIEEE Forum - Physics , Optics

bumba

i cannot understand in the problems related to magnification that suppose it is given that a concave mirror forms a doubly magnified image(for example) then if u be object distance and v be image distance then we will take:::
2= -v/u
or 2=-v/-u(taking u as - ve from the beginning)
please help!!!!!!!!!!

chimanshu_007

hey i think u are mistaken

u is negative na

m = v / -u or m = -v / u , 1 and the same thing as u is always -ive

v can b -ive or +ive

chimanshu_007

see some sign conventions

1) u is always -ive

2)if image is real , " v " is taken as -ive , if image is virtual , then " v " is taken as +ive

3)focal length of concave mirror is -ive and of convex mirror is +ive

4)wen image is virtual , magnification is +ive , wen real , it is -ive

5)in concave mirror , image formed may be real or virtual depending on position of object so magnification of concave mirror can b -ive or +ive

6)bt convex mirror only forms virtual image , so magnification is always +ive

hope this helps a little

mohnish_khiani

The formula is given as: m=-v/u.
This formuls is used differently for different sign convention methods.The most commonly used method is to consider that the direction in which light rays travel is considered +ve. In this method a real object always has -ve distance(u) and a virtual object always has +ve distance(v).
For the formula, m=-v/u,if the object is real i.e u is -ve the formula changes to m = -v/-u =v/u.For a virtual object u is +ve the formula then is m = -v/u.Whatever is the sign for magnification and image is to be considered so by the statement "The direction of travelling of rays is considered +ve."

devesh_l2k007

m=-v/u
u can be positive or neg conceptof real nd virtual image

devesh_l2k007

sorry
m=-v/u
u can be positive or neg conceptof real nd virtual object

iitkgp_bipin

Standard sign convention used :
The direction of light rays is taken positive and pole is considered as origin.

Apply this convention to find all the parameters.

Now substitute the algebric values (not the absolute values) of u and v in the formula m = -v/u to obtain the magnification.

ramyani

Probably u r using R P ( real is + ve ) sign convention which is not followed in most of the books.
I learnt this R P ( real is + ve ) sign convention in school ( class X ) as the prescribed text book followed this. When I registered in goiit , I asked specifically whether I can still follow this old sign convention. I was assured that I can.
       Now, in this R P ( real is + ve ) sign convention, all real spaces r taken as +ve. By real space, we mean the space where a light ray should NORMALLY be. In other words, all normal things r +ve and all abnormal things r --ve.
       In case of any mirror, the side from which the ray falls i,e, the side in which the ray goes after reflection is normal ( usual ) and is +ve. Therefore object dist and image dist of all real images r +ve in case of real image.
       In case of virtual image, these r formed in the unreal space (virtual space) in case of mirror. In other words, these images r formed behind the mirror where a light ray cannot pass. So space behind the mirror is virtual space. Images formed there r virtual. Image dist is --ve, but object dist is +ve. Ultimately, magnification is -ve in case of a virtual image.
       In case of a concave mirror, it can form both real and virtual image. For real image obviously, magnification m = +ve. For virtual image, magnification m = --ve.
   Thus the formual for magnification is m = + v /u for real image and m = --v /u for virtual image.
      But stick to any one sign convention.I am so told.

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