Question

In a concave mirror is the distance from the object to the pole is negative or positive??

Answer 1

object distance : negative

the concave and convex mirror u (distance between the object and pole) is always negative


Because distance measured is in the opposite directions to directions of Ray going

we all know that the pole ( the geometrical centre of a spherical mirror ) is called as origin and all distances are measured from pole that means a object is placed in front of a spherical mirror the distance from object to the spherical mirror should start from pole

simply understanding that if a object is placed in front of a spherical mirror the direction of incident ray from object is travelling towards the pole is taken as one direction now the measurement starting from pole is towards thee object

now in the above case both the directions are opposite to each other

so, u is always negative in spherical mirror


More detail

if you don’t understand the sign convention then is difficult to solve all these question.. so let me tell you sign convention first now,

(1)all distance should be measured from pole of mirror/lens.

(2) if direction of measurement of distance is opposite of direction ray then measured distance would be -ve. thats why u is always -ve. and concave mirror has focal length -ve.

(3) if you measure a distance in upward direction then it should be +ve , other wise -ve in cace of measured in downward direction. thats why when image is formed inverted the magnification is -v e because object is in upward direction and image in downward direction.

thats all the only things one should know to solve the this type of question of optics(mirror/lens).

At last let me clear you that why we take take -ve…..so ans is it is convention to solve the question. in physics -ve means not -ve things like maths . here -ve means shows some physical significance. in case of your question -ve means , where is the object or image ,is it front of mirror/lens or behind the mirror/lens.it is convention that if distance is front means -ve i.e. measured distance is opposite of ray direction.