Question

11. The distance of object from the pole in a spherical mirror is always positive or negative? Why?​

Answer 1

Explanation:

Since, object is always placed in front of the mirror hence object distance is taken as negative. Since, the centre of curvature and focus lies behind the convex mirror, so radius of curvature and focal length are taken as + (positive) in the case of convex mirror.

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Answer 2

Sign convention is a set of rules to set signs for image distance, object distance, focal length, etc for mathematical analysis of image formation. According to it:

Object is always placed to the left of mirror

All distances are measured from the pole of the mirror.

Distances measured in the direction of the incident ray are positive and the distances measured in the direction opposite to that of the incident rays are negative.

Distances measured along y-axis above the principal axis are positive and that measured along y-axis below the principal axis are negative.

Note: Sign convention can be reversed and will still give the correct results

Magnification is the increase in the image size produced by spherical mirrors with respect to the object size. It is the ratio of the height of the image to the height of the object and is denoted as m. The magnification, m produced by a spherical mirror can be expressed as:

m=

h

h

Here, h is the height of image and h is the height of the object.

Magnification is also equal to the ratio of image distance to the object distance.

m=

u

v

As the object is always above the principal axis, the height of the object is always positive. But sign for image height may vary according to the type of image formed. The height of virtual images should be taken positive while the height of real images should be taken negative.

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