CBSE Class 12-science Answered

Sign convention in the case of concave mirror:

• Since, object is always placed in front of the mirror hence the sign of object is taken as negative.
• Since, the centre of curvature and focus lie in front of the concave mirror, so signs of radius of curvature and focal length are taken as negative in the case of concave mirror.
• When image is formed in front of the mirror, the distance of image is taken as – (negative) and when image is formed behind the mirror, the distance of image is taken as + (positive).
• Height of image is taken as positive in the case of erect image and taken as negative in the case of inverted image.

Sign convention in the case of a convex mirror:

• Since, object is always placed in front of the mirror hence the sign of object is taken as negative.
• Since, the centre of curvature and focus lies behind the convex mirror, so sign of radius of curvature and focal length are taken as + (positive) in the case of convex mirror.
• In the case of convex mirror, image always formed behind the mirror, thus the distance of image is taken as positive.
• In the case of a convex mirror, always an erect image is formed, thus the height of image is taken as positive.

Following sign convention is used for measuring various distances during the formation of images by lenses:

• All distances on the principal axis are measured from the optical center.

• The distances measured in the direction of incident rays are positive and all the distances measured in the direction opposite to that of the incident rays are negative.

• All distances measured above the principal axis are positive. Thus, height of an object and that of an erect image are positive and all distances measured below the principal axis are negative.

The following table gives the sign convention for lenses:

Type of lensuvfHeight of the image (hI) Height of the object (hO)

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