There are only two possibilities of position of object in the case of a concave lens:-
(a) Object is at infinity
(b) Object is between optical centre, O and infinity
A highly diminished point sized, virtual and erect image is formed when object is at infinity by a concave lens at principal focus F1.
Position of image:- At F1
Nature of image: Virtual and erect
Size of image: Point sized, highly diminished.
A diminished, virtual and erect image is formed between principal focus F1 and optical centre, O; when object is placed between optical centre and infinity of a concave lens.
Position of image:- Between F1 and O
Nature of image:- Virtual and erect
Size of image:- Diminished.
Sign convention for lens is similar to that of spherical mirror. Signs are taken left of the optical centre as negative, right of the optical centre as positive, above of the principal axis as positive and below of the principal axis as negative.
The new sign convention is known as New Cartesian Sign Convention. In this sign is taken negative towards left and taken as positive towards right at X-axis from origin.
The sign is taken as positive (+) above the origin point at Y-axis; and below the origin point as negative (-) at Y-axis.
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