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The following rays are considered while constructing ray diagrams for locating the images formed by a concave lens for various position of the object.
- An incident ray of light coming from the object parallel to the principal axis of a concave lens after refraction appears to come from its focus.
- An incident ray of light passing through the optical centre comes out of the lens without any deviation.
- A concave lens always gives a virtual, erect and diminished image whatever may be the position of the object.
Let us now draw ray diagrams to show the position of the images when the object is placed
- at infinity and
- between O and F1 and
- any position between infinity and O.
When the Object is at Infinity
The image is -
- formed at F1
- erect
- virtual
- diminished
When the Object is placed at any Position between O and infinity
 The image is -
- formed between O and F1
- erect
- virtual
- diminished
It is used
- in spectacles for correcting myopia.
- along with convex lens it is used to overcome defects like chromatic aberration and spherical aberration (the failure of rays to converge at one focus because of a defect in a lens or mirror).
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