Question
Form the image in case an object is moved from infinity to the concave mirror.
✓
Answer
i. Object at Infinity. Two cases arise :
a. When mirror is in parallel plane to the object. In such a case, rays from infinity come parallel to principal axis. After
reflection they pass through principal focus F (Rule 1). Image is extremely small, it is real, inverted and at principal focus.
Object at infinity, real extremely diminished image is formed at principal focus.
b. When mirror is inclined so that the rays strike the mirror obliquely. The ray AB passing through F after reflection goes
parallel to principal axis towards BA' (Rule 2). Another ray DE through C striking the mirror at E is reflected back. The
two form an image at A' Image is real, inverted, extremely diminished and at F.
Object at infinity, image at F. It is real, inverted, very much diminished.
c. Object beyond C. A ray AD from A parallel to principal axis after reflection passes through F (Rule 1), Another ray from
A through C, ray AG is reflected back along the same path (Rule 3), forming real, diminished, inverted image of AB is
formed at A'B', between F and C.
Object beyond C, a real, inverted diminished image between F and C, inverted at C and is of same size as that of object.
d. Object at C i.e. at 2f. A ray AD from A parallel to principal axis after reflection from mirror passes through F (Rule 1).
Another ray AD' from A through F, goes parallel to principal axis i.e. towards D'A' (rule 2) forming real, inverted image of
AB at C i.e. at 2f. The image is of the same size as the object.
Object at C, Image is also at C. It is real.
Object between F and C (f and 2f) A ray AD from object going parallel to principal axis is reflected towards F (Rule 1).
Another ray AE through C is reflected back (Rule 3) forming image of A at A'. Similarly image of B is formed at B'.
Image is real, inverted, enlarged and beyond C (2f) i.e. as shown in fig.
f. Object at F. A ray AD parallel to principal axis passes through F. Another ray AE strikes the mirror normally at E is
reflected back as it passes through C (Rule 3). They form image of object AB at infinity. The image is very much enlarged
and is real and inverted.
Object at E. Real, inverted, extremely enlarged image is formed at infinity.
g. Object between F and P. A ray AD from A goes parallel to principal axis after reflection passes through F (rule 1). Another
ray AE striking the mirror normally through C is reflected back (rule 3). They form virtual image of the object behind the
mirror. The image is erect and enlarged.
Object between F and P. An erect, enlarged, virtual image is formed behind the mirror.
Images Formed by a Concave mirror
a. When mirror is in parallel plane to the object. In such a case, rays from infinity come parallel to principal axis. After
reflection they pass through principal focus F (Rule 1). Image is extremely small, it is real, inverted and at principal focus.
Object at infinity, real extremely diminished image is formed at principal focus.
b. When mirror is inclined so that the rays strike the mirror obliquely. The ray AB passing through F after reflection goes
parallel to principal axis towards BA' (Rule 2). Another ray DE through C striking the mirror at E is reflected back. The
two form an image at A' Image is real, inverted, extremely diminished and at F.
Object at infinity, image at F. It is real, inverted, very much diminished.
c. Object beyond C. A ray AD from A parallel to principal axis after reflection passes through F (Rule 1), Another ray from
A through C, ray AG is reflected back along the same path (Rule 3), forming real, diminished, inverted image of AB is
formed at A'B', between F and C.
Object beyond C, a real, inverted diminished image between F and C, inverted at C and is of same size as that of object.
d. Object at C i.e. at 2f. A ray AD from A parallel to principal axis after reflection from mirror passes through F (Rule 1).
Another ray AD' from A through F, goes parallel to principal axis i.e. towards D'A' (rule 2) forming real, inverted image of
AB at C i.e. at 2f. The image is of the same size as the object.
Object at C, Image is also at C. It is real.
Object between F and C (f and 2f) A ray AD from object going parallel to principal axis is reflected towards F (Rule 1).
Another ray AE through C is reflected back (Rule 3) forming image of A at A'. Similarly image of B is formed at B'.
Image is real, inverted, enlarged and beyond C (2f) i.e. as shown in fig.
f. Object at F. A ray AD parallel to principal axis passes through F. Another ray AE strikes the mirror normally at E is
reflected back as it passes through C (Rule 3). They form image of object AB at infinity. The image is very much enlarged
and is real and inverted.
Object at E. Real, inverted, extremely enlarged image is formed at infinity.
g. Object between F and P. A ray AD from A goes parallel to principal axis after reflection passes through F (rule 1). Another
ray AE striking the mirror normally through C is reflected back (rule 3). They form virtual image of the object behind the
mirror. The image is erect and enlarged.
Object between F and P. An erect, enlarged, virtual image is formed behind the mirror.
Images Formed by a Concave mirror
| Position of Object | Position of Image | Size of the Image | Nature of Image |
| At infinity | At focus F | Highly diminished | Real and inverted |
| At C | At C | Same size | Real and inverted |
| At F | At infinity | Highly Enlarged | Real and inverted |
| Beyond C | Between F and C | Diminished | Real and inverted |
| Between F and C | Beyond C | Enlarged | Real and inverted |
| Between P and F | Behind the mirror | Enlarged | Virtual and erect |
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