Image formation by concave mirror
(Image formation by spherical mirror)
To determine the position, nature and size of the image formed by a spherical mirror, it is necessary to consider the two rays of light coming from the object. For this, a ray parallel to the principal axis coming from the object to the mirror and a ray passing through the center of curvature of the mirror is taken for consideration. After reflection from the mirror of these two rays, the image is formed at the point of meeting.
Image formation by concave mirror
(a) When the object is at infinity
The rays coming from an object at infinity, which is parallel to the principal axis, after reflection from a concave mirror, converge at the principal focus.
Position of Image :- Since the rays coming from the object are reflected from the concave mirror and converge at the focus, hence the image is made at focus(F).
Size of Image :- Highly diminished, almost point sized.
Nature of Image :- Real and Inverted.
(b) When the object is at infinity by light rays are not parallel to the principle axis
Position of Image :- Since the rays coming from the object are reflected from the concave mirror and converge at the focal plane.
Size of Image :- Very small in comparison to the object.
Nature of Image :- Real and Inverted.
(c) When the object is beyond Centre of Curvature (C)
When the position of the object is beyond the radius of curvature of a concave spherical mirror, the rays coming from the object after reflection from the mirror pass through the principal focus. In this case the image is formed between the focus and the center of curvature.
Position of Image :- Between the principal focus (F) and the center of curvature (C).
Size of Image :- Smaller than object.
Nature of Image :- Real and Inverted.
(d) When the object is at the Centre of Curvature (C)
Position of Image:- At Centre of Curvature (C).
Size of Image:- Same size as that of the object.
Nature of Image:- Real and Inverted.
(e) When the object is between Centre of Curvature (C) and Focus(F)
Position of Image:- Beyond Centre of Curvature (C).
Size of Image:- Enlarged.
Nature of Image:- Real and Inverted.
(f) When the object is at Focus(F)
Position of Image:- At Infinity.
Size of Image:- Highly Enlarged.
Nature of Image:- Real and Inverted.
(g) When the object is between Focus(F) and Pole(P)
When the object is between the focus(F) and pole(P) of the concave mirror, a virtual, erect and magnified image is formed behind the mirror.
Position of Image:- Behind the mirror.
Size of Image:- Enlarged.
Nature of Image:- Virtual and Erect.
By concave mirror, only in this position, i.e. when the object is between focus(F) and pole (P), the image is made virtual and erect behind the mirror. In other cases the image is real/inverted and formed in front of the mirror.
| Formation of image in different positions of the object by a concave mirror | |||
| Position of the object | Position of the image | Size of Image | Nature of Image |
| At infinity(∞) | At focus(F) | Very small, dot-shaped | Real and Inverted |
| Beyond the center of curvature | Between F and P | Small | Real and Inverted |
| At the center of curvature(C) | At C | Same size as that of object | Real and Inverted |
| Between the center of curvature(C) and the focus(F) | Beyond C | Magnified | Real and Inverted |
| At focus(F) | At Infinity(∞) | Highly Magnified | Real and Inverted |
| Between pole(P) and focus(F) | Behind the mirror | Magnified | Virtual and Erect |
Next Topic :- Image Formation by Convex Mirror.