Edited by Sim, Jen Moreau
Is defined as a spherical mirror with a polished inner side.
The type of images formed by a concave mirror depends on the position of the object to the mirror. If the distance of an object say OO' from a concave mirror is changed, then the nature, size, and location of the image are also changed.
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Formation of different images depending on the position of the object are shown below:.
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Categories : Physics
Recent edits by: Sim
Formation of image depends upon the position of the object. There are six possibilities of the position of object in the case of concave mirror.
Since parallel rays coming from the object converge at principal focus, F of a concave mirror; after reflection. Hence, when the object is at infinity the image will form at F. Fig: Object at Infinity When object is placed between infinity and centre of curvature of a concave mirror the image is formed between centre of curvature (C) and focus (F). Fig: Object Between Infinity and C
Object between infinity and Centre of Curvature:
When the object is placed at centre of curvature (C) of a concave mirror, a real and inverted image is formed at the same position. Fig: Object at C When the object is placed between centre of curvature and principal focus of concave mirror, a real image is formed beyond the centre of curvature (C).
Object between Centre of curvature (C) and Principal Focus (F):
Fig: Object between C and F
Properties of image:- Larger than object
- Real and inverted
When the object is placed at principal focus (F) of a concave mirror, a highly enlarged image is formed at infinity. Fig: Object at F When the object is placed between principal focus and pole of a concave mirror, an enlarged, virtual and erect image is formed behind the mirror. Fig: Object between F and P
Object between Principal Focus (F) and Pole (P):
Positions of Object and Image in Concave Mirror Position of ObjectPosition of ImageSize of ImageNature of Image At infinity At focus Point sized, highly diminished Real and inverted Between infinity and C Between F and C Dminished Real and inverted At C At C Same size Real and inverted Between C and F Beyond C Enlarged Real and inverted At F At infinity Highly enlarged Real and inverted Between F and P Behind mirror Enlarged Virtual and erect
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3. Find the size, nature and position of image formed when an object of size 1 cm is placed at a distance of 25 cm from a concave mirror of focal length 20 cm ?
Sol: Here we have been given the object distance and focal length, so first of all we will find out the image distance which will give us the position of image.
(i) Position of image
Here, Object distance, u | = | − 25 cm (To the left of mirror) |
Image distance, v | = | ? (To be calculated) |
And, Focal length, f | = | − 20 cm (It is concave mirror) |
Now, putting these values in the mirror formula : | ||
1/f | = | 1/v + 1/u |
We get: 1/–20 | = | 1/v + 1/– 25 |
(or) – 1/20 | = | 1/v – 1/25 |
(or) 1/v | = | 1/25 – 1/20 |
= | 4 – 5/100 | |
= | – 1/100 | |
So Image distance, v | = | − 100cm |
Thus, the position of image is 100 cm to the left side of mirror or 100 cm in front of mirror (Minus sign shows the left side of mirror).
(ii) Nature of image
Since the image is formed in front of the concave mirror, its nature will be “Real and Inverted”.
(iii) Size of image
To find the size of image, we will have to calculate the magnification first.
The magnification produced by a mirror is given by : | ||
m | = | v/u |
Here Image distance, v | = | − 100cm |
Object distance, u | = | − 25cm |
So, m | = | – (–100) / (– 25) = –4 |
Magnification,m | = | −4 |
We also have another formula for magnification, which is : | ||
m | = | h2/h1 |
Here, Magnification,m | = | −4 (Found above) |
Height of image,h2 | = | ? (To be calculated) |
Height of object,h1 | = | 1cm (Given) |
Now, putting these values in the above magnification formula, we get: | ||
– 4 | = | h2/1 |
Thus, the size of image is 4 cm long. The minus sign here shows that the image is formed below the principal axis.
That is, it is a real and inverted image.
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