Table 10.1 contains image formation by a concave mirror when the object is placed at different places.in this topic we will cover the image formation with ray diagram.
On the basis of the table, we will discuss all the position of object one by one
Object is placed before the concave mirror
(i) image formed by concave mirror when object is at infinity
We know the sun or the stars are at infinity so these can be our object.
All the rays from these objects come parallel to the principal axis.
Parallel incident rays meet at Focus point after reflection.
Position of image – At Focus point (F)
Nature of image – Real and inverted
Size of image – Highly diminished
(ii) image formed by concave mirror when object is at focus point (F)
Imagine an object (↑) is placed at focus point of the concave mirror.
A light ray from the point ‘A’ of the object incident parallel to the principal axis and the second ray from the same point (A) incident at the certain angle.
After reflection, The parallel incident ray passes through the focus point (F) after reflection.
The second incident ray passes through point C after reflection.
Both the reflected rays meet at infinity so the image is formed at infinity.
Position of image – At infinity
Nature of image – Real and inverted
Size of image – highly enlarged
(iii) image formed by concave mirror when object is beyond C
Now we place the object beyond C (between infinity and C)
A light ray from point ‘A’ of the object incident parallel to the principal axis on the concave mirror and the other light ray from the same point (A) incidents on the mirror after passing through the centre of curvature (C).
The parallel incident ray passes through the focus point (F) after reflection.
The second incident ray goes back on the same path.
Both the reflected rays meet between F and C , so the image is formed between focus(F) and centre of curvature (C).
Position of image – Between Focus point (F) and C
Nature of image – Real and inverted
Size of image – diminished or smaller than the object
(iv) image formed by concave mirror when object is between C and F
Now place the object between C and F in front of the concave mirror.
A light ray from point ‘A’ of the object incident parallel to the principal axis on the concave mirror and the second light ray from the same point (A) incidents on the mirror at certain angle.
The parallel incident ray passes through the focus point (F) after reflection.
The second incident ray passes through point C after reflection.
Both the reflected rays meet beyond C (between C and infinity) that’s why the image is formed beyond C.
Position of image – Beyond C
Nature of image – Real and inverted
Size of image – enlarged (bigger than object)
(v ) image formed by concave mirror when object is at C
We place the object at centre of curvature (C)
A light ray from point ‘A’ of the object incident parallel to the principal axis on the concave mirror and the second light ray from the same point incident on the mirror after passing through the focus point (F).
After reflection, the parallel ray passes through focus point (F) and the other ray becomes parallel.
Both reflected rays meet at ‘C’ . so image is formed at C.
Position of image – At centre of curvature (C)
Nature of image – Real and inverted
Size of image – same size of the object
(vi) image formed by concave mirror when object is between F and P
Now place the object between focus point and pole of the concave mirror.
A light ray from point ‘A’ of the object incident parallel to the principal axis on the concave mirror.
The second light ray from the same point (A) incidents on the mirror at certain angle at pole (P).
The parallel incident ray passes through the focus point (F) after reflection.
The second incident ray gets reflected at the same angle as of angle of incidence (here )
Both the reflected rays never meet with each other but when we extend the rays behind the mirror, they appear to meet on the principal axis.
So image is formed on the upper side of the principal axis.
Position of image – Behind the concave mirror
Nature of image – Virtual and erect
Size of image – Enlarged
Frequently Asked Questions
1.Is image formed by concave mirror laterally inverted?
Ans – Yes, image formed by concave mirror is laterally inverted
2.Where is image formed in concave mirror?
Ans – Position of the image formed depend the position of the object placed before the concave mirror.
3.What type of image is formed in concave mirror?
Ans – Image formed in concave mirror can be real or virtual depending on the position of the object before the concave mirror.
4.How does a concave mirror form an image?
Ans – A concave mirror forms an image through the reflection of light rays. The image is formed where the reflected rays meet each other.
5.Why inverted image is formed in concave mirror ?
Ans – An inverted image is formed when the object is place beyond the focus point (F) and the reflected light rays converge and cross each other downside the principal axis.
- How to remember image formation by concave mirror?
Ans – when the object is placed beyond the focus point (F) then the image is real and inverted because the reflected rays converge and cross each other.
When the object is placed before the focus point (F) near the mirror, the image is formed virtual and erect
MCQs
1.Where is the image formed when the object is placed at infinity?
- At focus point
- At C
- Beyond C
- Between C and F
Correct answer – (a)
2.If the image is formed between C and F then what is the position of the object?
- At infinity
- At focus point
- Beyond C
- Between F and P
Correct answer – (c)
3.When the object is placed beyond C in front of the concave mirror, the image is
- Real and inverted
- Real and erect
- Virtual and inverted
- Virtual and erect
Correct answer – (a)
4.A concave mirror forms a virtual and erect image when the object is located
- At focus point
- Between F and P
- Beyond C
- At infinity
Correct answer – (b)
5.An image formed by the concave mirror is same size of the object when the object is
- At focus point
- At centre of curvature
- Beyond centre of curvature
- At infinity
Correct answer – (b)