Aim: To study the nature and size of the image formed by a convex lens using a candle
and a screen.
APPARATUS
Aconvex lens, a candle, a screen, three uprights, an optical bench, metre scale.
THEORY
The nature of the image formed by a convex lens depends on the position of the object with respect to the
lens. The image may be real or virtual. Convex lens produces real image for object at or beyond the focus. For
object between O and the image is virtual and erect.
The positions of the object and the relevant images are shown.
(i) When the object is at infinity (i) When the object is beyond 2F'
Convex Lens Convex Lens
A 2F
2F F
(üi) When the object is at 2r (iv) When the object is between Fand 2F
Convex Lens
Convex Lens
2F
2F A'
A
2E A
(o) When the object is at F"
(vi) When the object is between F'and O
Convex Lens
Convex Lens
B'2F
DIAGRAM
Screen Convex Lens
Image
Candle
10 20-U 30 40 50 607 70 80 9d100
cm
PROCEDURE
1. Obtain rough focal length of the given convex lens by obtaining a sharp image ofa distant object on a
screen. Set the lens in the middle of the optical bench.
2. Keep the candle and the screen on either side of the lens. Adjust the height of the candle and screen so
that they are on same horizontal level as the optical centre of the lens.
3. Mark the positions of F and 2F on the optical bench on either side of the lens.
4. Light the candle and keep it at maximum possible distance from the lens. The distance between the lens
and the candle should be more than double of its rought focal length. Change the lens if required.
Adjust the screen to be at the focus on the other side of the lens and obtain a clear image of the candle
flame on the screen with minor adjustments. The image is real, inverted and very small.
5. Move the candle closer to the lens so that it is beyond 2F, at 2F; between F and 2F and at F. Adjust the
position of the screen each time to get the clear image of the candle flame. Record the nature, size and
position of the image in the following table.
RESULT
The nature and size of the image formed by a convex lens is as given below :
S. No. Position of object Position of image Size of image Nature of image
At infinity At F Highly diminished Real and inverted
2. Beyond 2F" Between F and 2F Diminished Real and inverted
3. At 2F" At 2F Same size Real and inverted
4 Betweenf and 2F Beyond 2F Large Real and inverted
5. At F At infinity (very far) Very large Real and inverted
6 Between F and lens On the same
side of lens
Enlarged Virtual and erect
(not on screen)
It is significant, that even for a virtual image, its real image always
appears on the retina of the eye.
cONCLUsION
As the object moves closer to convex lens, the image moves
away from the lens and its size increases.
