[link] shows a convex lens. Light rays traveling through a
convex lens are bent
towards the principal axis. For this reason, convex lenses are called
converging lenses.
When an object is placed in front of a lens, the light rays coming from the object are refracted by the lens. An image of the object is produced at the point where the light rays intersect.
The type of images created by a convex lens is dependent on the position of the object. We will examine the following cases:
the object is placed at a distance greater than
from the lens
the object is placed at a distance equal to
from the lens
the object is placed at a distance between
and
from the lens
the object is placed at a distance less than
from the lens
We examine the properties of the image in each of these cases by drawing ray diagrams. We can find the image by tracing the path of three light rays through the lens. Any two of these rays will show us the location of the image. The third ray is used to check that the location is correct.
Experiment : lenses a
Aim:
To determine the focal length of a convex lens.
Method:
Using a distant object from outside, adjust the position of the convex lens so that it gives the smallest possible focus on a sheet of paper that is held parallel to the lens.
Measure the distance between the lens and the sheet of paper as accurately as possible.
Results:
The focal length of the lens is
cm
Experiment : lenses b
Aim:
To investigate the position, size and nature of the image formed by a convex lens.
Method:
Set up a candle, and the lens from Experiment Lenses A in its holder and the screen in a straight line on the metre rule. Make sure the lens holder is on the 50 cm mark.
From your knowledge of the focal length of your lens, note where
and
are on both sides of the lens.
Using the position indicated on the table below, start with the candle at a position that is greater than
and adjust the position of the screen until a sharp focused image is obtained. Note that there are two positions for which a sharp focused image will not be obtained on the screen. When this is so, remove the screen and look at the candle through the lens.
Fill in the relevant information on the table below
Results:
Relative position of object
Relative position of image
Image upright or inverted
Relative size of image
Nature of image
Beyond
cm
At
cm
Between
and
cm
At
cm
Between
and the lens
cm
QUESTIONS:
When a convex lens is being used:
A real inverted image is formed when an object is placed
No image is formed when an object is placed
An upright, enlarged, virtual image is formed when an object is placed
Write a conclusion for this investigation.
Experiment : lenses c
Aim:
To determine the mathematical relationship between
and
for a lens.
Questions & Answers
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