FOCAL LENGTH OF A CONVEX LENS DISPLACEMENT METHODS

TITLE: FOCAL LENGTH OF A CONVEX LENS DISPLACEMENT METHOD

APPARATUS: 

1 . Convex lens of focal length 10 – 50cm.
2 . Lamp box with cross wires as object.
3 . Screen.
4 . Ruler.

AIM OF EXPERIMENT:

To determine the focal length of a convex lens using the displacement method. To understand the major sources systematic and random error and comparing the measured with the known value.

THEORY:

A convex lens is thicker at the centre than at the edges. Rays of light that pass through the lens are brought closer together I.e they  converge. It is a converging lens. When parallel rays of light passes through a convex lens the refracted rays converge at one point called the principal focus.

In this experiment the focal length of a lens is found out. The focal lens of a lens is a distance from the principal focus to the optical centre. It is the point where the image comes into focus when seen through the lens. The focal length of a lens depends on the curvature of each surface. So the more highly curved the surface, the shorter is the focal length and vice versa.

     In this experiment the lens is first placed between the screen and the object. When the illuminated object comes in focus on the screen the distance from the illuminated object is recorded. Then the lens is placed at a position where the object again comes into focus on the screen. The distance from the object is recorded and therefore the distance.

The general lens formulae is as follows.

I/F =I/U + I/V

U = OBJECT DISTANCE

V= IMAGE DISTANCE

F= FOCAL LENGTH OF A SPHERICAL LENGTH

When object O is separated a distance D (It should be at least 4X the focal length of the lens) from its real image. I two positions A and B of the lens exist for which the real images are formed. The images formed at positions A and B and magnified and diminished respectively.

O and I interchangeable and are referred to as 

CONJUGATE POINTS

From the diagram above, it is obvious that;

PROCEDURE:

a . Determine the approximate focal length of your lens by focusing a sharp image of a distant object on the screen, the window in the laboratory may be considered a distant object.

b . Place the lens between the object and the screen. The object and the screen should be at least 4X the focal length of the lens apart; record this distance as D and keep it constant. Locate the two positions of the lens for which sharp images of the object are formed on the screen. Measure the distance of through which the lens is moved from the first to the second position.

MEASUREMENT:

GRAPH

SLOPE