# MEASURE THE FOCAL LENGTH OF A CONVEX LENS

IBDP                                                                                                               PHYSICS

MEASURE THE FOCAL LENGTH OF A CONVEX LENS

AIM:

Determine the focal length of a convex lens with three different methods and make a comparison among them which one is the best.

EQUIPMENT:

A convex lens, lens holder, cardboard screen, optical bench consisting a meter stick mounted on supports, candle, grid paper, ruler

BACKGROUND:

PART 1

When an object is placed at a distance O from a convex lens with focal length f, then if O＞f, a real image is formed at some distance I away from the lens (see Figure 1). For O＜f, a virtual image is formed where I is negative. A real     image can be focused onto a screen, whereas as a virtual image cannot. The relationship between these quantities is given by the thin lens equation.

http://www.iit.edu/~bcps/database/frontend/resource_center/manuals/physics_223/Lab2-lens.pdf

PART 2

See the graph:

O means the objective length and I means the image length. If O＞2f, f＜I＜2f. If f＜O＜2f, I＞2f.

PART 3

DIAGRAM:

METHOD:

PART 1

1 Set up the equipment as the diagram.

2 Move the candle or the cardboard screen until the image of the candle can be seen clearly on the screen.

3 Write down the objective length and image length.

4 Repeat from step 2 to step 3 four ...