Geometric Optics

Simulation Used: Geometric Optics from the PhET at the University of Colorado.

I. Theory:

The lens equation:

gives the relation between the distance from the object to the lens, do, the distance from the image to the lens, di, and the focal length of the lens.

The focal length of a thin lens is given by:

,

where R1 is the front radius of curvature, R2 is the back radius of curvature.

Here, the equations are written according to the following convention:

o Any distance to a real location is taken as positive

o Any distance to a virtual location is taken as negative

o The radius of curvature of a surface is positive if that surface is convex (the reality of the lens lies behind it)

o The radius of curvature of a surface is negative if that surface is concave (the imaginary region lies infront)

The power of a lens is defined as:

,

where f is in meters and P is in Diopters.

II. Initial settings:

1. Select “Virtual Image” and “Ruler” from the options to the right.

2. You can click on the “Change object” button to change the object, but it really doesn’t matter.

3. Set “curvature radius” to 0.65; “refractive indes” to 1.51, and diameter to “0.75” from the slidebars on the top.

4. Select the “Principal Rays” radio button from the top left.

III. Activity: Determine the focal distance of a thin lens and the image distance produced by the lens. Add below a screen shot of the simulation.

Perform the experiment:

i. Drag the object on the left to a distance 140 cm from the front of the lens.

ii. Measure where the image is located. If you measure from the tip of the pencil you will obtain better results. Write down the orientation and the type of image, that is, upright/inverted and real/virtual.

iii. Repeat the procedure for 3 other distances. The values are given in the data table below.

1. In your lab notebook, write down the data in the following format:

do (cm) di (cm) image orientation image type f (cm)

140

120

40

20

2. Results:

Calculate the focal distance of the lens by averaging your results. Give the power of the lens in diopters.

f1 (cm) f2 (cm) f3 (cm) f4 (cm) faverage (cm) P (D)

Acknowledgments: Tatiana Stantcheva, NVCC