Lenses

Objective: To observe characteristics of a converging lens when the object is placed on one side of the lens and the real, inverted image is placed on the other side of the lens.

Equipment:

• socket lamp with V-shaped filament
• Large converging lens
• masking tape
• Lens Holder
• piece of cardboard (or other flat surface)
• Track for lens
• Meter stick

Procedure: 

     The focal length was recorded by taking a source that was infinitely far (the sun) and arranging it around to find a point where the image was focused. A meter stick was used to find the distance between the lens and the focused image. This was 0.0485 ±0.0030 m.

     The following was set up by placing the lens into the lens holder and creating a track for the lens (using a meter stick and some stands). 

     The length of the arm of the image from the circle to the end of the line was take as the object image (9.2±2.0 cm). We place the image about 1.5 focal lengths away and used the cardboard to focus the resulting image. The image height and distance from the lens was recorded. From this the magnification could be found by dividing the image height by the object height. (If the lens was rotated the image remained the same). The image was always inverted.

This was repeated for various focal lengths.

	Object Distance	Image Distance	Image Height	Object Height.	Magnification
1	25	6	2.1	12	0.175	±	0.02
2	20	5.4	2.9	12	0.242	±	0.02
3	15	6.4	3.8	12	0.317	±	0.02
4	10	8.6	7.3	12	0.608	±	0.03
5	7.5	7.5	13	12	1.083	±	0.04

All of these values were measured in cm. The object distance and image height values were plus or minus 0.2cm while the image distance was about plus or minus 1.5cm.

Data Analysis: If the object distance was less then one focal point the object height was too great to record. A graph of image distance vs object distance was made and showed a nonlinear relationship. But when we graphed the inverse of negative object distance vs the inverse of image distance we got a somewhat linear relation (within experimental error arising from difficulties in finding the exact image distance). Do to it being an outlyer the fifth data point was removed from the calculation of the plot.

The y intercept was 0.2208 which represents the inverse of image distance as the object distance reaches infinity. This is the inverse of the focus! The relationship between Inverse object distance (x) and inverse of image distance (y) is given in the equation in the above image