In this lab, the student will determine the focal length of a given lens by using an optical bench to make measurements of object and image distances. Students will be asked to make additional observations to further their knowledge of lens properties.

 

 

 

Using your measured object height (h_o), measured image heigth (h_i), measured object distance (s_o), and your measured image distance (s_i), draw a ray diagram for each trial. Although you may scale your values for s_o and s_i, draw your object and image the EXACT heights that you measured in Part I.

 

 

Using two rays, confirm the location of the lens and measure your diagram's scaled focal length. Before reporting your focal length values in the chart below, remember to reproportion them to the original size of the optical bench's measurements. Each member of the group must complete at least one ray diagram. Place that member's name on the top of their diagram's paper.

 

 

 

Using the last setup on your optical bench (h_i > 5.0 cm) take a card and slowly cover the lens while watching the affect of the image.

 

Notice in the following diagram that the two green triangles are similar right triangles.

 

We can therefore state that:

 

Rearranging these ratios gives us the expressions:

 

 

Hence there are two alternative ways to determine the magnification of an image.

 

Using your measured values in the table from Part I, calculate both magnifications for each trial.

 

 

 

 

 

 

Take your lens and place it on the candle printed on your paper below:

 

 

 

Slowly lift the lens above to candle printed on your paper while looking through the lens, until you reach the focal length.