</noinclude> <!-- 5. Find the focal length of at least four lenses, one being a negative lens. --> [[Image:FocalLength.jpg|thumb|400px|Finding the focal length of a positive lens]] ===Positive Lens=== Set a ruler on end on a work surface (a table, sidewalk, or a wood board). Using a bright light (such as the sun) as the light source, hold the lens against the ruler and slide it up and down the ruler until the light is focused on the work surface. The point on the ruler that the focused lens touches is to be recorded as the Focal Length for that lens. {{clear}} ===Negative Lens=== [[Image:Sistema afocale 2.png]] {{clear}} Measuring the focal length of a negative lens is more difficult because light going through it does not converge to form an image. Instead, you will need to place a positive lens between the light source and the negative lens. Move the lenses and screen around until the light is focused.
