Lenses

PHYS 242 Lenses Lab Post-lab Knowledge Check

In order to predict the location of an image we can draw rays from the object using the path the light takes. The figure on the next page shows a scale diagram with a filament bulb as our object and a lens with a focal length of 50cm. Each square in the grid represents 10 cm X 10 cm.

A. Place an X on both sides of the lens indicating the position of the focal point.

B. We will consider light from the top of the long filament (the point is shown on the diagram). We know that light will travel in all directions from each point on the long filament, but for our model we will first consider three of those rays called the principle rays. Sketch the principle rays using the following rules (be sure to use a ruler):

• Ray 1 is drawn from the object (the top part of the filament) to the lens along a line parallel to the principle axis. At the middle of the lens this ray bends and travels in a straight path through the focal point.

• Ray 2 is drawn from the object along a line through the center of the lens and does not bend.

• Ray 3 is drawn from the object through the focal point to the lens. This ray bends at the middle of the lens and continues along a straight path along a line parallel to the principle axis.

Light bends when it strikes the lens (not in mid-air)! For simplicity in our drawings we can bend all of the rays at the middle of the lens.

C. Show where the image forms. Compare the image distance with the value you would calculate from the Thin Lens Equation.