Screen Number 1.
Using what has been learned from the previous refraction experiment (parallel edges), the double convex lens drawing can be set up on a 3'x8" drawing. Prior to starting, set up an experiment consisting of a light bulb, screen and lens with BOTH the screen and bulb CLOSE to the lens (on opposite sides of the lens). The screen and bulb BOTH should be moved EQUAL distances at a time away from the lens in opposite directions until the image is formed on the screen. The image should be the same size as the object, inverted, and REAL. The screen and object should be TWICE the focal length away from the lens as shown above. The drawing also reveals a second method for determining "F": using a light source more than fifty feet away will produce the image at "F" as shown above, too. Objects more than fifty feet away produce nearly parallel incident rays, A & B, or waves with little or no curvature that converge at F.
Screen Number 2.
Draw a line down the center of the 3' paper lengthwise. Construct a double convex lens, 7 cm thick, at the center of the paper with a radius of curvature equal to 19 cm. (RL & RR are approximately 33 cm apart.) The left face of the lens is drawn with the compass from the RR on the right while the right face is drawn from the RL on the left. Draw 2 additional rays of incident light 6 cm above and below and on BOTH sides of the lens but NOT through the lens. Label the points A,B,C, & D where the incident rays of light strike the lens.
Screen Number 3.
Construct normals ARR and BRR just barely visible in order to measure the angles of incidence. Always place the base of the protractor on the normal as shown here to measure the Øi angle. The incident angles, Øi, here are about 28 degrees between the red line (normal) and the blue line (incident ray).
Screen Number 4.
A Dutch scientist, Willebrord Snell, discovered in1621 the relationship shown above the purple line, called Snell's Law. By using n = 1.5 and Øi = 28, Snell's law finds the Ør ~ 18 degrees. Use the values measured with the protractor to plot Ør on your drawing by constructing refracted rays AE and BF. Note that those two refracted rays are BENT TOWARDS the normal when rays of light enter MORE optically dense media.
Screen Number 5.
Label lines AE & BF as refracted rays of light. Now construct the NEW normals RLE & RLF, the red lines shown here. NOTE: those two NEW normals do not go through points C & D.
Screen Number 6.
Note: there are two NEW angles of refraction, Ør = ØE and Ør = ØF shown here ~ 32 degrees. Also note: the blue protractors shown here are smaller than yours and there is a possibility that the new Ør's could be incorrectly measured off of the two incident rays of light on the left side of the lens. Your protractor will have a base radius of 3" which means that lines AE & BF have to be extended outward (blue arrows) towards the left so that those refracted rays will be about 3" long. ØAERL and ØBFRL are NEW Ør's as measured from the normals and are too small to be measured without extending AE & BF out of the lens to the left (the blue arrows). When the NEW Ør's (pink Ør's) are measured, those extended blue arrows should be erased.
Screen Number 7.
Again, using Snell's Law, a NEW Øi = 53 degrees is calculated from that new Ør=32 degrees. The NEW Øi is an angle transmitted out of the lens bending the light ray towards the center line. Note that the two transmitted rays of light are bent AWAY FROM the normals when the ray of light enter a LESS optically dense medium.
Screen Number 8.
At this point in the drawing, rather than clutter up the drawing with a duplication of the right side on to the left side, points can be measured such as "F" and E & F to complete the next picture.
Screen Number 9.
Place the focal point "F" on the left side of the drawing the same distance from the center of the lens as on the right side. Next plot G & H on the lens opposite from E & F. Next draw GC & HD, then draw FG & FH.
Screen Number 10.
Extend GF down to the bottom LEFT incident ray, then extend HF up to the upper LEFT incident ray. Next, extend EF down to the bottom RIGHT incident ray, then extend FF up to the upper RIGHT incident ray. Locate the "Object" and "Image" as shown above. Measure the distance from the center of the lens to the focal point, "F". Then measure from F to 2F and compare those distances.