## Sunday, May 21, 2006

### Ray Tracing - Lenses

When performing ray tracing, there are three rays that follow simple known paths before and after their refraction by the lens. Use two of the three construction rays to find the image.

• Draw a line from the object to the lens, parallel with the direction of light. At the lens, draw the line through the secondary focal point of the lens.

• Draw a line from the object, through the center of the lens (no deviation at the lens).

• Draw a line from the object to F1 and then to the lens. At the lens, draw the line parallel with the direction of light.

Click on image to enlarge.

Where the lines cross is where the image will be. By doing this, you can estimate the approximate location of the image, tell whether the image is erect or inverted as well as real or virtual.

Figures 14 show the construction of images produced by a plus lens:

Click on image to enlarge.

• Object farther from the lens than F1 , image is real
• Object lies in F1 plane, the image therefore is at infinity

• Object closer to lens than F1 , image is virtual

• Object at infinity, image at F2.

• Object lies to the right of the lens, (a virtual image projected by another optical system), image is real.

Figure 15 shows the construction of images produced by a minus lens:

Click on image to enlarge.

• Object real, image virtual

• Object at infinity, image at F2 plane

• Virtual object in F1 plane, image at infinity

• Virtual object closer to lens than F1, image real

• Virtual object farther than F1 , image virtual