PURPOSE: To calculate the position of a virtual image and the verify this by using parallax.

DESCRIPTION: A convex mirror and a concave lens with the same focal length are positioned one focal length apart and the object post is positioned one focal length in front of the lens. The location of the image, behind the mirror, can be calculated and then verified experimentally using this demonstration. The picture at the top right views down along the optic axis of the setup to show the optical elements as they are aligned.

The image is located at the position of the rod at the right end of the optical rail above (behind the mirror). This can be verified by parallax viewing between the image and the rod at that position, as seen in the photographs above. The three pictures directly above show the parallax alignment of the image with the alignment post, as seen from along the optic axis and from each side. In the center picture you can see the unfocused object wire as a blur in front of the lens. Note that you want to align the image post with the image seen in the mirror directly - not the image as seen through the lens, which can be seen near the bottom in the pictures.

SUGGESTIONS: This was a question on an examination for engineering physics, after which the students viewed the image using this setup to see that the system actually works!

REFERENCES: (PIRA unknown.) See Demonstration Reference File for further information, including the exam question and the complete solution.

EQUIPMENT: Parallax rod object, 27.3 cm focal length convex mirror, 30 cm focal length concave lens, and parallax rod to determine image position.

SETUP TIME: 5 min.