An arrow/disc object baffle lit from behind by a bright point source of light is used as the object, and various convex lenses are used to cast images of the key onto a screen. The effect of variation in focal length and object distance can be readily seen. Lenses with 10cm, 20cm, and 30cm focal length are provided.

Combinations of two convex lenses or a concave and a convex lens used together can also be demonstrated. Please request additional lenses if desired.

Parallel rays entering from the left in the photograph at the left above are focussed to a point by a plexiglass double convex lens. At the right above the same rays are shown displaced upward by rotation of a rectangular plexiglass block, but without the focusing lens.

Q: What will happen to the transverse (vertical) position of the focal point when the rectangular block is rotated while the lens remains at the same point? Will the focal point be displaced upward, be displaced downward, or remain at the same lateral position? A: The focus will remain at the same point, as seen in the photographs below. In fact, all rays parallel to those shown in the photograph will focus at the same point, in the absence of aberrations. This is a fundamental property of how lenses act to focus light.

The parallel rays are formed by a second double convex lens out of the photograph to the left.

Two 20cm convex lenses are available: a simple spherical lens and an achromat. Light from the point source is focused onto a slit by a cylindrical lens placed immediately in front of the source, which in turn is imaged by the 20-cm spherical lens onto a distant screen, as shown in the photograph at the left above. The screen is then rotated as shown at the right above to increase the dispersion at the image.

The image of the achromat, shown at the right below has virtually no chromatic aberration compared with that of the ordinary simple lens, seen at the left.