Physics Photo of the Week

November 30, 2007

Simple Lens
A simple magnifying lens inverts the image of a "distant" object.  Hold a simple magnifying lens at arm's length and look at a distant object through it.  You see an inverted image of the distant object through the lens.  The inverted image is called a "real" image.  That is because if a screen (paper, frosted tape, or frosted glass) were placed at the appropriate position between the lens and the viewer, the inverted image would be projected onto the screen.  Without the frosted screen, the viewer sees the image of the inverted object as if the object were originating at the image location between the lens and the viewer.

Focusing the camera to show this inverted image of the distant object is a bit tricky.  The auto focus camera "wants" to focus on the distant objects.  In the image at right, the auto-focus was used to focus on the distant spruce tree.  The image location was between the lens and the camera - so close to the camera that the image of the spruce tree is way out of focus.  To photograph the main picture above, care was taken to focus the camera on the lens itself, not the distant object.  That is a reasonable compromise - both the distant object and the close-up inverted image are in reasonable focus.  The poor focus was not discovered until the image was processed the following day.  Consequently, when I went out to re-do the image, the sky was overcast, and the lack of color renders the overall image not as attractive.

A similar inverted real image exists in all photography.  The lens projects a real inverted image onto the film - or solid state sensor in a digital camera.  Our eyes also work on the same principle.  The cornea and lens of our eyes make up the "simple" lens and the retina provides the "film" - or bioelectronic sensor.  The image in our eyes is real and inverted.

A future Physics Photo of the Week will feature a virtual ("unreal?") image produced by a simple lens.



Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.


Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: