Concave Lenses: Finding Focal Length

[Dekan]

Does anyone know a simple optical method to determine the focal length (FL) of a concave lens?

 

A method which doesn't require any kind of precision instrument, such a pair of callipers to physically measure the degree of curvature of the glass surface of the lens.

 

In the case of a convex lens, there is such a method. The convex lens converges light rays to a focus, at which an image forms.

 

So all you have to do, to find its FL, is hold the lens against a sheet of white paper. Then move the lens away from the paper, until a distinct image appears on the paper. When the image is at its sharpest, the distance between the lens and the paper, gives you the FL.

 

However, this method can't be used with a concave lens, since it doesn't converge the light-rays, it diverges them. So it won't produce a distinct image on a sheet of paper.

 

The only way I know of finding the FL of a concave lens, is this: Use it in conjunction with a convex lens of known FL, say 10", to make a simple Galilean telescope. The convex lens is the Object Glass (OG) of the telescope, the concave lens is the Eyepiece(EP). Then determine empirically the magnification power of the telescope - by comparing the magnified view to the naked-eye view.

 

The magnifying power is of course the result of the ratio OG/EP. So if the magnifying power is found to be 5X, the concave EP must have FL = 2" - Because 10"/2" = 5.

 

This complicated-sounding method works, but can only be used where the magnifying power is small, so that it's easy to compare the magnified view with the naked-eye view, and so estimate the degree of magnification.

 

I wonder, is there an alternative, simpler, optical method of measuring the FL of the concave lens?

[Klaynos]

If you put a collimated beam in and measure the diverging beam? Not sure it'd work without really thinking about it but my intuition is that it would.

[John Cuthber]

If you put a collimated beam through it you will get a diverging beam. If you trace the edges of that divergent beam back they will meet at a point. The distance from that point to the lens is the focal length.

Have a look at the diagram at the bottom of this page.

http://www.physics.uiowa.edu/~umallik/adventure/geo-optics/lightnw.htm

[swansont]

The collimated light measurement can also be done with a laser that is translated across the center of the lens. That may give a more precise measurement.

[Steobeast 01]

Hi does anyone know what the relationship between the focal length of any two different bi convex lenses and the thickness of those lenses, whilst the radius remains the same

[tamasflex]

Does anyone know a simple optical method to determine the focal length (FL) of a concave lens?

I wonder, is there an alternative, simpler, optical method of measuring the FL of the concave lens?

 

Dear Decan

 

The easiest way to determine the focal length.

But most accurate is when you make a Galilean telescope (known focal length of positive lens) and measure virtual exit pupil with a special optical instrument (what can be done at home).

[tamasflex]

A method more complicated for measuring the focal length.

No matter if lens is simple or achromat.

Edited August 10, 2011 by tamasflex