Natural Science Forum / Physics / Optics / September 2005

Just hold the magnifying glass far enough away.  But you won't get anywhere
near the intensity because it isn't large enough to capture that much light.

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The sun is 1/2 degree in diameter so you just need to run the basic
geometrical math to get the image to 1mm.  the Chief ray (the one that goes
through the center of the lens) is the one you want to do the math on.

--
Why do penguins walk so  far to get to their nesting grounds?

E Lasse Madsen:

Hi.
there are 2 problems u have to think about.
1st- image magnification.
since lamp filament has dimension,(for example 6X6 mm) your lens must
reduce it to the spot diameter u need.
for example if the lens configuration magnification is M=0.167 ,image
spot will be 6X0.167=1 .It is possible to have such magnification with
any positive lens with focal length f using the good old formula
         1/f=1/u+1/v ,
 where u is lens to filament distance,
  v is lens to wall distance ,
  and M=v/u.
for the above  example  if u=600mm,v=100mm and lens focal length is
85.7mm ,your spot size is exactly what u need, (neglecting abberations
which might have a small effect here).

2nd- image intensity depends on the ratio between lens focal lenght and
its  aperture diameter called F# , as smaller this number is ,the
better, meaning ,if you need higher spot intesity ,choose the lens with
the lower F# you can get.
Have fun!
Zvini

Try a 4X microscope objective about 150mm from the filament.