Re: 2 Sun Related Questions

Dear Bill,


> 1.  I know a magnifying glass can magnify sunlight to the point
> where it can cause an object to burn, but does a magnifying glass
> also magnify Ultra-Violet light, or for that manner, any other form
> of electromagnetic energy?

A magnifying glass used the way that you describe will focus light
from the large collecting area of the lens to a much smaller spot.
Packing the same light energy into a smaller region increases the
"intensity" of the light.

The ability of a lens to focus light depends on two characteristics:
the curvature of the surface of the lens and the index of refraction
of the material from which the lens is made.  The index of refraction
changes with the frequency, that is the color, of the light.
Therefore, a single lens can not focus all the colors in white light
at the same point; the focal point for red light is further from the
lens than the focal point for blue light.

The part of the electromagnetic spectrum that causes objects to burn
is called "infra-red" (IR) radiation because it has a frequency lower
than that of red light.  A glass lens will focus IR rays as well as
visible rays, but since the index of refraction of glass for IR
radiation is smaller than the index for red light, the focal point for
IR will be further from the lens than the focal point for red light.

A glass lens will not focus ultra-violet (UV) radiation simply because
glass is opaque to UV.  If a lens were made from a material
transparent to UV, then the UV radiation would also be focused to a
small spot.  Other materials such as paraffin can be used as a lens
to focus microwaves -- another part of the electromagnetic spectrum
with a frequency even lower than that of infra-red radiation.

> 2.  How far past pluto would a space ship need to go to not be
> effected by the sun in any manner at all.

Since the forces of gravity and of electricity and magnetism are
infinite in range, a spaceship would in principle always be affected
by the gravity and by the light of the Sun no matter how far the
ship journeyed.  In practice, there are technical limitations to
detecting the Sun's effects at large distances.

--Randall J. Scalise    http://www.phys.psu.edu/~scalise/