[Phys-L] Re: Ray Diagrams

[jbellina at SAINTMARYS.EDU (jbellina)]

I agree that students need to understand that the image is formed by
many rays other than just the so-called "principle rays" The work on
projecting light through a hole allows students to discuss how an
entire spot is formed, even though they can use geometry to predict the
size of the spot just using the rays on the edge.  That leads nicely
into all the rays forming the image when one uses a lens.
The principle rays are important because we can predict accurately,
without complicated calculations, what their path is.  That is why they
are important...then of course you can use geometry to do some
quantitative calculations.  So it is not the geometry piece, but rather
knowing the paths that is important.

cheers,

joe


On Aug 9, 2005, at 8:38 PM, Chuck Britton wrote:

> Please, when 'doing' ray diagrams, include MORE rays than the
> requisite 'principal' rays.
> The principal rays are only 'special' because we can do the geometry
> with them easily.
> I think that it is important to emphasize that ALL rays that share
> the same origin as the principal rays do - will end up at the same
> point as do all of the principle rays.
> It's awful easy to JUST do the principle rays - and this sometimes
> makes these rays seem overly 'special' or somehow the 'only' rays
> that behave in this manner.
>
> > I have found ray diagrams less then obvious to students unless
> > you spend some time working with them in a simple context.  I use
> > McDermott's...no relation??...material on projecting light through
> > holes to do this.  We take ray diagrams as simple and straight (pardon
> > the pun) forward.  My experience is that they are not to the
> > uninitiated.
Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556