Backscatter

when radiation strikes a surface, some of the beam is REFLECTED back again towards the source. This is called backscatter.

Taken from Dictionary of Science

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In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror. Backscattering has important applications in astronomy, photography and medical ultrasonography.

Backscattering occurs in quite different physical situations. The incoming waves or particles can be deflected from their original direction by quite different mechanisms:

• Rayleigh scattering of electromagnetic waves from small particles, diffuse reflection from large particles, or Mie scattering in the intermediate case, causing alpenglow and gegenschein, and showing up in weather radar;

• inelastic collisions between electromagnetic waves and the transmitting medium (Brillouin scattering and Raman scattering, important in fiber optics, see below;

• elastic collisions between accelerated ions and a sample (Rutherford backscattering)

• Bragg diffraction from crystals, used in inelastic scattering experiments (neutron backscattering, X-ray backscattering spectroscopy);

• Compton scattering, used in Backscatter X-ray imaging.

Sometimes, the scattering is more or less isotropic, i. e. the incoming particles are scattered randomly in various directions, with no particular preference for backward scattering. In these cases, the term "backscattering" just designates the detector location chosen for some practical reasons:

• in X-ray imaging, backscattering means just the opposite of transmission imaging;

• in optical fibers, light can only propagate forward or backward. Forward Brillouin or Raman scattering would violate momentum conservation, so inelastic scattering in optical fibers cannot be anything else but backscattering;

• in inelastic neutron or X-ray spectroscopy, backscattering geometry is chosen because it optimizes the energy resolution;

• in astronomy, backscattered light is that which is reflected with a phase angle of less than 90°.

In other cases, the scattering intensity is enhanced in backward direction. This can have different reasons:

• In alpenglow, red light prevails because the blue part of the spectrum is depleted by Rayleigh scattering.

• In gegenschein, constructive interference might play a role (this needs verification).

• Coherent backscattering is observed in random media; for visible light most typically in suspensions like milk. Due to weak localization, enhanced multiple scattering is observed in back direction.

Taken from Wikipedia

Retirement at 16?

By Ally Azzarelli

My earliest penny-counting days date back to the tender age of 7.

Silly as it may sound, I attempted to charge my family to use our bathroom. I hung up a bread bag on the doorknob and solicited my parents for donations. The venture lasted about an hour.

As the years progressed I moved on to selling three or four tickets for dance performances. Those weekly dancing and baton-twirling classes had finally been put to use! I'd round up my father's albums, put on my sequined dance costume and perform a talent show featuring my 3-year-old brother and me.

Truthfully, I can't think of a single show where my brother ever got his one-liner correct! The minute he saw my mother clapping her hands and "wowing," he was off climbing into her lap.

Most kids spent their summers on the beach or in the backyard pool. I, however, could always be found behind a picnic table on our front lawn.

Lucky for me, it was on one of the busiest streets in town. I can remember countless hot summer afternoons spent trying to "rake in a few bucks" selling lemonade or iced tea.

If it wasn't pouring sugary drinks, it was conducting a book and magazine sale. Then, as the months passed, it would be raking leaves, which would soon change into lucrative snow shoveling.

As I got older my strategies proved more successful. Too young to baby-sit alone, I decided to transform our basement into "Allyson's Play School."

Such an idea could not be complete without a strong curriculum including arts and crafts, snack time and show and tell. We even had a sports day (with three ribbons) in the backyard. The profits weren't much, but it was a big step from sweating it out behind an iced-tea stand.

What baffles me is what I--then a child of 10--ever wanted with so much pocket change. I can't recall saving it. In addition to the generous allowance doled out by my parents every Friday, I was always scheming new plans.

My next idea is one that I won't forget. While awaiting the age at which I could be legally employed, 16, I devised a final plan. At that time I was a fanatic of the '60s group, the Monkees. I arranged to put together a newsletter.

Thanks to numerous classified ads and the hundreds of fliers passed out at their beach concerts, this "fanzine" was subscribed to by 168 people across the United States. Charging as little as $5 a year for the quarterly newsletter, I made next to nothing after tallying up photocopying costs and postage stamps.

My motto at that age was: "Wow, I'm havin' fun and meeting new people. Who cares if I'm only making $40 a year?"

But now that I am 16 I should be ready for the business world, right? Wrong!

What really baffles my family is that now that I'm legally permitted to get a part-time job, I prefer not to.

Finally I can go confidently to the candy store owner for a position that has not been filled for months. Why haven't I?

Why is it that now when spending money is so important to a teenager, I am making do with an almost empty bank account? Has a new-found laziness taken over my mind? Not at all, I just feel like I'm ready for a hard-earned retirement plan!

Editor's Note: The author, 16, is from North Valley Stream, New York

Taken from Youth 90 magazine