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

Earthquake! What You Can Do to Protect Yourself

Mention the word earthquake and it strikes fear into the hearts of many. Few natural disasters are as terrifying. Few hit so suddenly and swiftly.

Experts tell us that a big one may soon occur in almost any part of the world. Are you ready for it?

After a major quake, telephone service will probably be cut off. Roads will be destroyed. For the first few hours, the victims will have to fend for themselves. Sometimes help may only arrive days later. To reduce injuries, suffering and even deaths, it is crucial that every family be prepared.

How can you help? Why not discuss the following suggestions with your family over dinner? Experts advice each family to have the following minimum preparations:

1. Large objects should be strapped down or bolted to the wall. Pay particular attention to the water heater. If it tips, it could break a gas line, possibly leading to a fire.

2. Have an evacuation plan for every member of your family during and after an earthquake. During a quake, seek protection under a solid table or by sitting on the floor of a hallway, bracing yourself against the walls and covering your head. Only stand in a doorway that has no door--a door could swing shut and crush your fingers. Do not run outside as you may be hit by falling objects. If you are outside, run to a clear, open space, away form any objects that could fall. As soon as the shock has stopped, walk to a preselected place to meet other family members.

3. Put away at least a three-day supply of food and drinking water. Remember the food you store will go bad after some time. So be sure to change it with a new batch when it reaches its shelf life. If you store canned food, remember to provide a can opener!

4. Have an earthquake kit that includes a first-aid kit, one or more flashlights, a battery-powered radio, extra batteries and a blanket.

5. Each family member who is old enough should know how to turn off the gas, water and electrical supply.

Preparation is the best defense. If you are living in an earthquake zone, think about being prepared. For more information, contact the fire department, an emergency relief agency such as the Red Cross Society or look for special booklets on preparing for an earthquake at your library or bookstore.

- By Keok C. Tan

Taken from Youth 90 magazine