# Monthly Archives - November 2010

## Reflection & Refraction

One day in my classroom…“My students were amazed by examples of colorful fiber optics but didn’t understand how the light goes from one end to the other; even when bent. After testing various angles of laser light through water and air up to 40 degrees, most students assume the light will continue to exit the other side. When the angle is increased to 50 degrees or greater, the light ray doesn’t come out the other side. Students finally see how fiber optics work through the phenomenon of Total Internal Reflection and have a great time exploring with this easy to set up lab!”Lori Andersen is a Physics and AP Physics teacher at Grafton High School in York County, VA.

Can a Trapezoid Trap Light?

Purpose: To discover the conditions that result in total internal reflection in a transparent material.

Summary: Students use the Laser Ray Box and trapezoidal lucite block to find situations that result in total internal reflection.

Question: How can a transparent medium be used to reflect light?

Materials: Laser Ray Box, trapezoidal block, graph paper, pencil, ruler, protractor.

Process: Ask students to predict what will happen if the laser light is directed toward the trapezoid along the normal to each side. Have each lab group sketch the predicted path of the laser beams.

Students trace the outline of the trapezoid on the graph paper. Use the protractor to draw a normal for each side of the trapezoid. Students point the laser along the normal. Trace the path of the rays (approaching the trapezoid, inside the trapezoid, and leaving the trapezoid) with a pencil and ruler. For each ray, use the protractor to measure and label the angles of incidence and refraction (or reflection) on the graph paper.

Did the observations agree with predictions? Encourage students to explain the differences between their predictions and their observations. Challenge students to find the critical angle for the lucite.

Results: Students will notice that for the angled sides, total internal reflection is observed when the laser light reaches the bottom side. This does not occur for the parallel sides.

Discussion: When light goes from lucite to air at an angle greater than the critical angle, there is no refracted ray, and all the light reflects back into the lucite! Show students using Snell’s Law why this occurs. Introduce examples of fiber optics applications that utilize this phenomenon, such as laparoscopic surgery and fiber optic communications.

Required Equipment
Laser Ray Box, trapezoidal block, graph paper, pencil, ruler, protractor.

Recommended quantity per lab group: 1

## Laser Ray Box and Lenses

In Stock SKU: P2-7680
\$99.95

## Magnetic Fields around Magnets

 Students will make observations of bar and horseshoe magnets, using iron filings or magnetic chips to map the field. The filings will be close together where the field is strong, and farther apart where it is weak. Lines will form, following the lines of the field, from the north pole to the south. Required Equipment Bar Magnets, Horseshoe Magnet, Magnetic Chips, Transparency Sheets Download Teacher Notes and Student Worksheets

## Steel Horseshoe Magnet with Keeper

In Stock SKU: P8-1128
\$3.95

## Magnetic Chips 50g/vial

In Stock SKU: P8-1140
\$12.00

## Pair of Steel Bar Magnets

In Stock SKU: P8-1125
\$5.90
 The “Transparent Sheets” required for this lab is readily available at your office or school supply store. Each lab group would need one sheet.

## The Firing Squad Demo… It’s Physics!

Every day we have to make choices. Sometimes these can be life changing. In the Firing Squad Demo the student you pick to be the target, will have to choose between two dart guns to be shot with!

One of the guns has a steel ball glued to the dart. Standing approximately ten feet from the two loaded guns he or she will need to think like a physicist to make the right choice. Both guns, being essentially identical, push the two different-mass darts with the same force. Which dart will they choose to be shot with?

Here the distinction between the inertia property of mass and its gravitational property is illustrated.

## Dart Gun

In Stock SKU: P4-1301
\$3.25