Trapping the Light

8/7/2019 Trapping the Light

http://slidepdf.com/reader/full/trapping-the-light 1/4

Revised 1/7/08

Name_____________________________________ Period_________ Date_________

CONCEPTUAL PHYSICS Activity

29.12 Reflection and Refraction: Total Internal Reflection TRAPPING THE LIGHT FANTASTIC

PurposeIn this activity, you will investigate the behavior of light as it passes from one transparentmaterial to another.

Required Equipment and Supplieslight beam viewing tank (or equivalent)opaque white tank insertstirring rodaccess to water access to scattering agent (Mop-N-Glo™, Pine-Sol™, powdered milk, or equivalent) laser pointer

LASER LIGHT WARNING: TO AVOID INJURY, DO NOT EXPOSE EYES TO DIRECTLASER LIGHT. DO NOT AIM THE LASER AT PEOPLE OR ANIMALS.

DiscussionWhen light passes from one transparent material to another, it undergoes refraction. If the lightcrosses the boundary between the materials at an angle, the light changes direction. In thisactivity, you will see how the direction changes when light travels from air to water and whenlight travels from water to air.

Most optical devices — including glasses, contact lenses, cameras, microscopes, and telescopes — rely on lenses that refract light in the manner to be explored in Part A of this activity.

The refraction of light as explored in Part B of this activity is employed in fiber optics. Opticalfibers are used in communication and medical technology.

ProcedurePART A: LIGHT BEAM TANK AND REFRACTION BASICSIf you shine a laser beam at a wall on the other side of the room, you will see the dot where the

beam strikes the wall. The light is reflected diffusely and visible from nearly any place in theroom. But the beam from the laser to the wall is not visible. Air is transparent. So the laser beam

passes through the air without being scattered. If smoke or some other tiny particles are added tothe air, the beam becomes visible. The ever-moving particles reflect the light of the beam in alldirections.

Step 1: Fill the tank about half way with water.

Step 2: Shine the laser into water, aim it through the length of the water and record your observations.

Thanx to

Dean Baird



Trapping the Light Fantastic page 2 of 4

Step 3: Add a small amount of scattering agent to the water and stir to mix it thoroughly.

Step 4: Shine the laser through the water again and record your observations. What — if anythingis different this time? Describe and explain.

Step 5: Shine the laser beam so that it passes from air to water as shown in figure 1 below.

The incident ray is labeled in the diagram. Sketch and label the refracted ray (in the water) inthe figure.

PART B: CRITICAL ANGLE AND TOTAL INTERNAL REFLECTIONStep 6: Move the tank a few centimeters over the edge of the table. Shine the laser beam so thatit passes from water to air as shown in figure 2. Sketch and label· the incident ray (in the water)· the refracted ray (in the air above the water)

· the reflected ray (in the water)

This arrangement is referred to as “subcritical.”

Fi ure 1

laser

incidentray

air

water

Figure 2laser

table

air

water




Step 7: Move and rotate the laser to change the angle of the incident ray as shown in figure 3.

Sketch the result in the figure above and describe it in the space below.

Question: This arrangement is referred to as “supercritical,” The phenomenon it shows is

total internal reflection. Why is this a good name? (Hint: which ray from figure 2 is nolonger represented in figure 3?)

Step 8: Now that you have observed the subcritical and supercritical arrangements, find thecritical arrangement. When the laser beam strikes the water-to-air boundary at the critical angle,the refracted ray travels along the surface of the water.

Going Further

Step 1: Obtain a clear, 2-liter plastic bottle. Cut off the top of the bottle and put a small, circular hole in the side of the bottle about five centimeters from the base of the bottle.

Step 2: Fill the container with water sothat a steady stream flows outward fromthe hole. Best to aim the stream into asink.

Step 3: Shine the laser beam throughthe water to the hole as shown in figure4. What happens to the beam when itgets to the hole? Add a line to the figureand describe below. Figure 4

laser stream

cut-off 2-Lbottle

Figure 3

laser

air

water




Summing Up1. When light passes from air into water, it does not travel continue on its original path. Does it

bend downward to travel deeper into the water or upward to travel at a shallower depth of water?

2. When light passes from water into air, it does not travel continue on its original path. Howdoes it bend?

Documents

Trapping the Light