MIDDLE SCHOOL EXPERIMENT: Light Beams!

Introduction:

Active remote sensors send out pulses of energy (for example, light or radio waves) to sense what is happening at remote locations. At the NOAA/Environmental Technology Laboratory, radars and lidars (laser radars) are active remote sensors which allow us to study winds, clouds, pollution, and the ocean. Lidars emit laser pulses which are absorbed, scattered and refracted by the atmosphere on their way back to the sensor.

National Science Education Standards Addressed:

  • Abilities Necessary to DO SCIENTIFIC INQUIRY/UNDERSTANDING About SCIENTIFIC INQUIRY
  • PROPERTIES and Changes of Properties in MATTER/STRUCTURE and FUNCTION in living Systems
  • ABIliTIES of Technological Design/UNDERSTANDING About Science and Technology
  • PERSONAL HEALTH/Science and Technology in SOCIETY

Materials Needed:

  • small flashlight with a narrow beam,
  • low power (Class 3) penlight laser pointer mounted on a wooden block,
  • 1 square bottle,
  • 1 round bottle,
  • water,
  • cocoa powder,
  • gelatin (clear, red and green) in petridishes (use 1/2 the recommended amount of water),
  • 2 sheets of paper,
  • white card,
  • black card,
  • ruler.

WARNING:

Be sure that you NEVER point the laser beam directly at your or any of your classmates eyes. Be careful not to bounce the beam off a shiny object into your classmate s eyes. Exposure to direct laser light can cause blindness, even if it is reflected light. The laser beam should point horizontally, well below eye level.

Investigation:

Contrast laser light with white light from a flashlight. Observe the effects of various materialsand container shapes on a light beam (optical scattering, absorption and refraction).

Activities I. The flashlight versus the laser.

Flashlight. What is the purpose of the reflector (silvery bowl) around the flashlight bulb? Test your ideas by shining the flashlight with and without the reflector at a white card.

The flashlight versus the laser. Move a white card forward and back along the path of both the flashlight beam and the laser beam. What happens to each beam s diameter as it gets farther away from the light source? We say that a light beam is collimated if its diameter remains constant as you go to further distances. Is one of the beams close to being collimated?

Beam expansion -- find the light source! Lay the flashlight on a table so that its beam travels parallel to the table; raise the back end if needed. Fold a piece of paper in half and lay it on the table with the creased edge against the flashlight. Slowly raise the paper and see cross-sections of the beam. Hold the paper parallel to the table with the creased edge slightly below the center of the flashlight. Draw two lines which trace the diverging outside edges of the flashlight beam. Now, unfold the paper and with a ruler, extend the lines until they cross. What is the position marked by the intersection? Check the flashlight to see if your measurement looks reasonable.


Activities II: Absorption, scattering and refraction.

Put some water in the square bottle. Shine the flashlight through the water. Set the white card behindthe container to view the beam. What do you see in the container and what do you see on the card? Stir a pinch of cocoa powder (or a drop of milk) into the water. What do you see in the container and what do you see on the card? Why? Repeat the process with the laser beam. Try other liquids and materials. What happens to sunlight on a dusty day?

In which of the colors of gelatin can the flashlight and laser light be seen best? Why? What role does the gelatin color play? What role does the gelatin material play? How well do you see the beam on a white card after the gelatin? What would you expect if there were some cocoa powder (or milk) in the gelatin? How do sunglasses work?

Hold the white card and then the black card in the laser beam. Which makes a brighter spot? Why?

Shine the laser through the curved side of the petri dish with red gelatin. How does the laser beam appear in the gelatin and on the card as you move the laser from the outside edge to the center? What happens when you shine the laser through the base of the petri dish? Why? Try shining the laser through the round bottle filled with water and a little cocoa powder. What everyday object works this way?