Intensity of Light

Teacher Instructions

Background Information:

    In this lab students are to discover the relationship between the brightness of a light and the distance from the light. Most people realize from experience that the brightness of a light seems to decrease as as they move away from the light. This is true but doubling the distance from a light source does not halve the amount of light as many people initially believe. As you move away from a light the brightness decreases by the square of the distance, which means if you double your distance from a light source you will receive one quarter as much light, and if you triple your distance you will receive one ninth as much light.
Important Information:
  • Students should be made aware of two different ways of looking at the brightness of a light.
    • How luminous a light is - how much light an object emits.
      The sun is a luminous body.
    • How illuminated an object is - how much light an object is receiving.
      The moon is an illuminated body.
  • Emphasize in this lab they will be experimenting with distance from a light source compared to how illuminated a body is.
Suggested Pre-lab activity
  • As an introduction to the lab allow the students to explore the concept of light brightness by using the light probe to measure the light levels in various parts of the room. (For this activity set up the calculators and CBL's as instructed in Calculator Set Up but use MONITOR INPUT instead of TRIGGER PROMPT)
  • Use a discussion of the students measurements to introduce the "Big Question".
  • Once the "Big Question" is presented allow the students to formulate their hypothesis. However make them be specific. Just stating the light gets dimmer as distance increases is not good enough.
Set-Up:
    The set - up could be given, determined as a class or left up to the individual student groups.
Procedures:
    The procedures could be determined as a class, left up to the individual student groups or the suggested procedures on the optional Student Instruction sheet could be used.
Data / Observation:
  • Students should record their data on their lab sheets as they perform the experiment.
  • They can also display their data on the computer by selecting "Data Table A" under the "Windows" menu in Graphical Analysis OR in the LoggerPro protram.
Calculations:
    Students should find a mathematical relationship between the amount of light and the distance from the light source.
    You should allow the students some freedom here. They could use either of the following methods:
    1. Trial and error.
    2. Transfer the data into Graphical Analysis or LoggerPro either manually or by using a link cable.

Conclusions:

  • Hopefully the students will discover that amount of light = 1/ distance squared.
    Whatever their conclusion it is most important that it is supported by their data and their calculations.
Questions:
  • Discuss questions raised by the students.
  • Assign students to find answers to questions raised.
Notes:
  • Candles seem to work better as a light source but do produce a major fire hazard.
    If light bulbs are used a small flashlight bulb in a base works best. Do not use a flashlight because the reflector will destroy the relationship we wish to see.
  • A larger bulb (20 - 100 watt) can be used. Place the bulb in a shoe box and cut a small round hole in one end to allow the light to exit as a point source of light.
  • The room should be kept as dark as possible and groups should be separated as much as possible to reduce ambient light that will confuse results. (Try to allow your students to determine this when discussing how they will set up their experiment)

Student Instructions for CBL with PHYSICS program

The Big Question:

    How is the distance from a light related to the brightness of the light?
Equipment / Materials:
  • Calculator
  • CBL
  • Light probe
  • PHYSICS program group
  • Meter Stick
  • Light Source (either candle or small light bulb - candle seems to work best)
Calculator Set Up
  • Set-up the calculator and CBL with the light probe in Channel 1. (The calculator should have the PHYSICS program group loaded)
  • Press the PRGM button and select PHYSIC. Press "ENTER" until you reach the MAIN MENU.
  • Select "SET UP PROBES"
  • Enter 1 for the number of probes.
  • Select LIGHT from the SELECT PROBE menu.
  • Enter 1 as the channel number.
  • Select COLLECT DATA from the MAIN MENU.Select TRIGGER/PROMPT from the DATA COLLECTION menu.
Set-Up:
  • Place the light source at one end of the meter stick.
  • Hold the light probe 20 cm from the light source pointing directly at the light.
Procedures:
  • Follow the directions for calculator set-up.
  • Place the light probe 20 cm from the light source.
  • Press TRIGGER on the CBL.
  • Type in 20 for the distance of 20 cm and press ENTER.
  • Move the light probe to 30 cm from the light source. Again when the reading stabilizes press TRIGGER on the CBL then type in 30 and press ENTER.
  • Continue increasing the distance of the light probe from the light source by 10 cm and recording data until you have collected data out to 100 cm.
  • You can then view your graph by selecting 2 for STOP AND GRAPH under the DATA COLLECTION menu.
Data / Observation:
  • Record your data on your lab sheet as you perform the experiment.
  • You can also find your data in L1 and L2 on the calculator.
Calculations:
    Find a mathematical relationship between the amount of light and the distance from the light source.
    You can use either of the following methods:
    • Trial and error.
    • Transfer the data into "Graphical Analysis" either manually or by using a link cable.
    Record any calculations on your lab sheet, attach graphs, or record results from your fit from the computer.
Conclusions:
  • Record your conclusion on your lab sheet.
  • Support your conclusion with your data and your calculations.

Student Instructions for CBL2 or LabPro with DataMate

The Big Question:

    How is the distance from a light related to the brightness of the light?
Equipment / Materials:
  • Calculator
  • CBL2 or LabPro
  • DataMate program
  • Light probe
  • Meter Stick
  • Light Source (either candle or small light bulb - candle seems to work best)
Calculator Set Up
  • Connect the calculator and CBL2 with the light probe in Channel 1. (The calculator should have the DataMate program group loaded)
  • Press the APP button and select DATAMATE.
  • The calculator should automatically recognize the light probe. If it does not, select SETUP (1), select CHANNEL 1 and choose the correct probe. Press OK until you are back to the main menu.
  • Press 1 to enter the SETUP menu and choose
  • Select COLLECT DATA from the MAIN MENU. Move the cursor down to MODE and press enter.
  • Select EVENTS WITH ENTRY and press enter until you return to the main menu.
Set-Up:
  • Place the light source at one end of the meter stick.
  • Hold the light probe 20 cm from the light source pointing directly at the light.
Procedures:
  • Follow the directions for calculator set-up.
  • Place the light probe 20 cm from the light source.
  • Press ENTER on the calculator.
  • Type in 20 for the distance of 20 cm and press ENTER.
  • Move the light probe to 30 cm from the light source. Again when the reading stabilizes press ENTER on the calculator then type in 30 and press ENTER.
  • Continue increasing the distance of the light probe from the light source by 10 cm and recording data until you have collected data out to 100 cm.
  • You can then view your graph by pressing STO.
Data / Observation:
  • Record your data on your lab sheet as you perform the experiment.
  • You can also find your data in L1 and L2 on the calculator.
Calculations:
    Find a mathematical relationship between the amount of light and the distance from the light source.
    You can use either of the following methods:
    • Trial and error.
    • Transfer the data into LoggerPro either manually or by using a link cable.
    Record any calculations on your lab sheet, attach graphs, or record results from your fit from the computer.
Conclusions:
  • Record your conclusion on your lab sheet.
  • Support your conclusion with your data and your calculations.
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