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Using a Light Source, LDR and Tracing Paper to Model Absorption of Gamma Rays by Different Thickness of Material - Physics IAA

Now, before I say anything, this experiment doesn't use actual gamma rays as they are dangerous and not allowed at school. Instead we will be investigating how the thickness of tracing paper affects the current produced from LDR (Light Dependant Resistor). This experiment is meant to mimic what will happen if we used gamma rays (the light ) and a different material (the tracing paper).

Firstly, let's identify what the independent and dependant variables are:
Independent Variable - Thickness of tracing paper
Dependant Variable - Current in the circuit

Independent variable causes a change in Dependent Variable and it isn't possible that Dependent Variable could cause a change in Independent Variable. So, the thickness of tracing paper causes a change in the current of the circuit.

Controlled Variables
Background light (meant to mimic background radiation), distance LDR is to light bulb, voltage, shroud, LDR, bulb, thickness and type of each individual tracing paper, ammeter, circuit and distance tracing paper is to LDR and bulb.

  • LDR& circuit board
  • Power supply
  • Wire leads
  • Milli-ammeter
  • Lamp & holder
  • Clamp stand, boss and clamp
  • Small squares of tracing paper (roughly 5cm squared) 

Set up
Circuit set up for Physics IAA experiment
1) Build a complete circuit as shown in the picture and set the power pack to 6V DC (direct current). Make sure the lightbulb and LDR are always the same length to each other throughout the whole experiment.
2) Test the lightbulb to ensure it works.
3) Connect the LDR with its black paper tube or shroud in series with a milli-ammeter and then connect the other ends of the leads across the same two terminals on the power supply. 
4) Turn the power supply on and turn voltage to 6V DC and check you get a reading for the current through the LDR using the milli-ammeter. 
5) Place your finger over the opening to the black tube or shroud connected to the LDR and see if the current reduces. If it does you are now ready to do the experiment.

1) From the circuit produced in the set up, place the light bulb on a clamp as shown, 1cm directly above the black tube (shroud) that is connected to the LDR. 
2) Record the reading on the ammeter to 3 decimal places. If the current changes alot, pick the mode number (number that appears the most amount of times for the longest period of time).
3) Repeat steps 1 & 2 but this time place one sheet of tracing paper over the black tube (shroud). 
4) Repeat steps 1-3 ten times.
5) Repeat the experiment 3 times.
6) Ignore any anomalous results and repeat to compensate for this.

Your graph should look something like this:

Pattern from the Graph
From the graph, it is clear that as the number of sheets increase, the current decreases. The whole graph shows negative correlation, with the line starting extremely steep and then eventually levelling out. This shows that after 'X' amount of tracing paper, any more sheets of tracing paper won't affect the current making the number of sheets a limiting factor.

The method was and could have been made more valid by (The point/Explanation for the point)
Keeping same amount of background light so the LDR won't cause an increase in current due change in background light. (Or turning off the lights in the room off to completely eliminate background light and to stop  the flicker from flurescent lights from affecting current levels)
Same light bulb so the same light intensity is trying to hit the LDR.
Same thickness of each individual tracing paper as different thickness will cause a change in light reaching LDR and therefore change in current.
Same circuit so the current of voltage around the circuit won't differ due to the circuit being different.
Same voltage so bulb emits same intensity of light.

The method was made reliable by
Using ten separate layers of tracing paper
Measure current to 3 decimal places
Use shroud to reduce background radiation
Repeating the experiment and averaging the results
Ignoring any anomalous data and repeat to compensate

(this is where you should use your results and scientific knowledge)
Ultimately, the thicker the tracing paper, the lower the current is. This is due to the tracing paper blocking some light from reaching the LDR (just like metal can block some gamma rays reaching its destination). This caused the LDR to absorb less light and therefore produce a lower current in the circuit. The graph starts off steep showing the thickness of tracing paper affects the current the most in the first few layers, then starts to level out towards the end. This just proves that the first few layers of tracing paper block out the most amount of light from reaching the LDR, while the layers after that have not as big an impact. 

About Will Green

A student in England studying Automotive Engineering with Motorsport, Will created Ask Will Online back in 2010 to help students revise and bloggers make money. You can follow AskWillOnline via @AskWillOnline.

60 comments so far:

  1. thank you sooo much!youre the best eveerrr:D

  2. YES!!:D it did thanks. Thanks very much once again:)

  3. Very useful for my IAA. Not exactly the same but very similar. Using light and glass slides instead. Cleared alot of blank spots for me.


  4. Glad to hear it proved useul to you :)

  5. Very helpful, thank you!

  6. Ugh thank you so muuuch
    You saved me ♥

  7. Thanks a lot! Cleared up some blank spots.


  9. this was extremely helpful thanks so much

    especially the conclusion

  10. This is the best site to go on, If you have internal IAA exams in school. Thanks you =D

  11. thank you...i hope that the conclusion will be the same

  12. Thank you sooooooooooo much this is exactly my IAA and it helped alot :'D
    Bless you xx

  13. Thanks a lot. great help!

  14. Thanks. helped a lot with my IAA!!

  15. This helps soooo much! THANKS!! :)

  16. This really helped. You literally gave me the whole write up for my physics iaa! Thanks loads!

  17. yay! i have to do research for the IAA for thursday and now i have it all B) thank you sooo very muchh

  18. heey! can you please relate the conclusion to if gamma rays were used? thank youu!

  19. It's pretty difficult for me t do so conquering I have not done this experiment in ages and don't have much knowledge on gamma rays, sorry.

  20. thannnkkkk youu soo uch hellped alot

  21. THANKS A LOT ! my stupid science teacher will get the credit for my grade when it's all down to you - you're a star. btw, i got an a* for my biology iaa all down to you too ! saved my future you have 8-) x

  22. hey! i have this IAA on thursday and i was just wondering about any safety aspects that i can note down? thank you!

  23. From this experiment, the bulb will become hot and if you look into it it could hurt your eyes. If this was the actual experiment, the gamma rays that can cause serious radiation will be the biggest source of danger.


  25. Dis will get me a better grade thanks to yu hardly knew anytin beforehand but yuve put things into perspective.. Your a great help onestli ta :D ..

  26. thank youuu so much <3

  27. how could you improve the gamma rays methof

  28. how was this experiment valid? thank you

  29. I've already stated how thr experiment has been made valid (look st the highlighted bits).

  30. im just scared about how their going to make us relate to the gamma rays D:

  31. could you please say the differences and advntges and disadv of using concrete or lead as a gamma shield?

  32. I don't really know about that sorry.

  33. Do you have any knowledge on the research question and the second experiment that may be asked on the iaa?

  34. DO you have any knowledge on the research question or the 2nd experiment method that is asked ?

  35. It is very similar to the first experiment and usually involves you answering questions how to make the second experiment valid reliable etc..

  36. how would the diagram/circuit look if you only had a ldr in a in a circuit board connected to the milliammeter

  37. The same as I have displayed above?

  38. Can this experiment relate to the LDR experiment?

    1. You will have to be a bit more specific what the LDR experiment is.

  39. thx bro dis was totes usefull safe G

  40. thankyouuuuuuuuuu

  41. thanks g, safe yeah