# Homework Help: How the intensity of light emitted by a lamp varies with wavelengt

1. Apr 22, 2007

### xraox

1. The problem statement, all variables and given/known data

Planning Exercise,

Special Lamps may be purchased to help plants grow indoors. these lamps produce light with wave lengths comparable to natural sunlight. a manufacture wishes to check these lamps by measuring the output of the lamps at different wavelenghths.

you are required to design a laboratory experiment using a ligh dependent resistor (LDR) to investigate how the intensity of light emitted by a lamp varies with wavelength.

you should draw a diagram of the arrangement of the apparatus, and in your account you should pay particular attention to..

A) the procedure to be followed

B) how the wavelength of the light falling on the LDR is determined

C) how a measure of the intensity can be obtained from the LDR

D) the range and precision of any intruments that would be used

E) the factors that would need to be controlled to ensure that it is a valid test

F) any safety precautions you would take when carrying out the investigation

G) particular features of the design that would ensure the accuracy and reliability of your results.

2. Relevant equations

3. The attempt at a solution

i know how LDR works and whats the experiment is about, but i dont have any clue of how to set the experiment up and what appratus i should use.

i need help with my practical planning exercise, i have no clue how to start, or what to do cuz, i did my 2 years of GCSE's in 3 months when i came from dubai to Uk, i have no idea of electronics..

sorry iam trying to attempt to work it through but, i am unsuccessful so far, maybe if someone can help with appratus and tell me how i can set it up..
thank you

2. Apr 22, 2007

### Mindscrape

How does a light dependent resistor work, the resistance changes depending on the incident light? I'd imagine that you can monitor the voltage drop across the resistor depending on the light, and then relate it to the intensity.

Hard to know what you have at your disposal.

3. Apr 22, 2007

### xraox

well its just a practical planning we dont have to conduct it, or anything, its just that if we have to carry out some experiment to measure light intensity on different wavelengths... just i dont know what appratus is needed and how to carry it out, please if you help me with that

4. Apr 22, 2007

### Mindscrape

So I assume you have a bunch of different lights with specified wavelengths. One idea would be to have a 15V voltage source connected to the resistor in a dark area. The resistor probably has a natural resistance, a resistance without any light hitting it, that will produce a voltage drop that you can measure with a voltmeter. Then you could turn on a lamp with a specific wavelength and determine the difference in voltage drops.

After many tests with different lamps are recorded, a best fit line of voltage against wavelength will tell you how much a change in wavelength will effect the voltage. Presumably, the resistor you have has a rating that tells you something about how the resistance is related to the frequency or wavelength of incident light that will further show something about the intensity of the light. Or if you aren't given the relationship, you could have an ammeter after that measures the current, and use Ohm's law to find a relationship.

I don't really know what facts you are given, so in the end you will know the best design, but there is at least an idea that you could modify to fit your needs.

5. Apr 23, 2007

### xraox

yes you are right, but it could be anything it can be a closed room or closed system where Light intensity at different wavelength can be measured with special lamps , but i dont know what appratus is required , how it works, and a diagram for it, the plan dont have to be very precise, i have no knowledge becuase i havnt done anything in GCSE. in UK.

i know certain things , lux metre is used, i dont know how, how to measure light intensity and stuff. i need to know what appratus is needed, and how to set it up to get the practical working

6. Apr 23, 2007

### Mindscrape

I do not know the equivalent of GCSE, E&M(?), but without much electronics experience all this might be a little tough.

If you want to measure the light intensity you could use a photovoltaic diode and a transimpendence amplifier to create some kind of photodetector.

Is this a university level experiment?

7. Apr 23, 2007

### xraox

no its a planning exercise (we dont have to perform this, just have to how to carry it out , its theory) for AS physics (college) , i was told something about lux metre , its just about measuring the intensity of light on an LDR with the change of wavelength..

i was told to measure the light intensity using a lux metre. i have no clue,

i just need a rough idea for this experiment, of appratus and how to make its diagram and set it up..

a photovoltaic diode and a transimpendence (these will be complicated words for the knowledge at my level )

8. Apr 23, 2007

### Mindscrape

Huh, I've never heard of a lux meter, but apparently it measures something with light (I assume intensity). Google showed a bunch of "light readings."

Anyway, all a photodiode does is produce a current based on how much light hits it, but I suppose it wouldn't be so good for uncollimated light like your light sources. A transimpedence amplifier takes the current from the photodiode and turns it into a voltage with a certain amount of gain (the ratio of output to input).

I suppose you might as well use a theoretical lux meter though. :)

You could probably use an apparatus similar to the apparatus for the photoelectric effect because it is essentially what you are doing -- measuring how a change in wavelength relates to energy. There are many ways you could conceivably perform this experiment.

9. Apr 24, 2007

### megegg

hi - im also doing the same planning thingy

hey
i was considering doing the prism option for my experiment and but i was wondering how i can measure the angle from the prism in just one colour - e.g blue no red and white

and if there is a equation to do this

i was also wondering how u connect your LDR into your circuit and what the whole point of using the LDR is for

i understand that the LDR is an input sensor which converts light into resistance - so the red light should have the highest resistance as it has the longest wave lengh?

i was also wondering where u would connect the LDR if it is to measure light does it have to be infront of the light

i think these are really stupid questions sorry but i would be grateful if you could help

thanks

10. Apr 24, 2007

### Mindscrape

I don't know if you are asking me or xraox. If you are asking me, then I have no idea what the prism option is and you would have elaborate.

Also, let me clarify (to one or both) that although I know electronics, I have never personally used an LDR, so I do not know how it functions. I assume that it is exactly as the name implies, and it changes its impedance based on how much light is incident. Though, I am not sure why you would use a LDR instead of a photodiode if that is, in fact, the case. If your paper says something, you could post it, and I could try to discern its functionality, but as of now I am merely guessing.

11. Apr 25, 2007

### xraox

no he is asking you, anyways mindscrape can you think of appratus list mate, and the way to carry the experiment out, or give me the link to photoelectric effect (if thats relates to this), can you email me .. but it has something todo with the resistance of LDR and light as well..

12. Apr 25, 2007

### megegg

hey this is the paper i am doing:
Special lamps may be purchased to help plants grow indoors. These lamps produce light with wavelengths comparable to natural sunlight. A manufacturer wishes to check these lamps by measuring the out put of the lamp at different wavelengths.

The above is the scenario given

You are required to design a laboratory experiment using a light dependant resistor (LDR) to investigate how the intensity of light emitted by a lamp varies with wavelength.

The requirement of what i need to do above

A diagram of the arrangement of your apparatus, and in your account you should pay particular atttention to

a. the procedure to be followed
b. how the wavelength of the light falling on the LDR is determined
c. how a measure of the intensity can be obtained from the LDR.
d. the range and precision of any instruments that would be used
e. the factors that would need to be controlled to ensure that it is a valid test
f any safety precautions you would take when carrying out the investigation
g particular features of the design that would ensure the accuarcy and reliability of your results.

i was woundering if anyone has any ideas on (c) , (f)

thanks :)

13. Apr 25, 2007

### megegg

lol actually ideas on any of the questions would be apprechiated

14. Apr 25, 2007

### xraox

hahaha Megegg you got the same problem as i do

15. Apr 26, 2007

### megegg

i hate physics lol :( boo hoo

16. Apr 26, 2007

### hpinderz

You need to use a prism or light filters, if you use filters for each colour of visible light you can use wavelengths of each colour, with a prism you need to use youngs slit experiment.
how do you measure the light intensity??

17. Apr 26, 2007

### xraox

lol yeah , what did you get in your Forces and Motion module?

18. Apr 26, 2007

### xraox

hpinderz please can you put the full detail mate, for that prism and young slit thing to measure the wavelength,

that would be great help
thanks

19. Apr 26, 2007

### Mindscrape

Something similar to Young's experiment would work too, but I think photoelectric is more on tune. Simply googling photoelectric effect apparatus gives many different labs that various schools have set up. I'm not going to do anymore than point you in the right direction. Figuring out how the pieces fit together is your job.

20. Apr 27, 2007