How the intensity of light emitted by a lamp varies with wavelengt

[xraox]

Homework Statement

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.

Homework Equations

The Attempt at a Solution

i know how LDR works and what's the experiment is about, but i don't 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

 

[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.

 

[xraox]

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.

well its just a practical planning we don't 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 don't know what appratus is needed and how to carry it out, please if you help me with that

 

[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.

 

[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 don't know what appratus is required , how it works, and a diagram for it, the plan don't 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 don't 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

thanks for your top reply Mindscrape

 

[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?

 

[xraox]

no its a planning exercise (we don't 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 )

 

[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.

 

[megegg]

hi - I am 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

 

[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.

 

[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 that's relates to this), can you email me .. but it has something todo with the resistance of LDR and light as well..

 

[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 :)

 

[megegg]

lol actually ideas on any of the questions would be apprechiated

 

[xraox]

hahaha Megegg you got the same problem as i do

 

[megegg]

i hate physics lol :( boo hoo

 

[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??

 

[xraox]

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

 

[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

 

[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.

 

[xraox]

that was great help you gave mindscrape thanks a lot buddy =D

 

[alip_93]

Im on the same paper aswell!

Woo go AS physics...well
heres what i managed to work out from it...
you get your LDR...which you need to research...and the wavelengths of visible light spread from violet - 400nm to Red 650nm
so need to find an LDR which will detect between them...(its quite hard)
then I am going to use filters to create the different wavelengths
so research coloured filters and find out what wavelengths they produce

then use a 'potential divider' circuit...with this 'lux meter' attached perhaps...

i was just going to measure the output voltage from the potential divider circuit...

but the lux thing sounds pretty good to me

good website for finding circuit parts - 'rswww.com'

but yeh...still don't know how far away to have my lamp...what lamp to use...still haven't found a suitable LDR...but one of my friends has

called a 'human eye LDR'...clever name

anymore ideas??

ali

 

[alip_93]

Violet Light
The visible violet light has a wavelength of about 400 nm.
Indigo Light
The visible indigo light has a wavelength of about 445 nm.
Blue Light
The visible blue light has a wavelength of about 475 nm.
Green Light
The visible green light has a wavelength of about 510 nm
Yellow Light
The visible yellow light has a wavelength of about 570 nm.
Orange Light
The visible orange light has a wavelength of about 590 nm.
Red Light
The visible red light has a wavelength of about 650 nm.

 

[xraox]

ohh thanks a lot my bro

 

[ad01]

hey, i am doing the idea with the coloured slides to change the wavelength, but can anyone tell me why a potential divider circuit is needed instead of a simple circuit??

 

[Hysteria100]

To obtain different wavelengths just use monochromatic filters. This sounds fancy, it just means they each let through a single/small range of wavelengths, eg red...

The resistance of an LDR (light dependent resistor) decreases with increasing light intensity; it is made from a semiconductor which frees more electrons with increasing light intensity, allowing more current to flow...

I would have thought that from this, the light intensity could be roughly estimated by using 1/R (or the inverse of the resistance). However, i don't know if the resistance of an LDR is proportional to light intensity, and so if it isnt, this probably wouldn't work accurately... it is probably worth looking into different types of LDR; a company must make one which is proportional, surely?

 

[Mindscrape]

Apparently everyone performing this experiment found this topic. Hah.

Whatever the relation between light and the resistance is, you can adapt to work however you want.I've only heard of LDRs in passing. I heard that they can be used to make a voltage controlled amplifier, but I've never seen anything that has one. Typically light sensitive experiments use photodiodes.

 

[seanna13]

photoelectric effect

hi,

From what I've put together, if the wavelength decrease the frequency increases. If the frequency increases, the energy increases so the intensity also increases. However, with the photoelectric effect one photon frees one electron no matter what its energy, so really the amount of energy that the photon has does not lower the resistance. Instead the intensity really concerns the amount of photons. But how does the amount of photons relate the wavelength of light?
Am very confused, and this has to be in Tuesday.

Any help would be appreciated
thx

 

[cristo]

hi,

From what I've put together, if the wavelength decrease the frequency increases. If the frequency increases, the energy increases so the intensity also increases.

This is not necessarily true. Here, the energy is the energy of one photon, found by E=hf. The intensity of the light is the number of photons (per unit time). Therefore light with high intensity (lots of photons per unit time) can have a relatively low amount of energy.

However, with the photoelectric effect one photon frees one electron no matter what its energy, so really the amount of energy that the photon has does not lower the resistance. Instead the intensity really concerns the amount of photons. But how does the amount of photons relate the wavelength of light?

Isn't the experiment supposed to be testing the relationship between the intensity of the light and the wavelength of the light? Therefore you don't need to know the answer to this question. If you are interested in the answer, then I got a graph in one of the first google hits.

 

[Kimioo7]

hmm i am doing this too.
the equation for intensity is power/ 4 pii r squared
where r is the distance between light and LDR.
i was thinking of having an LDR in a circuit with a voltmeter and an ammeter to then work out power, cos they would change with the different wavelengths, but i don't think that would be the right power would it? cos i need the power of the light surely, not the LDR??
can anyone please help

 

[Mindscrape]

One of you should just post the exact description of the problem, so that we know what you are working with.

 

[Osman]

I think some of u forgot rule 5 in the planning exercise paper, which states:
"Your Plan should be based on the use of STANDARD equipment, apparatus, chemicals and other materials available in a school or college science laboratory."

And the reason being, if ur school's 6th form or ur college doesn't have the equipment in ur plan then ur going to have some problems doing the experiment.

If we go back to the question it's pretty obvious I think:
"You are required to design a laboratory experiment using a light dependent resistor (LDR) to investigate how the intensity of light emitted by a lamp varies with wavelength."

-We have to use an LDR
-We have to investigate the light intensity change with wavelength

This is an OCR paper, and in my OCR physics AS textbook it says in the key notes after the Potential Dividers section:

"Thermistors and light-dependent resistors can be used in potential divider circuits to provide p.d.s that are dependent on temperature and light level respectively."

DING! I think that's a big enough hint to make a Pontential Divider circuit.

For those of you who still don't understand, take out the stuff about thermistors and concentrate on the LDR part:

"LDRs can be used in potential divider circuits to provide p.d.s that are dependent on light level (i.e. intensity of light emitted by a lamp)."

How can they be used? i.e. how is the circuit set-up? LOOK IN UR BOOK! They never test you on something they never taught you.

Your investigation will be based around the potential divider equation:
Vout = Vin * Rb / Ra+Rb
Say Vin (your source voltage) is 12v
Say the fixed resistor (Ra) is 10 ohms

Say the LDR (Rb) exposed to a certain light intensity has a resistance of 1 Ohm (for simplicity).
Vout will then be 12*1 / 10+1 = 12/11 = slightly over 1v output.

Now say the LDR has a resistance of 2 ohms at another light level:
Vout = 12*2 / 10+2 = 24/12 = a 2v output.

Once more, the LDR has a resistance of 3 ohms now:
Vout = 12*3 / 10+3 = 36/13 = almost 3v output.

Now using out knowledge of LDR's: The resistance drops as the light intensity increases.

What our calculations have shown is that a lower resistance of the LDR would result in a lower voltage output. Therefore, as light intensity increases, the voltage output decreases.

part (c) of the planning exercise asks "how a measure of the intensity can be obtained from the LDR" and I think using a potential divider circuit with an LDR answers this.

however, part (b) asks "how the wavelength of the light falling on the LDR is determined". But the question acknowledges it's worked out from a resistance (light falling on an LDR), therefore my best guess is using equations to work out the energy (perhaps W=ItV), and then E=hf => E/h = f to work out the frequency of the light, and finally c = f[lander] => c/f = [lander] (lander is the symbol for wavelength) to determine the wavelength of the light as the question asks.

that's where my discussion ends and I hope u have found this useful

 

[hpinderz]

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

I got an A, after 4 attempts!
Hope your planning went ok