Can anyone help me with my physics plan?

Question

Hello everyone,

I am having a little bit of a problem of how to do my physics planning excercise for this year, so I wondered if anyone can help?

Basically its all about LDRs' and the main point of it is to investigate how the intensity of light emitted by a lamp varies with wavelength.

So I was wondering if any of you big clever people could think of a way to investigate this?

Think about:

-How light intensity can be determined?

-How range + precision of any instruments that would be used?

Thanks everyone and much apprieciated! This is the only one I ever got stuck on. Its probably so simple I am thinking to hard.

Peace!

Mav

Answer 1

LDRs do have a spectral response so that at different wavelengths the LDR will become less resistant than at others.

A cadmium sulphide LDR has its highest response at 550 (see figure 5 on page 3 of link)

http://www.biltek.tubitak.gov.tr/gelisim/elektronik/dosyalar/25/LDR_NSL19_M51.pdf

It is probably easiest if you use the data on the LDR to correct your readings for different wavelengths. You are given the relationship between resistance and Lumens (figure 4, page 3 of link). To correct for wavelength, divide the calculated intensity by the percentage response for that wavelength.

The problem you are going to have though is this...
how will you filter out the unwanted wavelengths so you can investigate 1 wavellength at a time?

Answer 2

I'm a bit puzzled by "the main point of it is to investigate how the intensity of light emitted by a lamp varies with wavelength". Intensity (how bright a light appears) does not vary with wavelength. Intensity varies according to the amplitude of a light wave. [See source.]

Color varies by wavelength since c = Lf; where L is wavelength, c is speed of light in a given medium, and f is frequency. Thus, from c/L = f, we see that short wavelengths yield higher frequencies, and as frequencies go higher, the visual EM waves (aka light) seem bluer, the wavelengths determine what we see as color in the visual spectrum.

Perhaps you are mistaking light energy (E) for intensity. They are not the same thing; although they are related. A high energy photon can exist at low intensity because the amplitude of the wave is small even though its frequency is high. E = hf; where h is Plank's constant and f is f = c/L the frequency of the light. So, indeed, the energy depends on wavelength, but that is not the same thing as the brightness (intensity).

Being that intensity is just another name for brightness in the visual spectrum of EM waves, simply turn the light up and down (with a rheostat) to show the varying intensities. Then point out that the color of the light does not change, indicating the wavelengths (frequencies) did not change as the intensity changed. [This in fact proves your statement that intensity varies with wavelength is false.]

Answer 3

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