As Level Physics coursework- how Light intensity varies with wavelength?

Question

I have to design an experiment to determine how light intensity varies with wavelength, using an LDR to measure intensity. I have two questions really: 1) why do most people seem to have done this using a diffraction grating to get different wavelengths when simple filters would surely give a much more accurate single wavelength that you can determine by looking at its colour. Is there an advantage of a diffraction grating over filters that I am missing? 2) How exactly can the resistance of an LDR be converted to a value in kW/m^2 for the intensity of light hitting it? Is there an equation? Any help with these two things would be very much appreciated, especially if you are currently doing the same coursework. Thanks in advance!

Answer 1

Using a filter allows a bunch of wavelengths to go through, all yellow perhaps, or red, or blue. The colour can belong to a range of wavelengths, each a slightly different shade of red, say. Hence, this won't tell you what wavelength you are using.

The diffraction grating can isolate a single monochromatic wavelength, like 5893 Angstrom units, the yellow-orange line of the sodium lamp. Accompanying this is a paler red, green and violet wavelength.

You can use a range of discharge lamps: helium, neon, mercury. Select different colours of each lamp (some are not bright enough), and the values of the wavelengths can be known by looking up a book of physical constants or the Internet. There has to be a book in the lab. [e.g. by Kaye and Laby.]

You've got to plot a graph, otherwise how would you be able to show an intensity variation. Hence you need the values of the wavelengths for plotting.

Since intensity is measured as the energy per second falling normally on unit area, make sure the beam you are using falls on the same area on your LDR each time. Maybe the whole of it. In which case the beam you use may have to be broader than the LDR.

If the light beam is very narrow, there must be some means to keep the area of the spot of light on the LDR constant. Using the diffraction grating helps because the size of the slit on the collimator determines the size of the image. All the images are the same size anyway whatever the colour. So you can stand the LDR in the path of each image.

You have to calibrate the LDR with known intensities first, I guess, i.e. whose kW/m^2 is known. The LDR would be used in some circuit first, so that the current can be indicative of its resistance. Hence the current changes can be indicative of resistance changes.

I'm wondering if using a light-pipe of some kind might help to channel the light where you want it. Plexiglas is sometimes used for the light pipe.

I hope this helps. Learn how to google. You may get better answers than this.

Answer 2

This is a multiple type question that many will ignore. You need to break that down into separate ones.
Filters will absorb more of the frequencies that should come through. And looking at color is still one of the best way to determine it's purity and frequency. The human eye is a first class instrument.
As far as the LDR is concerned, you need to know its efficiency at various frequencies to get a true readout.
To your email:
The problem is that getting filters that are pure enough will be (almost) impossible, diffraction gratings or prisms do not have that problem.
Multiple filters will only worsen that situation, as the impurities will add up.

Answer 3

A diffraction grating provides a continuous spectrum while a specific filter would have to be manufactured for each color that you wished to investigate. What if there was an odd peak or dip at a frequency that you didn't get a filter for?

Also, I would think that some losses due to the filter would be unavoidable, whereas a diffraction grating would spread the colors (letting you select the one that you want) with very low losses.

Can't help with part 2. Doesn't the manufacturer provide some specs?