Number of modes in a cavity

[Cummings]

This might be a bit to specalised for some of you, but i am trying to calculate the number of modes that light can propegate in a material. The question is as follows

Consider a cavity that is a cube with the side length a = 1mm filled with water (refractive index(n) 1.33). Calculate the number of cavity modes that fall withing a bandwidth Delta f = 1MHz at the frequency of Ar laser (vaccume wavelength 488nm)

Now, we have been taught the equation that the numer of modes N in the material, N = (8Pi * n^3 * a^3 8f^3) / 3 c^3 where c is the speed of light in vaccume.

We have also been taught that The number of modes in a frequency interval between f and delta f is given by DeltaNf = (dNf/df)Deltaf

I get an answer of 827 modes in that interval. I got the frequency by using f=c/landa in the vaccume and remembered that the frequency of light is the same in every medium. (6.1475x10^14 Hz) Is this the right frequency to use in the water? And am I calculating the number of modes right?

 

[Almsco]

Yes, the frequency you calculated is the right one to use in the water. The equation you used is correct for calculating the number of modes, and 827 sounds like a reasonable answer. If you're still not sure, you could double-check it against some other calculations or sources.

 

[thepatientmental]

Your calculation is correct. The frequency you have used, f = c/lambda, is the correct frequency to use in the water. This is because the frequency of light remains the same in every medium, as you mentioned. Your calculation of the number of modes, using the equation N = (8Pi * n^3 * a^3 * f^3) / 3 c^3 and the given values, is also correct. The number of modes in the given bandwidth of 1MHz is 827 modes. This means that within a bandwidth of 1MHz, there are 827 different possible standing wave patterns that can exist in the cavity. This is a result of the quantization of energy in the cavity, where only certain discrete values of energy are allowed for the light to propagate. This calculation is important in understanding the behavior of light in a cavity and can have practical applications in laser technology and optical communication systems. Good job on your calculation!