# Search results

1. ### Understanding the physical meaning of multiplication, etc

Is there some kind of intuitive way to understand the physical meaning when mathematical operations are applied to equations in physics? What I mean is that, say we start with a 'starting point' equation, in this example Ficks law of diffusion (wikipedia:): J = -D \frac{\delta \phi}{\delta x}...
2. ### Optical microscopes and white light / laser light

I have a few questions regarding an optical microscope and their white light sources... So white light generally first hits a diffuser, some kind of ground glass lens. What is the purpose of this? Then the light goes through a field diaphragm, which we can open and close. I have heard that...
3. ### Newton's second law dp/dt version?

Hello, I am confused by the momentum version of newtons second law... So we know \bar{F}=m\bar{a}=m\left(\frac{d\hat{v}}{dt}\right) and that \bar{\rho}=m\bar{v}=m\left(\frac{d\bar{x}}{dt}\right) so is \frac{d\bar{p}}{dt}=m\frac{d\left(\frac{d\bar{x}}{dt}\right)}{dt} What I mean is this...
4. ### How to Fourier transform this expression?

I have this expression: f(\tau) = 4 \pi \int \omega ^2 P_2[\cos (\omega \tau)] P(\omega) \, \mathrm{d}w \quad [1] where P_2 is a second order Legendre polynomial, and P(\omega) is some distribution function. Now I am told that, given a data set of f(\tau), I can solve for P(\omega) by either...
5. ### Why does a light blinking quickly average out as 'on' to us?

Consider a light, an L.E.D for example, turning on and off once per second. For humans, we will look at it and think "clearly on, clearly off, clearly on, clearly off" for each 'state'. Our view of whether it is on or off will continue like this if its 'blink frequency' is increased up to a...
6. ### How can I think of rotational diffusion inverse seconds?

When thinking of a spherical shaped particle moving about under Brownian motion, one describes its motion by Diffusion. The units being \frac{m^2}{s} I can understand this physically as a distance it will travel from a certain point in space averaged over x-y and z direction. Now rotational...
7. ### Understanding derivation of scattering from Maxwells eqns

I am trying to follow a Maxwell's equations derivation for light scattering but don't understand 'why' the authors do the steps they do at this start bit. Help would be greatly appreciated... It starts with the incident electric field equation. \textbf{E}_{0}(\textbf{r},t) = \textbf{E}_0...
8. ### Programming physics books

I am looking for recommendations of books that teach physics, with an emphasis on solving real-world problems with code. I find I can only really fully grasp a concept if I can actually program it and visualize it in MATLAB, change variables and see what effect that has on the results...
9. ### Book recommendation on a variety of topics

Hi Guys, I am after recommendations of books that teach in a specific way. Basically I have come out of a 3 year physics degree with just doing the work but not really understanding the material (the mathematics) that well. I want to re-learn a bunch of topics in a way so I will actually...
10. ### Functions of time 0?

Hi Guys, In a lot of books dealing with spectroscopy, correlation functions or any kind of functions involving time sometimes take the form like this: \left\langle A[q,u(t)]A^{*}[q,u(o)] \right\rangle Where A is some function that depends on say q and u, and u is another function that depends...
11. ### Function multiplied by its complex conjugate

Hi Guys, I have two questions which kind of relate. The first relates to the complex conjugate of a function. Specifically, When a function is multiplied by its complex conjugate, what does that mean physically? For instance, I am reading a book on electromagnetic wave scattering, and often...
12. ### Inverse Problems in Scattering

Hi Guys, I am doing a bit of work with dynamic light scattering (DLS) data. It is one of the many areas of science where we encounter an inverse problem. The forward problem is: For a known sized particle, calculate its scattered spectrum (that is easy). The inverse problem is: from the...
13. ### Integral want same answer as book

Hi Guys, I'm doing some work on signals and power spectrums. The book I am using says the spectrum for this light scattering signal is given by: I(q,\omega ) = \frac{1}{{2\pi }}\int_{ - \infty }^\infty {\exp ( - i\omega t)} S(q,t)dt In the book an S term is given as: S(q,t) = N\left\langle...
14. ### Mathematical 'Moments'

Hey guys, I am trying to understand the concept of mathematical moments (as defined in this wikipedia page: http://en.wikipedia.org/wiki/Moment_%28mathematics%29), as some work I am doing relates to them. I am looking for more information to read up but when I search maths moments I get...
15. ### Dynamic light scattering laplace inversion

Hi, Those of you familar with dynamic light scattering (DLS), will know that a common method used to obtain a particle size distribution is via a laplace inversion of the autocorrelation function. What I want to know is why? What does Laplace space have to do with DLS (I've only learned...
16. ### Mathematica Mathematica symbolic equation simplifcation

Hi Guys, I am new to mathematica. I have gotten it to do an integration for me, then simplified the answer using FullSimplify. It does a very good job, but I can see that a futher symbolic substitution would make it even nicer, e.g. I would like to define A = b*x*t. So where ever there is...
17. ### Preparing biological samples for electron microscope

Hi Guys, Ive been doing some work with E. Coli recently and I would like to image them under a scanning electron microscope. There are lots of papers with lots of different methods to prepare them for such a feat... but has anyone here actually done it and gotten a decent image? If so what...
18. ### Autocorrelation of intensity fluctuations

Hi Guys, I am working on some fluorescence correlation spectroscopy and I have some data generated by the machine I use. The data has the correlation plot of G(tau) vs the delay time (tau) which is an exponential decay. The data also has the count rate given for the duration for the...
19. ### Integrating WRT d^2 x or d^3 x

Hi Guys, Ive gone through a 3 year physics degree but cant remember ever touching on this? Im doing some work and some of the theory contains integrals of d2x or d3R for instance. I have no idea how to do these. Any help? Thanks all