# What is Cavity: Definition and 273 Discussions

A body cavity is any space or compartment, or potential space in the animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid.
The two largest human body cavities are the ventral body cavity, and the dorsal body cavity. In the dorsal body cavity the brain and spinal cord are located.
The membranes that surround the central nervous system organs (the brain and the spinal cord, in the cranial and spinal cavities are the three meninges. The differently lined spaces contain different types of fluid. In the meninges for example the fluid is cerebrospinal fluid; in the abdominal cavity the fluid contained in the peritoneum is a serous fluid.
In amniotes and some invertebrates the peritoneum lines their largest body cavity called the coelom.

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1. ### A Cavity locking using Lockin+PID technique

Hello, So I am trying to lock an X cavity to a 1064 nm laser. Laser has a linewidth of ~100KHz, cavity linewidth is 37.65KHz, and the FSR is 3.125 MHz. When I try to scan the cavity, the peaks usually shift and after a few seconds, the resonance comes off. I am scanning the cavity at around 100...
2. ### A Question about PDH locking a cavity

Hello! I am building a bow-tie cavity and I am trying to lock it using the PDH method. From reading some stuff online I understand the concept but the figures they show in the tutorials online are different from what I see on my oscilloscope. I am attaching below two pictures (the second one is...
3. ### I What determines RF cavity size in particle accelerators?

In a particle accelerator what determines a RF cavities size? Do they follow the same regulations drift tubes do when calculating there size? What formulas would a physicist use to build a RF cavity?
4. ### Gauss' law for a cavity in an insulator

This is a problem from Yale OCW (Shankar). The solution he gives is as follows: Sure, this makes sense. However... Superimposed rho and negative rho with radius R/2 means there is no charge enclosed in the cavity... therefore no charge -> no flux -> no electric field.
5. ### I Does a Fabry Perot cavity charge in discrete steps?

I'm trying to understand the Fabry Perot interferometer and came across this amazing video. Basically it all comes down to adding E-fields together with each of them being delayed by the cavity round trip time. In the extreme case, either they all interfere constructively at the second mirror...
6. ### A Investigating the dielectric constant of a pure polar liquid

The model that he uses is a dielectric in which there is a spherical cavity with a dipole at its center. The dipole ##\vec{m}## has a component due to a permanent dipole and a component due to an induced dipole (because of polarization). In order to obtain the dipole moment in the cavity, the...
7. ### Density of a sphere that has a cavity

The sphere floats on water so we should have: ##F_b=F_g## The buoyant force is equal to the weight of the displaced fluid, so : ##\rho _wV_wg=\rho _sV_sg## (w: water, s: sphere) From last equation we have : ##V_w=\frac {\rho _s}{\rho _w} V_s \rightarrow V_w=5 V_s ## The volume of displaced...
8. ### A Confused about reflected signal in a bow-tie optical cavity

Hello! I am building a bow-tie optical cavity, of finesse about 10,000. When I measure the transmitted signal out of the cavity, after aligning it, I see clear peaks, significantly higher than the background and pretty much single mode. I do see 2 other peaks, probably from other modes, but much...
9. ### Calculating the energy savings of an insulated cavity in home wall construction

I am a client who bought a new house and the cavity in the wall was not insulated. I can calculate the U value of the uninsulated wall and the U Value of the wall if it were insulated correctly. front the two different U values I want to calculate how much extra energy is being lost through a...
10. ### A Error signal when locking a cavity

Hello! I am trying to lock a laser to a bow-tie cavity. I managed to align the cavity to a certain degree but when I lock it, the error signal looks as below. I do get a significant power enhancement (although about 10 times smaller than expected) and the lock is relatively stable, but the error...
11. ### A Weird beam behavior in a cavity

Hello! I am trying to align a bow-tie cavity and I noticed a behavior of the beam spot that I can't explain with my (limited) knowledge of cavities. For reference, I am scanning the current of the laser in order for the laser to follow the cavity. I attached below 3 frames from a video I took of...
12. ### A Unknown large linewidth of the fringes of a cavity

Hello! I am building a bow-tie cavity and for some reason the peaks I see when scanning the laser frequency are extremely wide and with a very long tail to the right. However, if I put a 50 Ohm resistance in between the BNC cable from the diode and the oscilloscope (using a BNC T connector) the...
13. ### I What sense does it make to "weigh" a Casimir cavity?

This Paper takes reference to quantum fluctuations within a Casimir cavity: Towards weighing the condensation energy to ascertain the Archimedes force of vacuum I. INTRODUCTION ... About a decade ago, it was pointed out that a possible way to verify the interaction of vacuum fluctuations with...
14. ### A How can I overcome alignment issues with a bow-tie cavity?

Hello! I am having some issues with a bow-tie cavity I am aligning and I would really appreciate any help. The cavity is not locked, so it does have some vibrations (it is on damping legs), and I want to lock a laser to it. I am seeing some peaks, but the output power is lower than expected...
15. ### Triangular cavity thermal radiation simulation

Hi, I'm trying to solve a problem involving radiation in a triangular cavity: As you can see, lengths and emissivities of all surfaces are given. For two of them, the heat flux is known and the temperature has to be found while for the remaining surface it's the other way around. I have the...
16. ### A How making the ions leave the cavity in a ion sources

Ion sources are devices that allow creating ion beams (e.g. argon ions) and to project them outside the device, for example to be further processed by a particle accelerator, or to irradiate materials or biological tissues etc. The ions are usually created by a plasma inside a cavity filled...
17. ### Cost/Benefits of an RF Cavity vs Quartz Oscillator

I just encountered an RF cavity in the wild for the first time. It was used as the frequency reference in the Agilent 8640B RF signal generator, which I believe dates back to the 70's. Were quartz oscillators not an option back then? Or were they worse in stability back then? I'm curious about...
18. ### I What is the function of the air cavity inside drums?

I'm trying to understand the function of the air cavity inside drums. I've read that 'The air cavity inside the drum will have a set of resonance frequencies determined by its shape and size. This will emphasize some frequencies at the expense of others.' Then what are the resonance...

20. ### A Fourier Transform MW spectroscopy in a FB cavity

Hello! I am reading about Fourier Transform MW spectroscopy in a FB cavity, which seems to be quite an old technique and I want to make sure I got it right. As far as I understand, this is very similar to normal NRM, i.e. one applies a MW ##\pi/2## pulse which puts the molecules in a linear...
21. ### A Photon-Phonon Cavity Spectrum

I am considering the following Hamiltonian: $$H = -\Delta a^{\dagger}a + \omega_m b^{\dagger}b + g_0 * a^{\dagger}a (b + b^{\dagger})$$ which is the interaction picture optomechanical Hamiltonian for a cavity with movable end mirror. The mirror vibrations are quantized, leading to phonons that...
22. ### A Vibrations in a bow-tie cavity

Hello! I am trying to build a bow tie cavity and it seems like I am dealing with a large amount of vibrations (I think!). When I scan the laser frequency over one cavity FSR (the cavity is not locked to anything), I see just one peak, which I think it means that I am at least mode matched...
23. ### A Accounting for Astigmatism in Bow Tie Cavity: Finding the Optimal Cavity Length

Hello! I am a bit confused about how to account for the astigmatism in a bow tie cavity. I did the math for the resonant frequency of the tangential and sagittal direction of a Gaussian beam, and I got in my case (the angle of the mirrors is 3 degrees) a difference of about ##862.6##kHz. However...
24. ### Point charge in cavity of a spherical neutral conductor

For (a) this problem, the only thing I can see changing is the distribution of the negative charge on the inner wall of the cavity. When the point charge is in the center of the cavity, you could say the induced charged is spread symmetrically on the inner cavity wall in order to oppose the...
25. ### Gauss's Law for a sphere with a cavity, solving for E(r)

I am not sure how to solve for E(r) for R1<r<R2.
26. ### A Mode Matching a Cavity: Best Practices Explained

Hello! What is the best way to mode match a cavity in practice? I know the laser spot size before it enters the cavity, but I don't know what the beam spot size should be inside the cavity, so I am not sure how I should adjust the size of the laser spot such that it matches the one that builds...
27. ### A Vibrations in building a cavity

Hello! I am trying to build an optical cavity and I am having some issues with vibrations. I attached below a plot of the acceleration in x, y and z as a function of time, when the accelerometer is placed on the optical plate (x is red, y is green and z is blue; I shifted them upwards for...
28. ### A Question about aligning a cavity

Hello! I have a laser that I want to mode match to a bow tie cavity. For my setup I only care about power amplification inside the cavity, not about the actual frequency that gets amplified. So the cavity itself is not super stable, but I want to use a servo to control the laser wavelength such...
29. ### I Question about a Servo setup (locking a laser to a cavity)

Hello! I am trying to understand the properties of a laser I am trying to lock to a cavity. I would like to adjust its wavelength in order to achieve the locking. I attached below a part of the properties that I am confused about (I emailed the company, too, but I haven't heard back yet). I...
30. ### Movement of a Ball Bearing within a cavity inside a Projectile

Summary:: Question concerning the behavior of a ball bearing inside a projectile fired straight up or at an arc. Within a projectile is a 1-inch cylindrical cavity, inside of which is a steel ball bearing that can freely roll along the length of the cavity. When the projectile is fired...
31. ### I Radiant energy in a cavity

How to demonstrate that about 25% of the radiant energy in a cavity is concentrated between 0 and maximum energy wavelength?
32. ### Find the wave function of a particle in a spherical cavity

(a) Let the center of the concentric spheres be the origin at ##r=0##, where r is the radius defined in spherical coordinates. The potential is given by the piece-wise function $$V(r)=\infty, r<a$$ $$V(r)=0, a<r<R$$ $$V(r)=\infty, r<a$$ (b) we solve the Schrodinger equation and obtain...
33. ### Microwave generator and a resonant cylindrical cavity

I tried to fix this physics problem. But i think I’m using the wrong steps.
34. ### What determines the frequency of an RF cavity?

I am reading a document describing RF cavities, in there it says that for the TM 010 mode cavities the frequency is only dependent on cavity radius but not length (which I assume is the length along the beam axis). Also I assume the TM 010 mode is the mode in which klystron cavities and particle...
35. ### Cavity and coals: brightness and its consequences

Cavities formed by red-hot coals appear brighter than the coals themselves. Is the temperature in such cavities appreciably higher than the surface temperature of an exposed glowing coal? At first i tried to interpret the "brightness" as a synonymum of intensity, anyway, a radiation emitted by...
36. ### A Number of thermal photons in a real (finite Q) cavity

This is probably a common questions, but I can't find it answered anywhere... It is of course well known that the average number of thermal photons in a mode can be calculate from from the B-E distribution: <n>=\frac{1}{e^{hf/k_BT}-1} The usual understanding of this is that what we are...
37. ### Confusion on the distribution of charge

The charges are q1,q2 & q. P,Q,O1,O2 refer to positions only. This is a conducting sphere with cavities containing charges. I'm interested in knowing how the charge should be distributed in the sphere. I know the charges induced on the charges of the sphere should be equal and opposite to the...
38. ### Charge Inside a Cavity in a Conductor

Let us say we have a cavity inside a conductor. We then sprinkle some charge with density ##\rho(x,y,z)## inside this surface. We have two equations for the electric field $$\nabla\times\mathbf{E}=0$$ $$\nabla\cdot\mathbf{E}=\frac{\rho}{\epsilon_0}$$ We also have the boundary conditions...

40. ### Image charge of source charge in spherical cavity

All are used to finding the image charge induced by a source charge outside a conducting sphere. The solution is supposed to also work for the case where the source charge is inside the conducting sphere, in which case the sphere is now a conducting cavity. But the solution suggests the image...
41. ### I Resonant cavity and circuit analysis

As I understand, if you solve the Maxwell's field equations for a resonant cavity, supposing a time dependence of the form ##e^{iwt}##, you get an eigenvalue problem and different modes as possible solutions. I'm reading some notes where the author states that each mode can be associated with a...
42. ### Electric field in the Spherical Cavity

a. For the question a the solution is If the uniform charge density is ρ then the charge of the sphere up to radius r is q = ρ * (4/3)*π * r3; Hence the electric field is E = (ρ *4π*r^3)/(3*εο*r^2); E = (ρ*r)/(3εο); b. I don't understand what is superposition? How to proceed? Please advise.
43. ### Electric potential of a spherical conductor with a cavity

Summary:: If the conductor is having a cavity and is provided with some charge, with the cavity too having some charge then how the potential will be affected on the outer surface of the conductor. The center of cavity and the center of hollow sphere does not coincide. As if their centers do...
44. ### I Cavity resonances between two long parallel plates

Consider two rigid and infinitely long parallel plates (say they are of infinite length in X direction, so running from -inf to +inf in X axis) separated by a distance d (say measured in Y-direction). Let the space between the plates be filled up with a fluid that supports acoustic waves. If we...
45. ### Gauss' law -- Conductor with a cavity

I really don't understand the theory of the above kind of questions. But from the little theory i understand the Electric field is 0 inside the conductor and all the charge goes to the surface and distributes equally. a. Since the E=0 inside the conductor the point charge distributes outside...
46. ### Electric field within the cavity of a hollow conductor polarized by external charges?

Suppose we have a hollow metallic conductor, just a thin metallic shell forming a large hollow cavity. It is then polarized by electric charges placed nearby externally. The equilibrium electric field must be parallel to the surface normals of the shell, there must be no tangential component...
47. ### I Input/output coupling of a cavity

Hello! I am reading some stuff about building optical cavities, mainly a bow-tie cavity. Is there anything special about the input/output coupling in terms of design? I understand that these are the mirrors used to send the light inside the cavity and read some signal out of the cavity, but are...
48. ### A Equipartition of energy in the modes of a resonant cavity

I think the answer for this may be straightforward, but I don't see anywhere that explains this from the scratch: A large resonant cavity with a small hole is used to approximate an ideal black body. I understand the conditions for the modes inside the cavity. But there are two points that...
49. ### Electrostatic Energy of a solid sphere with a cavity

I tried to use ##W = ε_0/2 \int E^2d\tau## for all space. So I find that ##E = \frac{(R^3 - b^3)\rho}{3ε_0r^2}## where ##\rho## is the charge denisty. So from here when I plug the equation I get something like $$W = \frac{(R^3 - b^3)^2\rho^2 4 \ pi}{18ε_0} \int_{?}^{\inf}1/r^2dr$$ Is this...
50. ### Quantum Introductory textbook for cavity quantum electrodynamics

Hi. I'm having a hard time learning the physical realization of quantum computers. I got stuck with the section of optical cavity quantum eletrodynamics. There are some concepts I am not familiar with. I think I should read some introductory textbooks which cover cavity quantum electrodynamics...