Recent content by bobfei

Density interpretation from "ray counting" Add for completion: The reciprocity relationship essentially is based on ray-counting. The reason that counting from d{S_2} to d{S_1} gives the correct result, but not vice versa, it because from d{S_2} \to d{S_1} the ray's "density" is...

Awesome, I got it!

Quantum Mechanics needed to resolve the contradiction? Could you give more details on the inter-molecule shielding effect? If we draw the projected cone as in the image, although it is oblique and has a smaller cross-section than when \theta = 90^\circ , geometrically no molecule blocks each...

Hi, https://www.physicsforums.com/attachment.php?attachmentid=60066&stc=1&d=1372909704 I would like to ask a question on thermal radiation. In the attached image, if all three surfaces \{ dA,d{S_1},d{S_2}\} are black body surface, and d{S_2} = d{S_1} = S, then does d{S_2} receive the same...

Aleph, This is what I cannot understand: why k(small angle, 10 coils) = k(large angle, 10 coils)? see the below extreme-case reasoning?

Hi, Does a compression spring’s pitch or rise angle have any relation with its spring constant k? I checked various sources and they differ on this. Some sites simply asks you to feed input into a simple formula: k=Gd4/3D3na in which k: spring constant G: material’s shear modulus d...

Studiot, I just read the wiki article and got a better understanding. Thanks! Bob

Then they are consistent. Thank you! Bob

Hi, Does battery internal resistance obey the general resistance-temperature rule? For metal, resistance usually increases with temperature (see wikipedia). For batteries however, its internal resistance decreases with temperature (also see wikipedia). Could anyone explain the...

Borek, It looks very close. I would agree with it. Bob

Nytik, Yes, exactly: This symbol represent radius from a point electric source, and is found on page 235 of Griffiths, David J. - Introduction to Electrodynamics (3ed). Dadface, Looks alike, but I am not sure. Bob

Hi, Could anyone tell me what letter the circled one is? I thought it might be “tau” τ or “gamma” γ, but none of the two looked exactly the same as it. Bob

berkeman, I just learned that DFT can be used to calculate frequency. The method is simply finding the DFT coefficient with the maximum magnitude (not including the average component), which corresponds to the frequency. Among the other transforms listed: Wavelet cosine/sine Hankel...

Dear firavia, Plastic lens for CMOS cameras. I have special requirement on focal length for shooting near objects, monochromatic, and that's all. Is this difficult? Bob

Hi, Does there exist any form for optical lens design? I am a beginner in this field and would like to find such forum, and have search physicsforums.com, but only found isolated threads without any dedicated subforum. Bob