An ideal lens in geometrical optics will have a refractive index $n$. However, other materials can have  $n^{\prime}=n+i k$. What are the physical effects of the complex part of the refractive index? Would it result in a shifting of the focal length? Would it attenuate the light passing through the lens?
120590
In complex refractive index on a material, $n=n'+ ik$, the imaginary part $k$ is physical meaning, as it shows absorption in the material but it is an imaginary. How we measure an imaginary values in physics with imaginary numbers?
73782
Optical density is a measure of the refracting power of a medium. In other words, the higher the optical density, the more the light will be refracted or slowed down as it moves through the medium. Optical density is related to refractive index in that materials with a high refractive index will also have a high optical density. Light will travel slower through a medium with a high refractive index and high optical density and faster through a medium which has a low refractive index and a low optical density. So to restate the question, why do optical media have different refractive indices?
55070
I'm fairly new to blender and not a graphics person by any stretch of the imagination. I've been having some success though modelling semi-reflective glass for a project I wish to build (physically) and I'm wondering if it is possible to model the effects of a lens. The lens I wish to model is a double-convex type. It has a diameter of 120mm and a focal-length of 301mm. Is this something that can be modelled in blender and if so, how? I'd like to be able to position the camera close to this lens and reproduce the magnifying effects of the lens on the scene behind the lens as well as depth-of-field effects.  Thanks a lot.
115726
I'm reading some texts that seam to assume knowledge of light that I'm not too familiar with. How does wavelength of light relate to the minimum distance span that can be observed (i.e. you cannot make a lens big enough to resolve individual atoms), and is this a light phenomena or an intrinsic wave phenomena?
111193
What is the relationship between the focal length and field of view for an equal-area ("equisolid") projection fisheye lens?  Alternatively, what is the relationship between the 180-degree image circle size and the focal length for such a lens?  Ultimately I want to find the field of view of a Tokina 10-17 mm fisheye zoom at focal lengths other than 10 mm on APS-C ().
110677
If we will use crop lenses only on crop bodies, why do we call crop lenses for example "10mm" when they are effectively "16mm" (with a crop factor of 1.6 in this case)?  I am familiar with the history and the difference between a full-frame and a crop body. My question was about why we call these lenses different in focal lengths since they both yield the same field of view in degrees (the crop lens on a crop body, and the FF lens on a FF body). I know the lens doesn't actually turn into a longer focal length optically on a crop body, but since the result is the same, why not just label it that way?
49273
It's said that 50mm is close to what the human eye sees and is a very versatile focal length in general, especially good for street photography which is what I'm most interested in. That being the case, if I have a cropped sensor (1.6) would I better off with a 35mm prime since it would give me a field of view similar to a 50mm lens (35 X 1.6 = 56) as opposed to a 50mm prime which would give me a field of view similar to an 80mm  lens (50 X 1.6 = 80)?
103029
There is no doubt that 50mm lenses, if all other variables are held equal, will beat any other lens when it comes to value for price (lower f/number). I want to know why.  Possible theories I have heard:   There some physical property of the lens that makes it fundamentally cheaper to produce (i.e. no real magnifying or odd wide-angle lens elements). The lens makers have had years and years to optimize the design of the lens    in order to make the manufacturing process cheaper.   The implication with the first possibility is that 50mm lenses will always fundamentally be cheaper. Unfortunately this will always plague cropped sensor camera owners with slight telephoto lenses if they want great valued lenses (i.e. 35mm lenses will never be as cheap at as low an f/number).  The implication of the second possibility is that one day, cropped sensor camera owners will be able to get absurdly cheap and high quality 35mm lenses.  Cropped sensors may one day fall by the wayside with Moore's Law, but for now they are going nowhere fast.  Any insight is much appreciated!
20546
I am looking for a prime lens to pick up, and I'm wondering if there is any difference between 35mm and 50mm in terms of the end product if I just stand back more with the 50mm. I use the Sony a6000 and am looking at the SEL50F18 and SEL35F18.  I understand that something like a fish eye lens will give a different effect, but is that what is going on in 35 vs 50mm? Or is 50mm effectively just taking a 35mm and cropping the center out (but with increased quality)?
6011
I wanted to get some cheap old lenses to use on my Nikon DX DSLR (D3100).  However, Nikon has one of the greatest flange distances of all manufacturers, which means that most lenses I can buy (M42 mount, etc) will not focus to infinity. I saw some adapters with correction lens to allow infinity focus, but also read they worsen picture quality.  Has anyone here used this kind of setup? (M42 cheapo old lens + adapter to Nikon with/without infinity focus correction lens)  If so, what can I be expecting, quality-wise? I read M42 lenses are not that bad, and I only have DX lenses, so I would not be expecting heavenly performance. Basically, I'd like a nice big tele lens (300mm+) and the ability to focus it to infinity.  Example pictures would be awesome, and particularly I'd like to hear some interesting opinions complemented by example photos of the setup I'm talking about — a picture is worth a thousand words.
90259
I understand that to reduce minimum focusing distances, macro lens design must focus on maintaining certain distances between the lens and the sensor.   However, is the placement of the lens and its elements the only difference between a macro and a prime or any other regular lens? OR are there other structural differences as well?  There are quite a few posts that address the differences but none that really bring out the "physical" differences.
8250
If APS-C and similar crop-sensor digital cameras have a focal length multiplying effect such that a 50mm lens has an apparent focal length closer to the field of view of an 80mm on a full frame camera, and yet at the same time the depth of field for the smaller sensor camera is more like the depth of field a 50mm lens would produce on a full frame camera (using the same aperture), then this would seem to suggest the concept of an "aperture dividing effect."  In other words, a 50mm f/1.8 lens on an APS-C camera would act more like a 80mm f/2.8 (approx. 1.8 * 1.6x) lens in 35mm equivalent — for depth of field, not considering exposure.  Can someone with a better understanding of the physics involved clarify this for me.  I've never seen this concept explicitly mentioned anywhere, so I am a bit suspect of it.
90934
The optical density of a medium is quantified by its refractive index $n$. Does it have any correlation with the mass/number density of the medium? In other words is an optically denser medium must have a greater mass density compared to an optically rarer medium?
82127
Consider a simple prism with a prism angle $A$, angle of incidence $\theta_1$, angle of emergence $\theta_4$ and the first and second angle of refraction as $\theta_2,\theta_3$. the refractive index for the prism (w.r.t the surroundings) is $n$. The angle of deviation is $\delta$.I wanted to derive an equation that could give the relation between $\theta_1$ and $\delta$, plot of which for a monochromatic light is as in the . Below is my failed attempt (equations 2 and 3 are from the :- $$\theta_4=\sin^{-1}n\sin(\theta_3)$$ $$A+\delta=\theta_1+\theta_4$$ $$A=\theta_2+\theta_3$$ $$\delta=\theta_1+\sin^{-1}n\sin(\theta_3)-A$$ $$\delta=\theta_1+\sin^{-1}n\sin(A-\theta_2)-A$$ $$\delta=\theta_1+\sin^{-1}n\sin(A-\sin^{-1}\frac{\sin(\theta_1)}{n})-A$$ Equation, when I plotted it on  for an equilateral prism with $n$=$1.5$ yielded the required plot in the limit $28.5^\circ&lt;\theta_1&lt;90^\circ$ (to avoid total internal reflection). But then, how do I use this equation, to analytically find the angle of minimum deviation, and the fact that at minimum deviation $\theta_1=\theta_4$. (I tried taking the derivative, but it turned out to be too complex).
100558
I have been shopping around for a prime lens for my Nikon D50. I noticed that the 1.8 is hundreds of dollars cheaper than the 1.4 AF-S. What will I gain for that cost difference? Is it really noticeable?
30122
Does the aperture ring lock the apertures beyond, say 5.6 at length lens's telephoto end? Does the lens introduce an obstacle to the aperture ring so the lens can't be opened anymore beyond the aperture at the telephoto end?  And why do lenses behave like this, anyway? Why don't they have constant apertures throughout their focal range?
109216
Consider a standard Michelson Interferometer (I took the picture from )    The incident beam is split into two parts, where the two parts travel on different paths to the screen. If the paths are of different length and the incident wave is a spherical one, one sees the interference ring pattern at the detector. I now have three questions:  1.) What happens if the path lengths are exactly identical. Do I then still see the interference pattern?  2.) How does the patter on the screen change if I then slowly change one of the path lengths?  3.) What happens if I use plane waves instead of spherical ones? If the difference between the two paths corresponds to a phase difference of $\Delta \phi= \pi$ do the plane waves cancel out at the screen?