# Tagged Questions

The subfield of optics in which light propagation is approximated in terms of rays. It mainly includes reflection and refraction on surfaces.

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### Under what conditions do reflected wave virtual images do not converge

Supposing we have a point source $p$ and multiple receiving points $r_i$ in $\Bbb R^3$ and there is a direct ray from $p$ to each $r_i$ and if there is a single mirror there is a single reflected ray ...
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### Transition between raytracing and physical(wave) optics

In the past I have done raytracing simulations for large things, and physical optics (FEM, FDTD) for small wavelength-scale things. Now I have a very large lens that focuses down on a waveguide. I ...
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### Is optical path length (OPL) usually in units of length, or wavelength?

When I do calculations, I usually define optical path length (OPL) of a path as the integral of index $n$ along a path divided by the vacuum wavelength, so that I can get the phase easily. So for a ...
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### In geometrical optics, how can we say that rays coming from a distant object are parallel to one another?

If two rays are not parallel in the start, how can they become parallel at the instant when they strike the lens of a telescope? If they don't become parallel, why do we consider them to be, in the ...
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### What is the purpose of glass in common mirrors? [closed]

We know that the image is formed by the silver layer, so why don't we use silver layer on some other material in household mirrors?
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### Increase the collection effciency of lens array

In an experiment I am trying to image a phosphor surface on a camera using a 3x zoom lens. The distance of phosphor surface to lens input surface is ~100 mm. The effective focal length of the zoom ...
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### Alternate derivation of the Thin Lens equation

Is there another, fairly straightforward way to derive the Thin Lens equation without using similarity of triangles?
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### What is the angular divergence of sun-rays?

I just want to find a number and I could not. Does anyone has it or can calculate it easily? Just in geometric optics, without diffraction, atmospheric effects and etc. Update: I think for me will be ...
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### What is the ray diagram for Schuster's focussing?

I haven't been able to find it online. Our course book also doesn't provide a ray diagram for this particular phenomenon. I feel like it's difficult to understand the process without the ray diagram.
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### What is the optimal distance to sit from a TV? [closed]

I was wondering how to calculate the optimal distance to sit away from a TV. I don't quite know the full set of parameters it will depend on, I would suspect it to include the following: Size of the ...
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### Analytical calculation for focus deviation [closed]

How can I analytically calculate the deviation in focus or focal length variation in a situation as shown in figure. When the Sensor is parallel to the face of the lens/ light; its easy to calculate. ...
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### Can we take photo of a mirage? [closed]

I do know that mirages are optical illusions . I know the phenomenon of its formation just tell me if we can ?
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### Question about Fermat's principle

Why when deriving the law of reflection from Fermat's principle of least time do I set $dL/dx = 0$? I am a 12 grade student with a little notions of maxima and mimima in one variable calculus.
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### What is the purpose of condenser lenses in the slide projector shown below? [duplicate]

I searched a lot on internet about this.I could not find the working of these condenser lenses in detail. Some websites say that condenser lenses make the light rays parallel and to pass through ...
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### what is the purpose of condenser lens in a slide projector?

What if we don't use any condenser lens? can't we use a single convex lens as a condenser? what difference will it make? kindly explain it in detail.
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### opaque to transparent based on distance of object?

If we look through a transparent glass window of an apartment, from a distance in day light, we can’t see what or who is inside. All the light is reflected off the surface of the transparent glass ...
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### why can't we see our image on a simple brick wall?

We can see our image in a mirror due to the reflection of light. We also know that a red color brick wall also reflects light. So, why can't we see our image there?
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### Does the curvature of the IMAX screen improve the view of 3D video projections in any way for viewers not sitting in the “sweet spot”?

When I think about a simple 2D projection on a screen with vertical-cylinder curvature and of viewing positions away from the "sweet spot" it seems to me pretty obvious that there would be geometrical ...
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### What is the angular dispersion of an optical surface grating and why does the textbook answer seem to violate reciprocity?

The behavior of a surface grating can be described by the grating equation $m\lambda=d(\textrm{sin }\theta_i+\textrm{sin }\theta_d)$ [1] with $m$: diffraction order, $\lambda$: wavelength of the ...
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### Lensmaker's equation and Imaging properties

In the textbook, usually the following construction (see the figure below) is used to determine the relative size of the image and the object. I understand that a ray parallel to the optical axis ...
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### Why don't reflector telescopes place their reflectors at an angle?

To avoid placing a secondary mirror in front of the objective mirror, why not tilt the objective and design the eyepiece in such a way that comatic aberration is minimized?
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### Optics to correct focal distance across a plane

I have a laser beam which is focused to a point at a certain distance. I'm then going to use a galvanometer to scan that beam across a plane. Obviously, as the beam scans across the plane, the ...
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### Would the moon be brighter if it were completely spherical?

I remember reading Galileo's 'Dialogue Concerning the Two Chief World Systems' where Salviati and Sagredo explain how the moon would be almost entirely dark if it were a perfect sphere but after ...
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### Where do rays that are not marginal or principal get stopped?

The picture is from an MIT lecture but the concepts are explained in many optical texts. The chief/principal rays go through the center of the aperture stop, hit the edge of the field stop, and the ...
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### Geometric optics, the size of the image

I have the following question from Snell's Law, I calculated the apparent location of the light source by: X = 20/1.47 = 13.6 So, this should be the apparent location of the light source. ...
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### What is the physical meaning of magnifying power of a telescope?

So the following question was given in the JEE Mains 2016 conducted throughout India on 3rd April. An observer looks at a distant tree of height 10 m with a telescope of magnifying power of 20. To ...
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### Is it possible to use a tapered fiber to collimate a single-mode light beam?

I would like to collimate a light beam stemming from a single-mode LED source. As the source is pretty big (a few millimeters), collimation with a single lens gives a bad result, i.e. a quite ...
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### Why does a specific static image “move” when moving my head? (glass user)

I am a daily PC user. As most users, I have a taskbar (using Windows) that shows icons of programs. My two screens are big (24") and I'm at ~60cm distance from them, so fairly often I need to turn my ...
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### Attenuation of light through a simple lens, and is it important?

I have an object with incident light rays traveling away from this object. Some of these rays are traveling from the left-hand side through a simple lens (say, a double-convex converging lens). As ...
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### Why is this laser beam being scattered(and not)?

I was shining a laser beam through a liquid filled test tube(an ester particularly),and I found this phenomenon rather intriguing.Have a look. Now when I passed the laser straight through the ...