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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189
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18answers
64k views

A mirror flips left and right, but not up and down

Why is it that when you look in the mirror left and right directions appear flipped, but not the up and down?
103
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2answers
5k views

Why do sunbeams diverge even though the sun is much more than a few kilometers away?

Consider this picture of sun beams streaming onto the valley through the clouds. Given that the valley is only (at a guess) 3km wide, with simple trigonometry and the angles of the beams, this ...
54
votes
4answers
6k views

Is it possible to start fire using moonlight?

You can start fire by focusing the sunlight using the magnifying glass. I searched the web whether you can do the same using moonlight. And found this and this - the first two in Google search ...
22
votes
5answers
2k views

Is it possible to focus the radiation from a black body to make something hotter than that black body?

My previous question wasn't specific enough. I'll try to be more specific. Let's imagine we have a hot body let's say 6000K hot that emits lots of thermal radiation. Let's assume 1kW of radiative ...
20
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3answers
87k views

Virtual vs Real image

I'm doing magnification and lens in class currently, and I really don't get why virtual and real images are called what they are. A virtual image occurs the object is less than the focal length of ...
18
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2answers
2k views

Does light reflect if incident at exactly the critical angle?

A lot of textbooks and exam boards claim that light incident at exactly the critical angle is transmitted along the media boundary (i.e. at right-angles to the normal), but this seems to violate the ...
16
votes
2answers
722 views

Can't seem to reconcile geometric optics and wave optics

I was looking at a physics situation involving light, and I can make the correct derivation assuming light is a ray of a given intensity (geometric optics), energy conservation checks out, everything. ...
11
votes
1answer
389 views

light ray 'entropy'

Is there something like an entropy law for light rays? I came up with the following experiment: A black box has two circular holes in it, a small and a large one. I don't care about there ...
10
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2answers
281 views

Why does a blue sky at dusk appear nearly black through a telescope?

Earlier this evening I was looking at the Moon through my cheap toy telescope (x150 magnification) when I noticed a (rather mundane) optical effect I couldn't explain. The Sun had just dipped below ...
8
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2answers
2k views

Why Does Passing Light Through a Slit Make it Coherent?

Related: Why incoherent source of waves have to be passed though a single slit for double slit interference pattern to occur? If you have a an incoherent light source, why does passing it through a ...
7
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8answers
2k views

Is it possible to make glasses that make everything brighter, but do not magnify or focus?

Corrective glasses are usually intended to help focus light on your retina. Supposing I had good vision already, but simply wanted more light, could I make glasses that would send more light into my ...
7
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2answers
1k views

Do all light rays from a point passing through a thin lens converge at the image?

I have often seen diagrams, like this one on Wikipedia for a thin convex lens that show three lines from a point on the object converging at the image. Do all the other lines from that point on the ...
7
votes
2answers
917 views

Can you shake your reflection's hand?

For a spherical mirror, an object at the mirror's center has an image that is also at the center. Its magnification is $-1$. For a video showing this, see here. If you stand slightly behind the ...
7
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2answers
109 views

Is light red shifted in optical tweezers?

This is a question I put to my supervisor during my PhD many years ago, and never really got a satisfactory answer to. In an optical tweezers, assume that a beam of light is used to move a glass ...
6
votes
3answers
906 views

reflection at intersection of 2 mirrors

Consider 2 mirrors which intersect at an angle. If we draw a light ray intersecting the mirrors at their point of intersection, how will it reflect? What will be the normal at that point?
6
votes
5answers
414 views

Trapping a lightray

Given a solid whose interior is a hollow sphere with perfectly reflecting mirrors. A small hole is drilled in the sphere and a photon is sent in at some angle. Will it always eventually exit through ...
5
votes
3answers
450 views

Will a prism widen a monochromatic beam?

The representation of the experiment which demonstrates that white light consists of many colors, invariably shows that the beam of white light is broadened inside the prism and is some what more ...
5
votes
3answers
4k views

Software for geometrical optics

Is there any good software for construction optical path's in geometrical optics. More specifically I want features like: draw $k \in \mathbb{N}$ objects $K_1,\dots,K_n$ with indices of refraction ...
5
votes
2answers
350 views

How to place a mirror parallel to a wall?

For one of my experimental setup I need to place a mirror perfectly parallel to a wall. It can be placed at any distance from the wall. I would like to use any method other than direct measurement. I ...
4
votes
2answers
8k views

Why do things that are far away seem smaller?

As you see things that are far smaller, a funny question about this: Imagine there are many people in a row (all are same height) ...
4
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2answers
405 views

Converting indices of refraction

If I know that the index of refraction of a given substance is 1.4 for the average wavelength (say 550 nm), and I would like to know what is the index of refraction with a wavelength of 832 nm, how ...
4
votes
2answers
518 views

Goldfish perspective

What does the world look like from the Goldfish point of view, from inside a spherical aquarium? If our eyes were inside, would we be able to see the straight lines, focus on different objects and ...
4
votes
2answers
2k views

How to make the projected image smaller by adding one ore more lenses in front of the built in projector lense?

I have a projector that creates a large image, even if the distance to the screen is short. The device is very small, approximateley 10x10 cm if you look from above. The height is only 3 cm. I ...
4
votes
4answers
440 views

Why do the passing objects' shadows run in the direction of the moving train?

While travelling in the train on the sunset, I have noticed the strange phenomenon. The shadows of the lonely trees and lampposts, that stay along the railroad, are moving on the wall in the same ...
4
votes
3answers
675 views

Virtual images in (plane) mirrors?

The following image is taken from teaching physics lecture Was man aus virtuellen Bildern lernen kann (in German): Now the cited paper claims that the left hand side is the correct picture to ...
4
votes
2answers
2k views

How to find the principal point in an image?

I need to find the principal point in an image. Its a point where the principal axis intersects the image sensor. Due to misalignment this point is not at the center of image always(or image sensor). ...
4
votes
1answer
200 views

On the optics of high-resolution surveillance drones

In this youtube video (which is an except from a documentation I believe) a DARPA development for a high-resolution surveillance camera is presented. This question is regarding the optics that are ...
4
votes
1answer
176 views

Reconciling total internal reflection and the evanescent Wave

I understand that light is guided in a dielectric waveguide via total internal reflection. My question is regarding the origin of power contained in the evanescent field traveling along the direction ...
4
votes
1answer
50 views

Options to correct rectangular laser beam profile?

We recently switched to a 405 nm laser diode which has a terrible beam profile -- it is a well-defined rectangle with an aspect ratio of about 10:1. The image below shows the collimated beam which ...
4
votes
0answers
75 views

Blonde hair looking green at certain angle

I have a friend who has blonde hair. And when look at an oblique angle, there's a tinge of light green in her hair. I've heard that the copper ion in pool water can make blonde hair green, but her ...
4
votes
0answers
49 views

How does a microscopes optics expose defects in the users vision?

I've got cataracts in both eyes. My vision is correctable to 20/30, so the cataracts are essentially a non-issue in daily living. But when I use a microscope, which I do daily, (binocular, zoom ...
3
votes
2answers
172 views

Is it possible to focus the sun in such way?

Imagine the sliding part of the mirror is controlled by computer and opens on intervals. Is it possible to increase the power of the beam by making it bounce between the mirrors thus going through ...
3
votes
3answers
5k views

Cladding of optical fibers

Why do optical fibers usually have a cladding? Ok, if you put make a bundle of optical fibers this prevents that light leaks from one fiber to another fiber in contact. However, are there other ...
3
votes
1answer
150 views

Why a star commonly exhibits 6 rays?

Have you realized, that often the photo of a star shows 6 rays spreaded symmetrically around it, independently of the camera chosen? Do you have idea of what kind of optical phenomena is behind it?
3
votes
3answers
2k views

Why should optical fibers be thin?

What are the reasons that optical fibers have to be thin (small radius of the fiber)? Is there a good picture which explains this in detail?
3
votes
2answers
131 views

Shack-Hartmann wavefront sensor concepts, pupil conjugate planes?

I am just starting to looking at the application of wavefront abberommtery to measure abberations in the human eye. I am confused over the pupil conjugate planes, do you use relay lenses to image the ...
3
votes
1answer
1k views

Beam splitters- Direction of use

There are two cases I'm asking about. The square in the middle is a cube beam splitter in the same orientation for both cases. We are looking at the beam splitter from the top. In the first case, ...
3
votes
2answers
228 views

How did ancient physicists around 500 BC decide that the earth is spherical?

One of the standard stories that is given for this (and is mentioned in Aristotle) is that the hull of a ship disappears first as it sails towards the horizon on a calm sunny day. Is this a myth, or ...
3
votes
3answers
635 views

Snell's law starting from qft? [duplicate]

Can one "interpret" Snell's law in terms of QED and the photon picture? How would one justifiy this interpretation with some degree of mathematical rigour? At the end I would like to have a direct ...
3
votes
3answers
5k views

How do contact lenses work?

I understand how telescope, microscope and glasses work. But how do contact lenses work?
3
votes
2answers
321 views

Redirecting light beams from beam splitters

I'm doing a project where I am taking a laser beam and sending it through a beam splitter. As I understand, approximately 50% of the light will go pass through and 50% will be reflected. So this means ...
3
votes
1answer
303 views

Which lens parameters determines it's format?

I'm working with a megapixel sensor with a 1/2" format. I would like to know which parameters have to be taken into account in order to design a lens that can be fit with 1/2" format. The light ...
3
votes
2answers
229 views

Is there a mathematical equivalence between gravitational optics and quantum optics or are they mathematically incompatible theories?

I know that massive gravitational bodies will curve the path that light travels. I think that quantum optical mediums also bend light. I am still confused of whether quantum optical mediums actually ...
3
votes
1answer
44 views

Measuring parallaxes from the moon

If one were to measure star parallaxes from the moon, what (if any) changes would you have to make compared to someone making the calculation from earth? What about on another planet in our solar ...
3
votes
2answers
1k views

Fermat's principle to prove the law of reflection

Fermat's principle tells that a light ray will follow a path from point $A$ to point $B$ so that the optical path length of this path is an extremum over neighboring paths. I wanted to use this ...
3
votes
2answers
96 views

Why is there no dark band below a rainbow?

I've read articles about Descartes' theory of the rainbow and I can understand why there is a dark band of about 8 degrees (Alexander's band) between the primary and secondary bows: for all the water ...
3
votes
3answers
487 views

What does it mean that a wavevector is null?

I have derived geometric optics for gravitational waves and I am trying to interpret one of the results. I have \begin{equation} k_{\rho}k^{\rho}=0 \end{equation} for the wavevector. For the case ...
3
votes
1answer
54 views

Why grating is an essential part of a monochromator?

From looking at these two images, it appears that the dispersed "rays" of the reflected light could be manipulated just as well if the reflecting mirror would be turned to a fixed angle. So why is ...
3
votes
2answers
97 views

Plano-convex lenses

When the spherical side of a planoconvex lens is towards the source we will see reduced aberrations normally we use it for the beam collimation. When we use two planoconvex lenses configuration why we ...
3
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3answers
60 views

How to improve the visibility of light without increasing the brightness?

I am not sure whether or not this is the right SE site to post this question on but I'm sure someone will inform me of my mistake soon enough if it is wrong. Basically I am an Electronic Engineer and ...