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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230
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21answers
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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?
120
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2answers
8k 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 ...
72
votes
5answers
8k 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 ...
41
votes
6answers
5k views

Do people wearing glasses have different field of view than those who don't?

There is one thing I sometimes wonder about ever since I was a child. Do people who wear eye glasses see objects in different size than those who don't?(Technically different size means different ...
39
votes
5answers
5k views

If a lens focuses all incoming light to a point, how do we get 2D images?

How do lenses produce 2-dimensional images, if a lens bends all incoming rays of light to intersect at the focal point? Shouldn't this produce a single dot of light on a screen placed at the focal ...
29
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4answers
3k 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 ...
26
votes
3answers
148k 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 ...
20
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2answers
3k 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 ...
18
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1answer
519 views

Do light waves precisely follow null geodesic paths in General Relativity?

In special relativity one may show that a plane wave solution of Maxwell's equations (in a vacuum), of the form $A^a=C^a\mathrm{e}^{\mathrm{i}\psi}$ has the following properties: The normal ...
16
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2answers
932 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
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3answers
1k views

How many times can light revolve around a black hole?

Take a light ray approaching a black hole from infinity which goes out again to infinity. What is the maximum finite rotation it can describe? (I know it can loop around indefinitely in the ...
11
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2answers
425 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 ...
11
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1answer
523 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
278 views

How do a microscope's optics expose defects in the user's 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 ...
9
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3answers
5k 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 ...
8
votes
2answers
134 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 ...
7
votes
8answers
3k 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
votes
3answers
550 views

What determines the sharpness of a shadow?

What are the factors that affect the sharpness of a shadow? I would think that the distance between the light source and the object, the distance between the object and the shadow, and the size of ...
7
votes
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
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2answers
1k 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 ...
6
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3answers
1k views

Reflection at intersection of two mirrors

Consider two mirrors, which intersect at an angle. Let us talk about geometric optics. If we draw a light ray intersecting the mirrors at their point of intersection, how will it reflect? What will ...
6
votes
2answers
638 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 ...
6
votes
4answers
9k 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 ...
6
votes
5answers
479 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 ...
6
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3answers
339 views

Examples in which the light maximizes the optical path length

I posted a similar question about geodesics on Math.SE. Many sources (Wikibooks for instance) claim that the light could maximize the optical path length in some cases. But I don't think it's actually ...
6
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1answer
298 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 ...
5
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4answers
315 views

Infinite total internal reflection

Suppose that I have a block of some transparent material, glass, for instance, with a certain index of refraction. Suppose that the transparent material is placed in air or in any other transparent ...
5
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3answers
506 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
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2answers
388 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 ...
5
votes
3answers
12k views

Clarification needed in the concept of apparent depth & real depth

I understood the concept of apparent depth from here: But one thing I didn't understand is, will there be difference in the real depth and apparent depth when we are looking not at an angle as ...
5
votes
1answer
320 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 ...
5
votes
1answer
144 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
3answers
8k 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 ...
4
votes
2answers
366 views

Why would the reflection of a lamp by a bowl look like a heart?

I have this bowl roughly the shape of a circular truncated cone, and the lamp shines on the bowl at an angle. The reflected lamp light hit the bottom of the bowl and created a light ring that looks ...
4
votes
2answers
14k 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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3answers
4k 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
2answers
953 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
3answers
2k views

Calculating light's lateral shift in a glass slab

I have tried to compute light's lateral shift after passing through a glass slab In terms of the incidence angle $\theta_1$, the slab thickness $d$ and the refraction index $n$, I have found that ...
4
votes
4answers
719 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
1answer
187 views

How is Fermat's least time principle proven?

How is Fermat's least time principle proven? Or it is what usually is observed and is basis for the theories?
4
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3answers
909 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
343 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 ...
4
votes
2answers
3k 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 ...
4
votes
2answers
3k views

Fresnel distance and Geometrical limit

I read about the geometrical limit of wave theory. The source from where I read had a slightly different explanation to provide than here(The more rigorous answer is too complicated for me to ...
4
votes
2answers
4k 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
209 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
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0answers
237 views

How to calculate desired light path in continuous medium with gradient refraction index

See the Figure below. $O:(0,0)$ is the disk center of light source $\odot{O}$ with radius $3$. Then the profile light rays of disk $O$ from the view point $B:(-14,0)$ is defined by segments $DB$ ...
3
votes
2answers
192 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
1answer
155 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
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1answer
2k 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, ...