# Tag Info

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This is probably a better question for photography, not physics. I believe that green dot is a lens flare. It's the effect of the sunlight reflecting off of the optics that, in an ideal world, would not reflect at all. Its position and size are based off the angle between you and the sun and the particular position of the elements in your lens. As for ...

7

The lens elements in a camera contains anti-reflection coatings (thin films). Although they are intended to remove all internal reflections, they are not perfect. Their effectiveness generally depends on incident angle and wavelength. When you photograph an intense source of light, such as the sun, the little bit of light that is reflected is enough to show ...

4

Susskind's original argument doesn't work. Alice just needs Bob to send a message to her saying "I'm still alive!" She doesn't have to illuminate Bob. Of course, it's hard to get a message out from the near-horizon region of a black hole because of the redshift, but there's no theoretical reason that this shouldn't work. Suppose you don't have a ...

3

As mentioned in comments, a photon is "delocalised", so it feels the whole system. You may imagine a photon as a long-long wave (to have a defined frequency) and as such it interacts with the whole material. More strictly, one can say that the source of photon is the whole set of charges, so the photon is a collective mode of excitation of a given system. ...

3

As far as the second part of your question is concerned you can directly see the image improvement with squinting. If you have a DSLR with aperture settings you keep the camera slightly defocused and now reduce the aperture you will see that the image is becoming sharper. However at the same time the image will become darker because you are collecting less ...

2

Witness that the invariance of each invariant separately follows immediately from the $\propto \,S^2$ scaling of the area of the entrance / exit window with the distance $S$. The invariance of each is really a restatement of this scaling law. So it now remains to prove the equality of the two potentially different invariants i.e. the one calculated for the ...

1

The easiest way to do this is via the complex beam parameter $q$, which is given by $$\frac1q = \frac{1}{z+iz_R} = \frac{1}{R(z)}-i\frac{\lambda_0}{\pi w(z)^2}.$$ In your case you know all three parameters on the right hand side, so you know $q$, and therefore you can determine $z$ and $z_R$ as its real and imaginary parts. Doing this explicitly, \begin{...

1

When attacking a system of equations with square roots it is often helpful to get rid of the square roots until the very end (if possible). Good idea garyp. Let's start with... $$w^2 = w_0^2 \left( 1 + \frac{z^2}{z_R^2} \right)$$ $$z_R = \frac{\pi w_0^2}{\lambda_0}$$ $$R = z \left( 1 + \frac{z_R^2}{z^2}\right)$$ When I look at this system of equations I ...

1

No, even a lens don,t have a single focal point. they have a range of points in their principal axis . but for a very very large radius of curvature of lens compared to compared to its aperture, these range gets smaller and smaller eventually it tends to become a single point it is what we normal call a focal point. with irregular surfaces having many ...

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EDIT Please see the comments following my answer, regarding the paragraph below , as it is incorrect. Strabismus is the clinical name for squinting. The squint is simply compression of the eye muscles to compensate for problems with focusing and / or astigmatism. END EDIT In a somewhat similiar way, telescopes using adaptive optics can distort the ...

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Interferometer "questions": clearly the mirrors can impart energy to the light and vice versa. But I think that a part of the answer is more fundamental than this: according to Feynman the passage of a detectable gravitational wave imparts energy to any coupled massy environment through which it passes, and indeed this is necessary for gravitational waves to ...

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You are not the first to question this; see for example the work by Leiner et al. from 2013 and 2014. Here the Ray Tracing technique is interfaced to Finite Difference Time Domain (FDTD) Maxwell simulations with ''the Poynting vector representation of either rays or wave propagation directions''. As noted in these works you should be aware of the absence ...

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From a recent paper, Direct detection of a single photon by humans. JN Tinsley et al. Nature Comms 7, 12172 (2016) (open access). Abstract: [...] Here we report that humans can detect a single-photon incident on the cornea with a probability significantly above chance. This was achieved by implementing a combination of a psychophysics procedure ...

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