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60 votes

Why doesn't a light ray bend again when emerging from a lens?

They technically should "bend" because of refraction, and a more accurate drawing would be this: But drawings like the one that you show usually just tell you the net effect of the lens, i....
SuperCiocia's user avatar
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44 votes
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How is a (rifle scope) reticle in focus?

The image of the distant object is formed in the plane of the graticule. The eyepiece is then focused on the image of the distant object and the graticule which is in the same plane. . The ...
Farcher's user avatar
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39 votes

Why doesn't a light ray bend again when emerging from a lens?

You are right. The drawing shown in your question is quite poor. Here is a much better drawing, which correctly shows the refraction of rays on both convex surfaces. The rays bend towards the normal ...
Thomas Fritsch's user avatar
34 votes
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How does a telescope make an image larger by shrinking it?

While a telescope can make an image larger, the diagram shown above doesn't really show that happening. Your eye perceives the size of an image based upon the angular extent it takes up in the visual ...
BowlOfRed's user avatar
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28 votes

Why is an arrow pointing through a glass of water only flipped vertically but not horizontally?

Because it is a cylinder, which is a lens, along one axis, and homogeneous along the other. :) As in @fectin's comment: if it were a sphere, rather than a cylinder, then it would flip through the ...
paul garrett's user avatar
25 votes
Accepted

Why do convex lenses not disperse light like prisms, given that entry and exit points aren't parallel?

They do. It's called chromatic aberration - each different frequency has a slightly different focus point, blurring the image by different amounts for the different colors. Modern lenses of high ...
Sean E. Lake's user avatar
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23 votes

What can we deduce by the fact that mirrors cannot get a ray hotter than Sun's surface?

What can we deduce by the fact that mirrors cannot get a ray hotter than sun's surface? We can deduce that the laws of thermodynamics reign supreme, even with regard to radiative heat transfer. The ...
David Hammen's user avatar
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19 votes
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Why are the edges of the shadow so bright?

Look at it (pun intended) this way: your diverging lenses are making the central area darker because the irradiance, i.e. energy per unit area, is reduced. But that energy doesn't disappear: it has ...
Carl Witthoft's user avatar
19 votes
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How does an endoscope form the image?

Optical fibres can be accurately aligned in an array such that the order of the fibres at one end is identical to the order in the other end to form what is called a coherent bundle. If an image is ...
Farcher's user avatar
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18 votes

A lens inside a liquid is not visible. Why?

A lens suspended in a refractive medium would not be visible if it has the same, or at least very similar, optical properties. This means that the refractive index, opacity, and absorption spectra of ...
GenericHumanoid's user avatar
15 votes

Starting a fire with a lens without the sun

When you use a lens (or a focusing mirror) to increase the "power" of a light source - whether it be the sun, or another light source - what you are really doing is making the light source "look ...
Floris's user avatar
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15 votes

Why does the focus point of the eye does not burn the retina?

In ordinary life, only a point object will focus to a point image. Otherwise the light is spread out, just as the object is spread out. But a very intense object can injure your retina: don't stare at ...
John Doty's user avatar
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15 votes
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Why does the focus point of the eye does not burn the retina?

The following diagrams show how the eye focuses: There may be people with vision problems such that the focal point is a single point on the retina, but this can be corrected with glasses.
Xavier's user avatar
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14 votes
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What is the "tangential" distortion of OpenCV actually tangential to?

Take the position of a point P on the image relative to the geometric image center C. Assume C remains undistorted in the lens produced image, but P is distorted into P'. If the position of P' is ...
udrv's user avatar
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14 votes

What can we deduce by the fact that mirrors cannot get a ray hotter than Sun's surface?

The colliding proton beams at LHC , page 29: In the collisions, the temperature will exceed 100 000 times that of the centre of the Sun. The electric currents running the LHC can easily be ...
anna v's user avatar
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14 votes
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Why do contact lens work when they're inside out?

The key mechanical property that enables the optical function of the lens is that its thickness varies from the center to the edge. That doesn't change.
John Doty's user avatar
  • 21k
13 votes

Why doesn't a light ray bend again when emerging from a lens?

That diagram shows what's called the "thin lens" approximation. In real life, lenses have thickness, and light rays get refracted both when entering and exiting the material. But in practice,...
Draconis's user avatar
  • 633
12 votes

A lens inside a liquid is not visible. Why?

This is simply because the Refractive index of the material of the lens is equal or nearly equal to the Refractive index of the liquid medium. The geometrical bending of light rays do not take place ...
Safikul's user avatar
  • 131
11 votes

How does a telescope make an image larger by shrinking it?

I think the problem with the original image is that it does not show the subject you are focusing on, so it cannot demonstrate what portion of it you see magnified. Also, the original picture design ...
Andrej Lucansky's user avatar
10 votes
Accepted

Is there a more accurate form of the mirror equation $\frac{1}{f}=\frac{1}{u} + \frac{1}{v}$?

There is more than one approximation here. Some apply to mirrors and lenses. Some apply to the propagation of light in general. Lenses are more common that curved mirrors. Much more attention has been ...
mmesser314's user avatar
  • 38.7k
10 votes

How does a telescope make an image larger by shrinking it?

Your diagram's mirror configuration corresponds to Cassegrain reflector telescope. The rays that it shows are from a faraway point source situated exactly in the middle of its field of view (FOV), and ...
Ruslan's user avatar
  • 28.9k
9 votes

What can we deduce by the fact that mirrors cannot get a ray hotter than Sun's surface?

I think it is quite counter-intuitive that some lenses or mirrors focusing sunlight to a single spot cannot produce a temperature higher than Sun's Surface. It is indeed counter-intuitive because it ...
Level River St's user avatar
9 votes

Starting a fire with a lens without the sun

You can't focus light onto a region so that it increases the temperature to a value larger than the temperature of the source. This has its origin in Fermat's principle, and since it is a variational ...
Lawrence B. Crowell's user avatar
9 votes

Why do convex lenses not disperse light like prisms, given that entry and exit points aren't parallel?

Sean E. Lake's answer is right: convex lenses disperse light like prisms and that effect is known as chromatic aberration - which is easily noticeably by zooming in in the corners of photographs taken ...
Pere's user avatar
  • 2,149
9 votes

How do we see virtual images?

Your picture of the eye is incomplete. You need to add the retina and to consider the effect of the lens. The lens refracts the rays coming from the virtual object $B'$ so that they actually arrive ...
Thomas Fritsch's user avatar
8 votes

What temperature can I reach using magnifying glass at night?

SIGNIFICANTLY UPDATED ANSWER TL;DR: you can get to almost 200 °C because the component of reflected sunlight adds significant power. This calculation contradicts the assertion "no hotter than the moon"...
Floris's user avatar
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8 votes
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Why do glasses have a greenish reflection?

The lens have an anti-reflecting coating on them. The process of putting the coating on is called blooming. The thickness of the coating is $t=\dfrac{\lambda}{4 n_1}$ where $n_1$ is the refractive ...
Farcher's user avatar
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8 votes
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Why can't concave lenses be used as magnifying glasses?

In your image it is not clear how you actually end up with a "big image". It is not enough to take into account just one light ray per point on the object. You also need to consider how we ...
user1583209's user avatar
  • 4,292
8 votes

How does a telescope make an image larger by shrinking it?

I gave the answer above +1 but wanted to mention the concept at play. Étendue Étendue is a property of a light beam. At best, étendue is conserved as the beam propagates through the optical ...
boyfarrell's user avatar
  • 3,299
8 votes
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How to measure the strength of a prescription eyeglass lens?

For a positive lens: focus sunlight (for safety, preferably on dark, inflammable object such as a brick) and measure the distance in meters from the lens to the brick. The optical power of the lens in ...
Han-Kwang Nienhuys's user avatar

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