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....
- 25k
44
votes
Accepted
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 ...
- 98.6k
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 ...
- 40.3k
34
votes
Accepted
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 ...
- 41.9k
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 ...
- 441
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 ...
- 22.7k
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 ...
- 41.7k
20
votes
Eye Floaters Optics
They are visible because they cast shadows onto the retina, either by completely occluding the path of light or via refractive means. Imagine a dark box forming a camera obscura. If we place an object ...
- 4,062
19
votes
Accepted
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 ...
- 11k
19
votes
Accepted
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 ...
- 98.6k
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 ...
- 201
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 ...
- 119k
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 ...
- 21.8k
15
votes
Accepted
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.
- 284
14
votes
Accepted
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 ...
- 10.4k
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 ...
- 235k
14
votes
Accepted
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.
- 21.8k
14
votes
Using Thin Lens Equation to find how far 1972 Blue Marble photo was taken
You made two mistakes: the image is indeed cropped, and you calculated the distance from the center of the earth instead of the surface.
Here is the original image:
The source is this nasa.gov page, ...
- 28.2k
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,...
- 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 ...
- 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 ...
- 211
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 ...
- 42.7k
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 ...
- 29.4k
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 ...
- 2,498
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 ...
- 12.9k
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 ...
- 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 ...
- 40.3k
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"...
- 119k
8
votes
Accepted
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 ...
- 98.6k
8
votes
Accepted
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 ...
- 4,302
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