Hot answers tagged

75 votes
Accepted

If a lens focuses all incoming light to a point, how do we get 2D 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...? (In your diagram, the source image is at infinity. I will continue the ...
user avatar
  • 2,987
59 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....
user avatar
  • 22.7k
51 votes

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

Yes. I am myopic and I see a slight double image along the edge of my glasses: This means that the field of view inside the frame is bigger - zoomed out - than what it would be with the same frames ...
user avatar
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 ...
user avatar
  • 76.9k
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 ...
user avatar
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 ...
user avatar
  • 33.6k
27 votes

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

A convex lens does not focus all the rays to a single point. It focuses the all axis parallel rays on the focal point. It also focuses all the rays emanating from a given point to a corresponding ...
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 ...
user avatar
  • 19.3k
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 ...
user avatar
  • 39.2k
21 votes

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

Assume two people have identical (size) eyes, but one has a weaker lens than the other. In order to see an object at a certain distance properly, this person needs a second lens to get the focus. This ...
user avatar
  • 116k
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 ...
user avatar
18 votes

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

Your diagram shows parallel light beams originating from infinity. Light entering the eye in the real world isn't all parallel. In the real world, all the light that bounces off a single point will ...
user avatar
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 ...
user avatar
17 votes

Can spectacles converge sunlight to an extent that it burns the eyeball?

In bright sunny conditions, the eye's pupil shrinks to about 1mm diameter. If you look straight at the Sun with the pupil in this state at noon, when intensity is of the order of $1000{\rm W\,m^{-2}}$,...
user avatar
15 votes
Accepted

Glass that only allows light of specific amplitude to pass through it

Yes, there are. Such materials are called "saturable absorbers," and are (or at least, have been) used as switches in some laser designs. The one I recall is a nickel acetate dye, although there are ...
user avatar
15 votes
Accepted

How do I find the right lens for my laser?

You can't do this with a single "normal" lens. Because the beam width needs to be 4.25 inches you need a lens wider than that (which is huge compared to normal optical components). The focal length ...
user avatar
  • 860
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 ...
user avatar
  • 116k
14 votes

Why do nearsighted people see better with their glasses *rotated*?

This is a real effect, but it doesn't have anything to do with coma or any of the optical aberrations. It is caused by the fact that the effective focal length shortens as you tilt a lens. When your ...
user avatar
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 ...
user avatar
  • 9,763
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 ...
user avatar
  • 220k
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,...
user avatar
  • 543
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 ...
user avatar
  • 131
11 votes
Accepted

Difference between convexo-concave and concavo-convex lenses?

In both types, (convexo-concave or concavo-convex) the lens has one convex and one concave side. Convexo-concave : The concave face has a greater degree of curvature than the convex face. Concavo-...
user avatar
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 ...
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 ...
user avatar
  • 25.5k
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 ...
user avatar
  • 26.3k
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 ...
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 ...
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 ...
user avatar
  • 1,949
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 ...
user avatar

Only top scored, non community-wiki answers of a minimum length are eligible