# Tag Info

73

...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 analysis along that idea.) It is true that all rays parallel to the axis focus to that single dot. Not all rays, however, are parallel to the axis: Rays ...

51

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 but without the lenses. Similarly, objects are slightly but perceptibly smaller than they are without the glasses, as is clear from the size of the mugs at ...

44

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 objective forms an inverted image of the distant object in its focal plane. The next lens combination forms an erect image in the plane of the graticule (reticle in ...

33

I don't know of any research to find out if skin sunburns faster when wet, though someone did a comparable experiment to find out if plants can be burnt by sunlight focussed through drops of water after the plants have been watered. You need to be clear what is being measured here. The total amount of sunlight hitting you, and a plant, is unaffected by ...

27

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 point on the other side. That point is the image of the original point. The standard diagram shows that a lens sends all axis parallel incoming rays to the focal ...

24

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 quality have multiple elements added specifically to address the issue of chromatic aberration. What happens with flat glass isn't chromatic aberration - that's an ...

23

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 second law of thermodynamics would be false were it possible to use lenses and/or mirrors to make some object get hotter than the source of the thermal radiation....

22

I'm no expert in biology, and biology may describe this phenomena? I am applying what I know about physics. Sunburn is caused by excessive exposure to skin-damaging UV light. I tend to believe that wet skin by itself does not cause you to burn faster, one merely feels cool in water, so one is prepared to stay out in the sun longer. I can't see any reason ...

21

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 lens will typically be some distance in front of the eye (not touching). If I understand your question correctly, you are asking whether that results in an ...

18

This is mainly an engineering & economics question; and we can deal with those aspects of it over on the Sustainability Stack Exchange, if you want. And there is one conceptual physics aspect too. No, fresnel lenses are not widely used for solar power. Occasionally, but rarely. Concentrated solar power (CSP), including concentrated photovoltiacs (CPV) ...

18

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 both the lens and the liquid should have values very close to each other. Additionally, the lens in question should transmit much more light than it reflects, ...

17

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 hit the sensor (eye-cone, digital sensor, etc) at a single point, assuming that point is in focus. However, light from different points will strike the sensor ...

16

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 $1000{\rm W\,m^{-2}}$, your eye will therefore focus about $0.7{\rm mW}$ onto the retina. This is actually considerably below the amount of heat the superbly densely envasculated ...

15

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 others. Basically, the molecules absorb single photons at the laser wavelength, but when the intensity is great enough that two photons are absorbed ...

15

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 of the lens would need to be 4.25 in/(2*sin(60 degrees)) ~ 2.5 inches = 63.5 mm which is smaller than the width of the lens, and you can't really make normal ...

14

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 provided by a large series of solar panels. Now to go to the lenses part of the question: A series of lenses concentrating on the same spot can add up to a ...

14

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 bigger". Which is a lot like "being closer" to the source. For example, if you say the sun normally is a disk that spans about 0.5° in the sky, then if I have a ...

13

The effect, aperture give to the depth of field is caused by the "used part of the lens". As the a system of lenses can only make a certain point being focused, there is the need of a trick to gain a high depth of field. This is (not only but also) done by the small aperture. The reduction of the used part of the lens leads to less aberrations for the ...

12

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 eyesight gets worse, you need a stronger focal length lens in your eyeglasses, and tilting the lenses has this effect. The problem with doing this all of the ...

12

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 as the velocity of light in both the medium is same, which is clear from the idea of equal Refractive index.So the lens is not visible. So lenses inside liquid ...

10

When lenses are modeled (with the geometric optics approximations) what is usually shown is a single, mathematical, point source emitting equally in all directions (or in all forward directions), and the lens then transforms that point source to another point behind the lens. When the rays are coming in parallel, as shown above (courtesy of Wikipedia), you ...

9

If you want an intuitive answer, the depth of field indicates the interval of distances in which the image will be approximately on focus. When you reduce the aperture, the light cone narrows. This means that you would observe that the confusion circles are smaller. Hence, the range of distances where the image is on focus has increased. A formula for the ...

9

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 only distorted radially along direction CP, the distortion is said to be radial. If the position of P' is also displaced tangentially relative to CP (along the ...

9

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 is not possible to focus sunlight to a single spot. As a simplification, let us consider the sun to be a perfect black body. Then if we know its surface ...

9

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 calculus it carries to much of physics. This is primarily Lagrangian and Hamiltonian mechanics. The fundamental upshot is that the volume a system occupies in ...

9

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 with cheap cameras. I would add to his answer the reasons why usual convex lenses disperse light much less than a prisma does. For example, it's difficult to ...

9

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 paid to lens design than mirror design. But the techniques are similar. First, the small angle approximation assumes angles are so small that it doesn't ...

8

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-convex : The convex face has a greater degree of curvature than the concave face. The images are as follows: Now a days these lenses are referred as convex-...

8

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 actually see something, because if you think of it, at any moment there is light hitting our eye that comes from all kinds of directions and all kind of objects. However ...

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