# Questions tagged [lenses]

A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction.

797 questions
Filter by
Sorted by
Tagged with
830 views

### Does focal length of lens change with change in refractive index?

If there are two identical lenses L1 and L2 but made with different materials such that refractive index of one is greater than the other (Say for L1 = 1.33 and L2 = 2.13). Will it affect in any way? ...
175 views

### Can't figure out the other focal length for this plano-convex lens [closed]

The original question: A plano-convex lens has a thickness of 4 cm. When placed on a horizontal table, with the curved surface in contact with it, the apparent depth of the bottom most point of the ...
956 views

### Is real image formed by a single optical element always inverted and virtual image always erect?

Question: Is real image always inverted and virtual image always erect with respect to the object? Is there any single optical device (a mirror, or a lens, or anything else) which could produce a ...
113 views

189 views

### How to determine which number is $r_1$ or $r_2$ in the given equation?

A physicist is designing a contact lens. The material has an index of refraction of 1.40. It also had: an inner radius of curvature (surface that touches the eye) = +2.60 cm an outer radius of ...
376 views

### The sides of a spoon as concave/convex mirrors

I recently saw a lecture that said the following regarding the sides of a spoon as concave/convex mirrors: The concave inside of the spoon must be a positive focal length mirror because it would take ...
861 views

### Lens focusing a collimated beam into a disk of some material. Focus shift due to movement of material?

I encountered a geometric optics problem that gave the example of a lens focusing a collimated beam into a disk of some material with refractive index $n$. It then claimed that, if the disk moves ...
319 views

### Spot size for objective (not infinity corrected)

A fundamental axiom of imaging in optics is the resolving power of the lens system, which is directly related to the smallest possible spot a lens can form. For an ideal lens with a numerical aperture ...
170 views

### LED flashlight Lens / Reflector Choice for Collimation (Large throw)

I am trying to build a LED flashlight with a large throw (beam of light should reach very far, be as parallel as possible). I am using a led with approx 84watt (10x10 COB, 100Watt rated) which should ...
485 views

### Can we see a real image with naked eyes? [duplicate]

If we can't see a real inverted image without the help of a screen, why are we able to see an inverted image on the concave side of a spoon and in the convex lens when the object is far from the focal ...
36 views

### Phenomenon of waves transforming at focus

In the following image, we have a plane wave that travels through a lens, such that it comes to a focus: As we can see, the lens transforms the plane wave into a convex wave. But once the convex wave ...
998 views

### What happens in a two lens system with two converging lenses when the object is placed at the focus of the first lens?

I was working on my physics homework when I incorrectly copied part of a table for an optics problem with a two lens system, and I found myself trying to find the total magnification of a system of ...
30 views

### Rays and lenses in a viewfinder

An electronic camera viewfinder has a LCD screen and a combination of lenses. When looking into the viewfinder the image looks much bigger than the actual LCD though. In fact when looking from a bit ...
51 views

### Formulas with complete sign conventions for spherical mirrors and thin lenses

I'm looking for two things: Something like the third equation in the chosen answer in the link, but for spherical mirrors, and Something that covers conventions for the radii of curvature and focal ...
226 views

### Why do we need an objective lens in a scanning electron microscope?

I've been studying scanning electron microscopes quite a bit but can't seem to get my head around why we need objective lenses. On paper I don't see a reason, if we would want to focus the electron ...
720 views

### Difference between electrostatic lens and electromagnetic lens

Imagine an electron-beam in a scanning electron microscope. From what I've gathered usually electro static fields are used to accelerate the electrons to a desired speed. At the same time electro-...
170 views

### What kind of lens does the opposite of magnifying?

I know nothing about physics, so I figured I would ask here. I am doing an art project, for which I need an lens that has the opposite effect of a magnifying lens, that is, to make whatever you're ...
54 views

### In non-technical terms, how does the scanning electron microscope function?

I've watched a few videos like this one and know the basic structure. We have an e-source, an anode which pulls electrons out, then a condenser which starts to focus all beams, scanning coils and ...
99 views

### Why do I need a longer focal length to take a picture far away?

Can someone tell me why I need to use a lens with a longer focal length when aiming to take pictures of distant objects?
239 views

### How do we see virtual images?

Here in the diagram rays coming from B' point falls on two different places on retina of eye.So we should see B' at two different places. But it doesn't happen like that in reality. Is my logic wrong? ...
32 views

### The change of focal distance as a result of adding a substance [closed]

I have a given focal distance between lens and an image sensor (in air). Now I add a protective glass with a known refractive index and width between the lens and the sensor. How can I calculate the ...
740 views

### How to shift light to a longer wavelength / lower frequency energy level so that it becomes a different "color"?

I need to shift light from a certain wavelength/frequency to another, lower, level. For example, green light down to yellow or orange. I want to reduce the energy level of the transmitted light, for ...
116 views

### Question about zooming out with optical lens

I need some help. I am working on a project and I am wondering if it is possible to make an object to look 100 times far away than it is actually using lens (or more than one lens). The lens (or ...
1k views

### What happens when one side of a convex lens is made fully reflecting?

Assume that I have got a bowl half-filled with mercury. In this bowl, I place a convex lens which make one side of the lens fully reflecting. What will happen to the lens, will it behave like a ...
115 views

### Image formation of object on itself by a system of a lens and a mirror

Suppose there is a lens and a mirror. An object is placed in front of this system (nearer to the lens). If the final image of object (refraction, reflection and again refraction) is formed on the ...
438 views

### What would a lens-less world look like to us? [closed]

The images we perceive through our eyes are formed by focusing the incoming light with the eye's cornea and lens. The information is "captured" by the retina and then transmitted to our brains through ...
92 views

### Problem related to combination of lens & equivalent focal length and lens maker's formula [closed]

in between two equiconvex lens( made of glass) of focal length of 20cm there is a layer of water which also acts as a convex lens.The whole figure is surrounded by air. Find out it's equivalent focal ...
54 views

### Overall power/focal length of an eye and corrective lens system

Hey. I got this question wrong and I don’t get the explanation. I know that to fix myopia you need a concave lens to diverge the light rays so that the eye lens converges them further back on the ...
95 views

### How do I explain the below phenomenon I am seeing along the edges of my eyeglass lenses?

I am near-sighted. Here is the phenomenon: I stand in front of a light source that is situated on the ceiling so that it reflects off my eyeglass lenses and appears circular. It is a white light ...
34 views

### Formula for blurring out a grid of holes

Place a camera lens with focal length f_l mm and camera sensor size w mm and h mm behind an ...
106 views

### If I have an IR source, would glass lenses work in focusing them at a point?

So I'm using near-infrared light of the wavelength of order 700-800 nm. I need it focused at one point. I was wondering if using glass lenses would do the job or not?
23 views

### If final image of object is forming at viewer's eye, where will viewer see that object?

If an object is kept at centre of curvature of convex lens and a viewer is present at centre of curvature of other side then where will he see that object? I think he will see the object at it's ...
274 views

### Why do parallel light beams converge at the focal point of a curved mirror?

Can someone prove that all parallel lights beams reflect back and meet at point the point $F$? Any help would be appreciated.
47 views

### Can all aberrations be fully removed by a lens system in theory?

I recently came across an interesting argument (in Mack's "Fundamental Principles of Optical Lithography", beginning of Chapter 3): By adding more and more lenses to an imaging system, more and more ...
641 views

### Why is the reflection of CFL bulb a green spot on my glasses

Why is that if i try to see the reflection of a CFL bulb (the bulb is on) using my glasses i see it as 2 green spots instead of a white light. plus this happens on the outerside of the lenses...not ...