# Questions tagged [lenses]

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### validity of plane waves in thin lens approximation

If you have radiation emitting at the focus of a lens, why is it that on the other side of the lens, rays are essentially parallel within a region $\approx \frac{D^2}{\lambda}$ , where D is the ...
168 views

### How do those cheap shoebox-phone projectors throw any “decent” image?

You've probably seen them; shoebox phone projectors. They throw a very dim image, but I'm surprised it throws any image at all, since the lightsource is completely diffuse. Why does it still ...
188 views

### Finding a Lens System Matrix for a Planar Concave Thick Lens

How can I go about determining the lens system matrix for a planar concave thick lens? I analyzed that a light ray first enters the lens and refracts, then it goes through the lens, and then it ...
242 views

### Why do things seem smaller when seen through spectacles?

When I see through someone's spectacles, things seem very small to me? Why does it happen? Do people wearing spectacles see things smaller than we do?
76 views

### Combination of lens

In the above diagram, let the refractive index of concave lens be $n_1$ and of convex be $n_2$.So in order to apply lens maker formulae for it, will I have to assume air separation between the lens? ...
320 views

### Why is eye accommodation necessary?

Why is eye accommodation necessary when infinite number of light rays come from a specific point of an object and we can use any pair of rays we need and the curvature of the lens need not to be ...
2k views

### Sign convention in lenses and mirrors [closed]

Can we change the sign convention (distance measured in the direction of incident ray positive and opposite negative) midway solving a problem. According to me we should ,as the mirror/lens formula is ...
158 views

### Location of lens having effective focal length

We know a/c to Gullstrand's equation that the effective focal length of two lenses separated by a distance $d$ is given as $$1/feq=1/f1+1/f2-d/f1f2,$$ but the equation doesn't clarify on the position ...
790 views

### Does the position of the final image in a multiple lens system found by the thin lens formula always agree with the ray diagrams?

For example, in the figure given below let the object be at O, the eye of the observer at E and the pole of the convex and concave lenses be P and P' respectively. Assume the object is placed such ...
496 views

### How is it possible that a converging lens can form a virtual image if its function is to make the light converges?

If the function of a converging lens is to do the light converge, how it can form a virtual image, which as far as I know is formed when the light diverges ? I have another question, that may seems ...
434 views

### Imaging fidelity in a 4F setup

In Fourier optics, a 4$f$ setup is an arrangement of 4 lenses like so: The idea is that the beam waist $\omega_f$ at the last position (at $x = 2f_1 + 2f_2$) is equal to the waist $\omega_i$ at x = 0 ...
446 views

### Focusing LED using a Fresnel lens at different distances

Simple setup: A Red LED in front of a small fresnel loupe/lens (credit card size). Using a screw I can adjust the distance between the LED and the lens. The LED is centered with regard to the lens. ...
60 views

### How can I make a Telescope with the lenses I have

List of lenses I have Convex - 200mm F.L - 50mm Dia Convex - 50mm F.L - 50mm Dia Convex - 40mm F.L - 40mm Dia Convex - 45mm F.L - 25mm Dia Concave - 200mm F.L - 50mm Di I tried to make a ...
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### I have a plano convex lens and I want to increase the magnification

I have a plano convex lens and I want to increase the magnification but I cannot increase the focal length can I do this by increasing the curvature and dome height of the lens?
46 views

### transmission electron microscope beam divergence (limits on resolving power)

I have learnt about TEM recently and have a few questions. I've searched all across the internet but can't find the answer to whether or net electrons in the beam will repel each other thus diverge ...
2k views

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

I understand that light entering a parallel block of glass at a non-90 degree angle will cause dispersion of colours within the block but that these will be refracted by the same degree upon exit so ...
165 views

### How do I experimentally find an image projected by a lens?

I know this is a simple question, however, I have never been taught the practical side to Physics properly so here we go. I know I can use the lens equation to find the image distance, but if I ...
855 views

### How do I find/calculate the radius of curvature of a convex lens? [closed]

I have a convex lens, and I want to calculate it´s radius of curvature. It´s focal length is 24cm. how do I do it?
72 views

### Why can't rainbows form at the start of a storm?

I was recently at a park looking at the gorgeous scenery. I looked above and saw thick grey clouds covering up 3/4 of the sky. The sun's light is still visible for 1/4 of the sky, and it looks low ...
200 views

### How does a compound lens reduce chromatic aberration?

A compound light microscope has two lenses: the objective lens (the lens closest to the specimen) and the eyepiece lens (the lens through which an image is viewed). My textbook states that chromatic ...
232 views

### Proof of Validity of Thick Lens Model by Hamiltonian Formalism or Otherwise

The paraxial imaging geometry of any axially symmetric optical system can be described by three real parameters: the axial positions of the two principal planes and one focal length (i.e. $f_f=f_b$ in ...
172 views

### When does a lens go from 'linear' to 'fisheye'

I recently ordered some lenses in an attempt to gather more light from an object being imaged in very low light conditions. I'm trying to use a converging lens with the object > 2f from the lens so it ...
282 views

### Why bi-convex lens give ignition?

Bi-convex is mostly used type of lenses.It can burn paper with help of sunlight.But why? Why bi-convex lens can give ignition when other lenses can't?
2k views

### Focal length of a pair of lenses

My textbook claims that for a pair of thin lenses separated by a distance $d$ the combined focal length of the system is: $$\frac{1}{f}= \frac{1}{f_1} + \frac{1}{f_2} - \frac{d}{f_1f_2}.$$ Can this ...
3k views

### Magnification - When is it negative?

In magnification, I keep on confusing the signs. From what I understand currently, magnification is positive when the image is erect. An image is only erect when it is a virtual image, therefore ...
534 views

### Calculate the diameter of of Gaussian beam at FWHM (or at 1/e2)

I need to collimate a gaussian laser beam out of a single mode fiber (NA0.15) with a single lens so that the full-width half maximum (FWHM) of collimated beam is 10mm. This question may sound naive, ...
276 views

### Using Standard Parabolic Mirror With Off Axis Source

I am trying to reflect as much light from a device as possible into a camera located 90* off angle from the light source. Currently I'm using a standard flat mirror placed 45* from the part and camera....
70 views

### Is it possible to construct a lens which focuses all the light rays from an extended object in one point?

A perfect lens focuses the light rays from an extended object (in a plane at a constant distance from the lens), in the focal plane corresponding to the distance of the object. Now there is an ...
109 views

### Spherical aberration entity vs. distance from the lens?

Consider a positive lens and spherical aberration, which causes the formation of that region of intersection of different rays as in picture. Named $q$ the distance of the image from the (idealized) ...
206 views

### Does a lens with a given numerical aperture (NA) increase etendue, even if it follows a source (e.g. fiber) with lower NA?

It is well known that the etendue of an optical system cannot be decreased - i.e. the brightness of a source cannot simply be increased by using lenses to try and focus something more tightly. ...