# Questions tagged [lenses]

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### How is a (rifle scope) reticle in focus?

The lens zooms and focuses something very far away, yet the reticle inside the lens assembly is in perfect focus just like the far objects. How?
0answers
44 views

### How do I determine the lens to set the focal length of a converging or diverging beam?

I have a gaussian beam going through two lenses. The first lens is focusing a collimated gaussian beam to a focal point either before or after the second lens. What formula do I use to get the focal ...
0answers
36 views

### Geometric optics limit in lens design [closed]

What is the rule-of-a-thumb ratio of lens' thickness to wavelength at which they can be designed using ray optics approximation? (The thickness is more relevant to gradient index lens, homogeneous ...
0answers
53 views

### Will a combination lens have symmetric focal length on either side?

In this discussion on finding the focal length for combination lens, the image described in the answer shows the measure of focal lengths. Description of the Gullstrand equation also talk about front ...
2answers
476 views

### Focusing white LED light into a spot as small as possible

An aperture (about 1mm) is placed 2cm in fount of the LED, collimated by a f=50mm lens, then focused by a f=100mm lens into a 3x3mm spot. The focal distance of the second lens cannot be decreased. ...
0answers
111 views

### How do hypermetropic people see objects beyond the focus of their convex corrective lenses?

The way I have developed intuition about optics suggests that humans can only see objects if two rays of light that originally emanated from the same point reach the eye in a way such that when ...
2answers
290 views

### Depth of field of the human eye

When reading about the basics of photography and the physics involved, I learned about  depth of field. Long story short, the message is that with a smaller aperture, the length of the depth of field ...
2answers
88 views

### Difference in focus between lenses and glasses [closed]

I have both glasses and contact lenses. The prescriptions are both recent and up to date. I am 46, so I'm starting to have a harder time to focus on things that are very close to my eyes, compared ...
0answers
65 views

### Biconvex lenses in 2 media

The question is what is the formula of the focal length of a biconvex thin lens if object medium is water and the image medium is air. I know that we have: 1/f = (n-1)*(1/R1 - 1/R2) , if the medium ...
0answers
31 views

### Screen distort for corrective lens

Can we apply a screen distort for corrective lens for eye ware instead of using a lens maker? Just test at home with a phone screen and buy at shop. To reframe the question can we bring to effect a ...
1answer
695 views

### Focal length of Convex lens

Is it true that, twice the focal length of a convex lens is equal to the radius of curvature? Please give your opinion.
2answers
66 views

### What's the difference between these two approximations for thin lenses?

I have seen the thin lens equation, which describes the following relation. $$\frac{1}{\text{image distance}} + \frac{1}{\text{object distance}} = \frac{1}{\text{focal length}} \tag{1}$$ However, I'...
3answers
165 views

### Does density of light power (w/m²) increase after passing a convex lens ?

Does density of light power (w/m²) increase after passing a convex lens ? and if so i need the equation that represent that increase
1answer
70 views

### Can we see through the central obstruction of a lens?

I have a very basic question in optics inspired by this article from the University of Stanford which clearly states that one should see a light source through a mask in the middle of a lens if the ...
1answer
54 views

### For nearsightedness how does the concave lens not shift the focus point for near objects?

In case of myopia, the light focuses in front of, instead of on, the retina when observing far away objects. while for near objects the eye is able to focus the light onto the retina. For most people ...
0answers
29 views

### In this situation, is an endless sequence of images (theoretically) formed?

I was dealing with an optics problem, where an object is placed at $u= -60$ cm in front of a biconvex lens of radius of curvature $R=30$ cm and refractive index $\mu=1.5$ . Simple calculations show ...
1answer
46 views

### Lens metrology: How to measure a double-sided thick aspherical lens optically?

In lens metrology, how do people measure a double-sided thick aspherical lens optically? By "optically" I mean for example, using wavefront sensing, interferometry or the interferogram methods. I ...
1answer
54 views

### How does the human eye not get a distorted image from an irregular object?

Suppose I have a sheet of cardboard, or for that matter any irregularly shaped object. Now we do know that reflection on cardboard is irregular and not specular as it is a rough surface compared to a ...
1answer
38 views

### Determining a spherical mirror setup just from the image and object heights

I am working through some questions to prepare for an upcoming exam and got stumped by a question involving a spherical mirror where we are only provided with the image height (1 cm), object height (0....
2answers
196 views

### Confused with image and object distance in lens

I am not sure why S is negative in this image. How do we determine whether it's negative or positive?
2answers
403 views

### Light focusing on long distances

If I have a green laser (500 nm wavelength) with initial beam diameter 3 mm, with proper lenses I can easily focus it to 1 mm or 0.5 mm for small distances like 10 cm. My question is can I focus it ...
2answers
102 views

### Human eye dispersion [duplicate]

As we know dispersion is caused by a convex lens.our human eye also has a convex lens so dispersion must take place in eye also this leading to a formation of blurred image on retina.then why are we ...
1answer
24 views

### Making a lense using an array?

Would it be possible to build a lense out of array elements with a higher refractive index inter spaced with void elements eg a matrix of glass with air. I imagine that the spacing would have to be ...
1answer
114 views

### Shape of a virtual image

There are a lot of posts regarding real vs virtual images. I 'think' I understand the distinction clearly, however, there is one thing that I can't quite wrap my head around. What is the "shape" or ...
2answers
3k views

### How to measure the refractive index of a lens?

how to measure the refractive index of a lens? (a simple method) I want to find a simple method to measure the refractive index of a lens.
1answer
853 views

### Should the thickness of a plano-convex lens affect its focal length and intensity of light at the image?

Using Lens maker formula for thick lens, where f is the focal length R1 and R2 are the radii of the two sides d is the thickness n is the index of refraction For a plano-convex lens, R2=∞. So the ...
1answer
91 views

### Why does the human eye-lens have a constant magnification for a fixed object distance?

The human eye-lens flexes to change focal length and thereby bring objects of various distance into focus. The magnification of the lens $M$ is always constant for an object with distance $d_o$ ...
1answer
97 views

### Geometric optics question (from 2001 GRE)

In the diagram above, two lenses with focal lengths $f_1 = 20$ cm and $f_2 = 10$ cm are placed $40$ cm and $70$ cm from an object $O$, respectively. Where is the final image formed by the two-lens ...
1answer
22 views

### How can I achieve small field of view by 2 or more lenses?

I have the following problem. I have a sensor with 1mm diameter and I need to have 500 urad (microradian) field of view. If I understand correctly, that would generally mean that I need to use 2meter ...
1answer
36 views

### Distorted Converging Lens

You have a converging lens made from a certain plastic that focuses sunlight when the lens is 12 cm from the ground. The lens is curved identically on both sides. You leave it out in the sun so that ...
1answer
109 views

### Why is an image formed when two light rays coming from an object intercept?

Why is an image formed when two light(one to the focus and one to the lens perpendicularly from the top of the object) rays coming from an object intercept?
1answer
45 views

### The Final Image? [closed]

I can't understand that if these lines are parallel to each other how and where the final image would form?
0answers
44 views

### How big a lens would you need to light a fire with moonlight? [duplicate]

Randall Munroe posted an interesting what-if in which he claims that you can't use moonlight to light a fire, because it is impossible for reflected light to raise the temperature of an object higher ...
2answers
175 views

### Achromatic doublet lens

I am trying to build an achromatic doublet lens with a focal length of 40mm and a diameter of 40mm. I cannot seem to find this commercially available anywhere and I do not want to order a custom lens ...
1answer
47 views

### Laser filamentation in air

For the non-linear optical process of filamentation in the atmosphere propagated by intense laser pulses (1TW) of ps-fs pulsewidths, what is the optimum method, through lens set-ups (or otherwise) to ...
1answer
35 views

### Effective strength of prescription glasses for arbitrary angle

I am trying to understand the optics of prescription glasses. Prescriptions have a spherical and a cylindrical (with associated axis) component specified in dioptres which are roughly additive. For ...
3answers
437 views

### Plane wave focussed by lens to a point

I want to mathematically recover the situation in the following picture, that is a plane wave is incident on a (thin) lens, such that the outgoing beam focusses to a finite spot size at a distance ...
1answer
80 views

### Optics and lens

I am nearsighted. So i am using glasses. In glass i see a convex shaped lens. But to correct my vision i need divergengt lens that is concave. When i am using the glass in correct position the upper ...
1answer
69 views

### Do photons make contact with the atoms in a lens?

We can all agree empirically that a stream of photons encountering an arrangement of atoms forming a lens bends. But do the photons actually make contact with the atoms in the process of bending? I ...
1answer
42 views

### Vertical scale of spherometer

Does the vertical scale of the spherometer have any use? I only depend on the circular scale of the spherometer.
1answer
117 views

### What would happen if the aqueous and vitreous humour of a human's eye are replaced by some harmless transparent liquid of different refractive index?

The title says it all. What would the person be able to see? Note By harmless, I mean that it is non-corrosive, non-toxic, etc. The liquid doesn't cause any biological damage to the eye or any other ...
1answer
93 views

### Why doesn't the brain flip retinal image horizontally as well?

I was reading about the Camera Obscura and the physics of apertures, and I read that "The camera obscura produces a flipped (horizontally) image as well as an inverted (vertically) one." I understand ...
1answer
175 views

### How to calculate the focal spot size of concentrated sunlight

I know that concentrated photovoltaic systems use Fresnel Lenses to concentrate incident sunlight to a point. I also know that this point isn’t infinitely small and has an actual size since the lenses ...
1answer
53 views

### How are progressive lenses made to allow for continuous changes in prescription?

I recently got progressive lenses in my glasses and have been wondering how they achieve the (at least seemingly) continuous change in prescription? Is the glass formed to have a continually changing ...
1answer
3k views

### Refractive index and focal length

When the refractive index of medium is greater than the lens then what will happen to the focal length of the lens?
3answers
553 views

### Is it possible to collimate a point source of concentrated light?

So I am wondering whether it would be possible to create an optical system that does and consists of the following: Use a Fresnel lens to concentrate direct sunlight to a point focus. I assume that ...
1answer
189 views

### Numerical Aperture

I have some questions regarding numerical aperture. Let's say there is a glass lens and a light source to the left of the lens. You can then draw a cone to the lens. Let's say the light rays get ...
1answer
134 views

### Effect of two lenses after single diffraction grating

I am having problems undestanding the effect of two lenses after a diffraction grating of slits whose separation is unknown. The setup is like this: 3cm away from the slits there's a convergent lens ...
2answers
3k views

### Is it possible to calculate image distance by knowing magnification and focal length?

There is an equation in Physics about photography which is all about lenses. ${\rm magnification} = \frac{\rm image\,distance}{\rm object\,distance}$ Assuming we know the magnification, in order ...
1answer
1k views

### Lensmaker's equation for thick lens in two different media

I've been looking around and I've found the lensmaker's equation for thick lens to be \frac{1}{f} = \frac{n_{lens} - n_{m}}{n_m} \big( \frac{1}{R_1} - \frac{1}{R_2} + \frac{n_{lens} - n_m}{n_m} \...