# Optics, image at infinity

I've been trying to look at other questions here but I cannot find to late on. I'm trying to replicate Young's original experiment, trying to find a fraunhofer diffraction, this is basically diffraction in infinity.

To solve this problem I'm using a lens. There I will have my light (collimated), striking the object, then through the lens and into the camera. He says I need f-f setup, that is the distance f from the object to the lens, and f from the lens to the camera. But when I use the thin-lens approximation, all I can get is that the length from the object to the lens should be f, what am I missing here ?

• It's an experiment we are doing, which is to recreate Young's original experiment, and we were trying to recreate the far-field diffraction i.e. the fraunhofer diffraction. This basically means that we need our image of the diffraction pattern at infinity, only there is it fraunhofer diffraction. The problem is how to get there, we got the hint that we should use a lens system, from left to right that is: Object (slith of card in our case) Lens Image The focallength of the lens is f, and the hint is that we should use the distance f from object to lens, and f from lens to imag – John Skeet Apr 26 '18 at 12:22
• What I cannot understand is why I need the distance f from lens to image, as far as I can see, from the thin-lens approximation, only the first length should be f, that is from the object to the lens. Why does the length from the lens to the image have to be f? – John Skeet Apr 26 '18 at 12:23
• The lens is simply to make sure that the image gets in the infinity, I'm aware that the setup will not be identical to Young's, but close enough and it will be easier for me to show others by using the setup im talking about above. My concern is the lens, as I said, we got the hint from the professor that we should use : Object -> lens (distance f, focal length), lens->image (distance f, focal length). My question is simply, why do I need the distance f from the lens to the image, this is not what think-lens approximation predicts, what am I missing? – John Skeet Apr 26 '18 at 13:03
• 1. The experiment I want to replicate is Young's experiment. 2. I'm looking for the effect Fraunhofer diffraction. 3. I want to use a lens, so it will not exactly be Young's experiment, but close to it. 4. I want a lens between the object and the image, the focal length is f, I believe it's around 120 mm, but keep it as f. 5. Fraunhofer diffraction is bascially an effect occuring in infinity ->I want infinity. 6.My professor tells me, use the distance f = 120mm from the object to lens, and f=120mm from lens to image. 7. I dont understand why I want f=120 mm from lens to image. – John Skeet Apr 26 '18 at 13:56

## 1 Answer

You're focusing (pun intended) on the "f" from the slits to the lens. That'll make the positions of the slits into angles past the lens, casting them out to focus at infinity.

But the part of the experiment that's interesting isn't the slits; they just sit there. It's the bright and dark fringes that emerge from the slits at various angles. So you want an optical setup that converts angle at the lens to position (an image) at some point that you can then photograph. That's the "f" spacing behind the lens: The lens focuses rays of different angles to different points at a distance "f" behind it.

Here’s a found image that might help visualize it (ignore the caption, please):

• Sorry for not being able to upvote, I have yet another question, is it possible to show this mathematically somehow? I tried with the thin-lens approximation but couldn't really understand. – John Skeet Apr 26 '18 at 17:58