I'm interested in doing small projects for curiosity. I'm designing an optical structure as shown below. Please tell me what kind of lenses (concave, convex, plano concave etc.. ) to be used at different points of the structure.
I'm designing this to see objects indirectly and I tried this using plano-convex and plano-convex at both the ends but it didn't work. I'm trying to get a magnified vision of the farther objects but its not working. I've to get a magnified vision at the first mirror itself so suggest me what kind of lens to be used in front of object mirror.
It looks like you're trying to make a periscope that magnifies, effectively a bent refracting telescope. I assume that the two optical components at 45 degrees are plane mirrors.
Measure the light path (total distance between your object lens and eyepiece). Pick a convex lens of similar focal length to form an image at your eyepiece position. Another convex lens of shorter focal length here will act as an eyepiece, although you'll have to play around with the separation between the lenses until you get a clear(ish) image. You could also try a biconcave lens at the eyepiece position, making a Galilean telescope. You'll get a lot of spherical and chromatic aberration.
First of all, you need to be sure that your two lenses 'work' without the kink in the system, due to the mirrors (not lenses, as n the diagram, I think). A simple telescope can be made using two convex lenses and the magnification will be equal to the ratio of the two focal lengths (Google 'astronomical telescope) with the long focal length being the objective and the short one being the eyepiece. To focus them, they need to be the right distance apart (about the same as the sum of the focal lengths). Any mirrors you put in between must maintain this same optical distance so you will need quite a long focal length for your objective lens (at least 0.5m). I could suggest getting hold of a +1 Diopter (convex with 1m focal length) lens from an Optician because you may be unlikely to be able to but one from an ordinary 'Science Shop'. Several years ago I went into an Opticians shop and he just gave me a +1/2 Diopter lens when I told him I was a teacher and needed one!! A 20cm focal length lens as eyepiece will then give you a X5 magnification with a 1m focal length as objective. Start with a modest magnification because it may be hard to get it to work with any more. You need to mount things firmly on a board and make sure it's all lined up well or you will not find the image. If you use a bright light (not the Sun) and focus it on a card first, you will see where to put your eyepiece, because that image position is what you need to look at through the eyepiece. The image will be upside down, of course, in such a simple telescope.
Once the telescope works ok, you can get clever with the mirrors but keep everything firmly mounted and, ideally, in a black box.