# Difference between plano-convex and biconvex lens

1. Jun 6, 2009

### klng

Hi all,

I am implementing a lab activity for a 2 lens set-up that will serve as a simple telescope.

In the teacher's manual, it lists down 2 plano-convex lenses, with the 2 curved surfaces facing each other. The plane faces will respectively face the light source and the observer's eye.

A simple net search shows that regardless of which side (i.e. plane or curved) of the planoconvex lens is facing the light source, the point of convergence of the light rays will be the same on the opposite side of the lens.

Which then begs the following 2 questions, what then is the key difference(s) between a plano-convex lens and a biconvex lens? Will my 'telescope' fail to work if i use 2 biconvex lens instead?

Thanks in advance. Physics Forums rock!

2. Jun 6, 2009

### Lambduh

It won't matter at all. For a set up like a simple telescope all that matters are the focal lengths of the lenses.

There shouldn't be much of a difference at all.

3. Jun 6, 2009

### Redbelly98

Staff Emeritus
Biconvex lenses will be worse than plano-convex in terms of spherical aberrations.

Using two plano-convex lenses, with the plane surfaces facing each other, would minimize spherical aberrations in a telescope. It seems the teacher's manual got it wrong, or the author was not considering aberrations when they made the telescope diagram.

4. Jun 6, 2009

### Andy Resnick

Redbelly98's correct- the idea is to have each surface participate equally in the optical power of the combined instrument. For example, using a plano-convex singlet to expand and collimate a light coming out of a pinhole (a spatial filter), putting the plane side facing the collimated beam side will result in considerably more spherical aberration than placing the plane side towards the pinhole- in the first situation, the plae surface contributes zero optical power, while in the second it contributes power.

5. Jun 6, 2009

### klng

Thanks a lot to all those who have applied. This forum is a great resource haven.

So to conclude, a better way to have a better telescope will be to have the 2 plane surfaces facing each other. The curved surfaces will then face the light source and the observer's eye respectively. Is this the right conclusion?

6. Jun 7, 2009

### Redbelly98

Staff Emeritus
Yes, that's right.

7. Aug 18, 2009

### Artie84

What's written here is correct in many cases, but it is dependent on what your application is:

1. If your 'telescope' is for imaging distant objects:

In this case the object is near infinity and the incoming field can be considered nearly collimated. Plano-convex lenses are best, with flat surfaces facing the inside of the telescope. The goal is to have both sides of the lens active in deviating the light direction.

2. If your 'telescope' is being used for image relay of small, nearby objects about a focal distance away from the lenses:

Plano-convex is fine, but the flat surfaces should now be facing outward. If the magnification of this image relay system is close to 1:1, then the use of biconvex lenses will eliminate most abberation. If the magnification is far from 1:1, then biconvex lenses will make things worse.

In any case, orientation and use of plano-convex vs. biconvex lenses won't matter terribly much. You will still form a fine image so long as the lenses are the correct distance apart. These details only affect small distortions in the image.

8. May 27, 2011

### udayag

hi to alll

I am new to this forum. I have small doubt regarding converging light beam. I have highly diverge light beam of diameter of 1cm. I want to focus or converge the light beam with the distance of 20cm.How can i achieve maximum focus without much loss? which type of lens shall i use?.

I planned to use of planoconvex lens. expecting the your earliest response guys

9. May 28, 2011

### Artie84

Hi Udayag,

This is probably not the right thread for your question, but I'll try to answer it anyway.

First, your question is not very clear. My guess is that you are trying to focus a large diverging beam, and you want the focus to be 20cm away from your lens.

If you are focusing a collimated beam, this is easy: Just use a 20cm focal length lens.

If you are focusing a diverging beam, it is more difficult. If you know where the previous focus is, you can use the lens equation, $$/frac{1}{f}=/frac{1}{d_o}+/frac{1}{d_i}$$ to determine the focal length. If not, you can try trial and error.

The most typical way to focus a diverging beam is in a "4-f system", meaning four focal lengths. This actually uses two lenses. The first lens collimates the diverging beam, and the second focuses it. This is used for systems where it is important to preserve the wavefront shape across distances. If your beam is a simple gaussian beam (the standard output of most solid-state lasers), you don't need to worry about the 4-f configuration.

Either way, to minimize loss you need to make sure you have an appropriate coating on your lens. An uncoated lens will typically give you 8% loss (a 4% reflection on both the front and back surfaces). It is standard to sell lenses with broadband coatings that reduce loss to less than 1% over a range of wavelengths.