# Some questions on lenses

1. Jan 16, 2010

### ap_cycles

1. Are all convex lenses also converging lenses?

The attempt at a solution
Convex refers to shape. Converge refers to action. My answer is yes, provided that the material the lens is made of is denser than air, or denser than some other surrounding medium. Am i correct? OR are there other explanations?

2. What are the characteristics of a lens with a short focal length?

The attempt at a solution
We should divide the lens into 2 categories: namely converging and diverging. For a converging lens, the centre should be much fatter than at its edges. For a diverging lens, the centre should be much thinner than at its edges. Am i correct?

3. Suggest an application when an object is placed at the focal length of a lens

I saw a textbook. It says that a real image is on opposite side of the object at infinity, in which case it is used in a spotlight. Another image which is virtual is on the same side of the object, also at infinity, in which case it is used in the eyepiece of a telescope. Can someone please explain how the real image (on the opposite side of the object) is formed please?? It really bugs me!

2. Jan 20, 2010

### ap_cycles

Anyone able to help please? I am a school teacher about to deliver this topic soon. :surprised

3. Jan 20, 2010

### Redbelly98

Staff Emeritus
1. Yes, I agree.

2. The converging/diverging issue seems unrelated to the matter of short focal length. I'm not sure exactly what they are trying to get at, it may have something to do with aberrations.

3. Again I'm not sure exactly what they are after. The spotlight is a pretty good answer. An object at the focal length will form an image "at infinity", as you are saying. That means that rays coming from a point on the object will come out of the lens parallel to each other. So their "intersection" point (the image location) is at + or - infinity.

If you are going to teach a class, perhaps you can just avoid the subtle questions and concentrate on basic calculations using object and image distance, focal length, and magnification? What grade level is the class?

4. Jan 21, 2010

### ap_cycles

Hi RedBelly,

Thanks for the reply. To clarify the points you raised:

2. The converging/diverging issue seems unrelated to the matter of short focal length. I'm not sure exactly what they are trying to get at, it may have something to do with aberrations.

I interpret this question as what are the physical characteristics of a lens that will enable it to have a short focal length. Using this as the main reasoning, a thick fat converging lens will have a shorter focal length. So will a thicker diverging lens. (I made a subtle mistake in my first post, the overall diverging lens should be thicker, not thinner.)

Another physical feature i can think of is the refractive index of the lens. The higher the refractive index compared to the surrounding medium, the shorter will be the focal length.

If you are going to teach a class, perhaps you can just avoid the subtle questions and concentrate on basic calculations using object and image distance, focal length, and magnification? What grade level is the class?

Well, true. But the students i am teaching are a bunch of high-ability students.(They are fifteen year olds.) That is the caveat. As a teacher of such students, i naturally have to be prepared.

5. Jan 21, 2010

### Redbelly98

Staff Emeritus
Good points. Another way to put it is to say that short focal length lenses have greater surface curvature, while long focal length lenses have flatter surfaces.

That's true. Though within a given commercially available product line of lenses, the lenses will be of the same material (hence refractive index), and have varying surface curvatures to produce different focal lengths.

True. Good luck, and feel free to post again with questions. I'll suggest your future posts let people know that you are a teacher up front, then people may be more forthcoming with help.

6. Jan 25, 2010

### ap_cycles

"True. Good luck, and feel free to post again with questions. I'll suggest your future posts let people know that you are a teacher up front, then people may be more forthcoming with help."

Thanks for the encouragement !