With any type of lens, the object and image?

Question

A. will be the same size.
B. may be on the same or opposite sides of the lens.
C. must be on opposite sides of the lens.
D. can be virtual or real.

Answer 1

Any type of lens - 

I have given an explanation of the answers based only on two types of lenses - either bi-concave (diverging) or bi - convex (converging). Other types of lenses (plano convex, plano concave, diverging and converging meniscus) 

may not agree with all of the explanations given below.

 

A. The image and object have the same size only if they are at the same distance away from the center of a converging lens. Thus, if they are at different distances away from the lens center, they would have different sizes. In the case of a diverging lens, the object distance is greater than the image distance, and the magnification (size of image/size of object) is less than 1. This means what ?

 

B. For a converging lens, a real image and the object are on opposite sides; a virtual image and the object are on the same side. For a diverging lens, the image which is always virtual, is on the same side as the object.

 

C. This contradicts the explanation in B.

 

D. Converging lenses can produce either virtual or real images; virtual images are formed if the object is located between the principal focus and the lens; real images are formed if the object is on the other side of the focus.

Diverging lenses produce only virtual images.

Answer 2

The two principal types of lenses are converging, or convex, and diverging, or concave (there can be more detailed types, but these usually cover what is seen at this level).

 

The identifying characteristic of a real image is that light rays coming from the object need to be brought together and really cross in space (hence, "real" image).  If you put a screen at the site of a real image, you would see the image on the screen, because light rays are actually being brought together there.  To make a real image, then, a lens must be able to bring light rays together (to "converge" them).

 

This means that a (single, thin) diverging lens can NEVER make a real image, because it only ever diverges rays, sending them away at even greater diverging angles.  This is the recipe for making virtual images, which is all a (single, thin) diverging lens can do.  This shows one of the answers above not to be true.

 

If you look at a conventional film projector, the picture on the strip of film is being projected onto the big screen in the theater.  The film strip "cel" is the object, and the real image forms on the big screen.  The projector lens is a type of lens, and what is the relationship between the object (film cel) size and the image size?  This shows another of the answers not to be true.

 

A magnifying glass works by creating a larger, virtual image of what is being read (typically, by using a single converging lens where the object is closer to the lens than the focal length).  This image is on the same side of the lens as the object, as all virtual images must be for a single thin lens.  This shows a third answer not to be true.

 

In fact, in general, for a single thin lens, real images are always on the opposite side of the lens from the object, and virtual images are always on the same side.  This points to one of the answers above.

 

I hope these examples help a little!