Brightness of real images from converging lens

[BeauGeste]

Hi,

Have a question about images from a converging lens. I assume that the brightness of the (real) image projected on a screen depends upon the amount of light rays going through the lens - more rays -> brighter. So a larger lens should bring about a brighter image.

Ok, now let's go the the extreme situation of making the converging lens smaller and smaller thereby making the brightness of the image dimmer and dimmer. Eventually only one ray passes through the lens which would be equivalent to having no lens at all.

My question is, can you think of a real image being formed even when there is no lens but since the real image only contains one ray, it is completely dimmed and therefore unobservable?

To think of in another way. Consider no lens. Consider the rays emanating from some point on an object. One of those rays goes to a point on the wall. We see no image though. However if we 'take' other rays coming from the object point, we can see an image (this is what the lens does, right?). So not seeing the image due to the one ray is really a matter of dimness.

Any thoughts on this would be appreciated.

 

[Valerie Park]

Thanks. Yes, it is possible to form a real image with only one ray. This phenomenon is known as the Eikonal approximation, which states that when light passes through a medium, its wavefront will tend to travel along the path of least time. This means that when light passes through a lens, it will tend to take the shortest route through the lens, which may lead to an image that is composed of only one ray. This kind of image is usually very faint, and can be hard to observe. However, it is still a real image and can be used for certain applications.