# I How can I have an image in space that I cannot reach out and feel with my hand?

1. Jan 26, 2017

### barryj

There is this science gadget that consists of two parobolic mirrors, one with a hole in it and you place an item in the bottom of one, turn the other mirror with the hole on top of it, and you can see the image of the item standing above the hole in the top mirror. How is this done? Are the mirrors parobolic, I am not sure. What does the focul length have to be. I do not know of any other way that I can see an image but when I try to touch it, it is not there. Explanation Please

2. Jan 26, 2017

### phinds

3. Jan 26, 2017

### barryj

It would be interesting to see a ray diagram showing how this works.

4. Jan 26, 2017

### Staff: Mentor

The diagram they have in phinds' link shows two rays emerging from the object and converging at a point just above the device. These rays (and all the others from the rest of the object) pass right through each other and start to diverge. These rays diverge the same way that rays from a real object diverge and when you put your eye into their path the rays appear to be coming from an object hovering in air (because that's the closest point the rays are diverging from).

This imaginary object is known as a 'virtual object' in optics terminology.

5. Jan 27, 2017

### pixel

If by "imaginary object" you mean the image that is hovering above the device, then that is not a "virtual object." It is a "real image."

6. Jan 27, 2017

### Staff: Mentor

Whoops, you appear to be correct.

7. Feb 1, 2017

### Staff: Mentor

Okay, upon reviewing my class notes I figured out why I was confused. The example in my notes has a marginal and a chief ray intersecting and then being intercepted by a lens before they could form an image, thus forming a virtual object on the right side of the lens. I thought it was two chief rays or two marginal rays or something.

8. Feb 1, 2017

### rumborak

Off course the annoying part of these gadgets is that the object always hovers just below the hole, not above it.