Spectrum Scientifics Blog

What is this strange picture you may ask? Well, its an image of the front of our store, projected with a simple lens onto the rear wall of our store. How did we do it? We did it with nothing more than a simple magnifying lens.

In an age where plasma screens, LED, and other electronic systems reign one can easily forget that simple image projection with lenses can be surprisingly satisfying and fun.

In this image you can see our setup was little more than a mounted magnifier and a box for it to sit on. You don’t even need anything this fancy, we just set it up that way to take the photographs. All you would need is a magnifier, a wall to project on, and an image source that is bright enough to project. In this case we found the large windows and windowed door of the front of the store to work nicely. To project an image properly, the projected object must be significantly brighter than the wall or other place where you are projecting the image.

We put the magnifier close to the wall and positioned it until it was pretty close to being in focus. This is the easy way to set things up. If you want to be more complex about it, there is actually a formula for calculating where the image will be

B = (F x A)/(A – F)

Where B = the distance from the lens to the projected image, A = the distance from the object being projected, and F = the F=focal length of the lens (the focal length is the distance at which light coming from an effectively infinite distance (I.e. The sun) is concentrated by the lens.

In this case, the magnifier has a focal length of about 10” or .25 meters, the distance from the front of the store to the magnifier is about 45 ft. or 13.7meters. So we plug in these figures:

B = (.25 x 13.7)/(13.7-.25)
B = 3.425 / 13.45
B = .254m or 10”

You might notice that the image distance is the same as the focal length. This is fairly typical for objects that are very far away; the images distance is usually the same as the focal length.

So how big is out projected image compared to the original object? That’s a simple calcluation:

Size = B / A

So in this case it is:

Size = .25m/ 13.7m
Size = 0.018x

You may have already noticed what you are doing seems familiar, it is because what we have set up is a very crude camera! There’s no film or CCD to capture the image in this case, but the principle is still there: we have made an image using a lens.

What have made here is not only similar to a camera, it is also known as a camera obscura which was invented by persons unknown around the 16th century (maybe earlier in a cruder form). Camera Obscura were very popular in public parks in the Victorian era, but now only a handful remain. The most famous being in San Francisco.

If you enjoyed this post, make sure you subscribe to my RSS feed!

Filed under: Optics, Physics