How to Use a Convex Lens

Convex lenses, also called converging lenses, are the most common lenses used in optical systems. These lenses are thicker in the middle than they are at the edges and have surfaces that curve towards the light rays. This causes the beams to bend together and focus at a point behind the lens and then eventually flip upside down, much the way an image in your eye or a camera would.

Instructions

1. Set up an object you wish to make an image of. Make sure it is sitting steadily on a solid surface, like a sturdy table. If the object is wobbly or unsteady, it will be difficult to obtain a clear image, so secure it carefully.

2. Shine extra lights on to the object you are imaging to make it easier to obtain a clear picture. Because you are employing a very rudimentary lens set-up, the more light the better. You can use simple incandescent bulbs en masse or powerful bulbs designed for photography. Either will do so long as there is sufficient light to produce the image, so use as many lights as you can ably employ.

3. Mount a focusing screen, such as a semi-sheer piece of white silk, tautly about a foot away from the image. Make sure you can see an image projected on to the screen from the other side. Hanging this screen is very much like hanging something to project a movie on to, so it can be as simple as a thin white sheet tacked to the floor and ceiling or tied to posts.

4. Position the convex lens between the object and the focusing screen while you are looking at the screen from behind through the lens.

5. Position yourself behind the screen with the lens in hand. Align the lens so that an imaginary straight line connects your eye, the lens and the object you are imaging. The lens should be perpendicular to this line in order to focus the image and make it clearly visible to your eye.

6. Vary the placement of the lens slowly and carefully until you can see a sharp and upside down image of your object on the focusing screen. Move the lens back and forth and up and down, rotating it slightly if necessary. You will probably need to try several positions before you find one that obtains an image.

7. Mount or suspend the lens in place. This will allow you to create a semi-permanent imaging station. Any object placed at the same point as the object you originally imaged will also appear focused, so you can use this set-up several times in different ways.

References

• The Worsley School: Convex Lenses
• University of Virginia Physics Department: Convex Lenses
• The Physics Classroom: The Anatomy of a Lens