How Do Magnifying Glasses Work?

Magnifying glasses permeate the world in various sizes and forms, and have applications ranging from the comparatively mundane — say, making otherwise difficult-to-read magazine text large enough to discern — to the scientifically profound — for example, bringing fantastically far-away elements of the universe into clear focus and allowing people to see microscopic organisms. Magnifying glasses work thanks to the simple principles of optical physics.

In addition to making reading easier by functionally enlarging the words on printed pages, magnifying glasses broaden humankind's understanding of nature by allowing people to see in great detail what they otherwise wouldn't see at all. The magnifying lens of a powerful microscope reveals the appearance of tiny bacteria and even viruses. The magnifying lenses in astronomical telescopes afford breathtaking images of distant planets, galaxies and other heavenly objects. Birdwatchers and other naturalists enjoy enhanced views of their targets using binoculars. Each of these instruments takes advantage of the same essential magnifying lenses found in hand-held units, and differ primarily in their arrangement and power.

A magnifying glass is a convex lens. Convex means curved outward, like the underside of a spoon or the dome of a sports stadium. It is the opposite of concave, or curved inward. A lens is something that allows light rays to pass through it and bends, or refracts, them as they do so. A magnifying glass uses a convex lens because these lenses cause light rays to converge, or come together.

A magnifying glass, in effect, tricks your eyes into seeing what isn't there. Light rays from the object enter the glass in parallel but are refracted by the lens so that they converge as they exit, and create a "virtual image" on the retina of your eye. This image appears to be larger than the object itself because of simple geometry: Your eyes trace the light rays back in straight lines to the virtual image, which is farther from your eyes than the object is and thus appears bigger.

See the resource for an interactive demonstration of this process.

The magnifying lens is a critical aspect of modern technology. Without it, you would not be able to take advantage of cameras, watch movies on a screen or use gadgets such as the night-vision goggles that are vital in certain military operations. Going back to the early 17th century, Galileo assembled the first astronomical telescope, and discovered previously unknown features of Earth's moon and nearby planets, and also revealed that Jupiter has multiple moons of its own.