A total magnification of 40x is the lowest power setting on a standard compound light microscope, providing a wide-angle view of a specimen. This magnification is achieved by combining the standard 10x ocular lens (eyepiece) with the 4x objective lens, often called the scanning objective. Scientists use this low-power setting primarily for initial scanning, allowing them to locate and center a specimen on the slide before moving to higher magnification. This level offers a complete look at larger objects, providing context that higher powers cannot.
Understanding Magnification Levels
Magnification in a compound microscope is a product of two lens systems: the ocular lens and the objective lens. The ocular lens, or eyepiece, generally provides a fixed magnification of 10x. Total magnification is calculated by multiplying the power of the ocular lens by the power of the objective lens being used.
The 40x total magnification is often confused with the 40x objective lens, which provides 400x magnification when paired with a 10x eyepiece. For 40x total magnification, the calculation is 10x (eyepiece) multiplied by the 4x (scanning objective). This low-power 4x objective is the shortest lens on the revolving nosepiece and is designed to bring the entire slide into broad focus, making it the starting point for microscopic examinations.
The Scale of 40x: Viewing Whole Objects
One characteristic of 40x magnification is the expansive field of view, which is the circular area visible through the eyepiece. At this setting, the field of view is large, often ranging between 4 to 5 millimeters in diameter, allowing the viewer to see the entirety of many small specimens at once. This wide perspective contrasts sharply with higher magnifications, where the field of view shrinks to a fraction of a millimeter, showing only an isolated section of the object.
The 40x setting also benefits from an excellent depth of field, which is the vertical range of a specimen that remains in focus. Since depth of field is inversely related to magnification, this low power maintains a greater depth of focus than higher powers. This keeps more of a three-dimensional object clear without constant fine-tuning of the focus knob. This combination makes 40x the optimal setting for viewing objects in the millimeter size range that possess a degree of thickness.
Common Biological Specimens
The 40x magnification is suited for observing organisms and structures too large to be fully appreciated at higher powers. In pond water samples, this setting allows for the observation of larger, active invertebrates. Examples include the entire body of a water flea (Daphnia) or the complex cilia of large rotifers. These organisms, often visible to the naked eye as tiny moving specks, are seen as whole, recognizable animals at this magnification.
Prepared slides of plant and animal tissues are also viewed effectively at 40x, providing a clear outline of the complete structure. A cross-section of a plant stem or a leaf is seen in its entirety, allowing for the identification of major tissue layers like the epidermis, cortex, and vascular bundles. Small insect parts, such as an entire wing, a detached leg, or an antenna, can be viewed whole, revealing the overall structure before a higher-power examination of specific details.
Non-Biological and Everyday Samples
Beyond biological samples, the 40x setting reveals detail in many common, non-living materials. Textile fibers, such as cotton, wool, and synthetic materials, become distinguishable by their unique structures. Cotton fibers appear as twisted, ribbon-like strands, while wool fibers show overlapping, scaly surfaces. This magnification is sufficient to identify the source of the fiber by its texture and structure.
Small crystals, such as table salt (sodium chloride) or sugar, display their distinct geometric shapes, which are otherwise too small to resolve. The 40x power is also high enough to distinguish individual grains of sand, revealing their varying mineral composition and color. When examining printed materials, this magnification reveals the minute, individual dots of color—cyan, magenta, yellow, and black—that printers use to create a full-color image, a process known as dithering or halftoning.