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A compound microscope is an optical instrument used for magnifying small objects that are not visible to the naked eye. It is a combination of two lenses: the objective lens and the eyepiece (or ocular lens). The compound microscope uses the principle of optical magnification to produce a magnified image of a specimen. Here's how it works:

Principle of a Compound Microscope:
The principle of a compound microscope is based on the idea of using two lenses to magnify an object. The objective lens is positioned close to the specimen being observed, and it forms an enlarged real image of the specimen that is located inside the microscope. The eyepiece then acts as a magnifying lens to further magnify this real image, allowing the viewer to see a highly magnified virtual image.

Image Formation in a Compound Microscope:

Objective Lens: The objective lens is the lens closest to the specimen. It collects and magnifies the light that passes through or reflects off the specimen. This lens forms a real, inverted, and enlarged image of the specimen just inside the microscope tube.

Real Image Formation: The objective lens creates a real image at its focal point, which is inside the microscope tube. This real image is inverted, meaning it is upside down compared to the actual specimen. The magnification achieved by the objective lens is determined by its focal length and the distance between the real image and the objective lens.

Eyepiece (Ocular Lens): The eyepiece is located at the top of the microscope tube, where the real image formed by the objective lens is situated. The eyepiece acts as a magnifying lens to further enlarge the real image. It creates a virtual, magnified, and upright image that the viewer observes when looking through the eyepiece.

Total Magnification: The total magnification of the compound microscope is the product of the magnification of the objective lens and the magnification of the eyepiece. It is typically written as "total magnification = magnification of objective × magnification of eyepiece." The eyepiece typically has a fixed magnification of 10x, while the objective lenses are available in various magnification powers (e.g., 4x, 10x, 40x, and 100x). By switching between different objective lenses and using the same eyepiece, the microscope can provide a range of magnification levels.

Focus: To observe a specimen clearly, the microscope's focus is adjusted by moving the objective lens or the stage (where the specimen is placed) up or down. This movement changes the distance between the objective lens and the specimen, ensuring that the real image formed by the objective lens is sharply focused.

In summary, the compound microscope employs the principles of optical magnification and real and virtual image formation to allow scientists and researchers to observe tiny specimens at high magnification levels. The combination of objective and eyepiece lenses results in a powerful tool for studying microscopic objects and structures.