There are a few cells that can be viewed with the unaided human eye. The yolks of bird eggs, red cell in watermelon, and fish eggs all are cells that can be seen with the human eye. Most cells can not be seen without a microscope. The surface to volume ration constrains increases in a cell's size. Each type has a different resolution, which is defined as the ability to distinguish between two points.
Resolution depends on the wavelength and numerical aperture.
Light microscopes work through bending light. In a compound light microscope, two or more sets of glass lenses bend incoming rays to form an enlarge image of a cell or specimen. It helps if the cell parts differ in color and density and in many cases stains are used. Transmission Electron microscope is based on accelerating the flow of streams of electrons that have wavelengths of about 0.005 manometer that is about 100,000 times shorter than visible light. Electrons are particles, but they behave like waves.