The resolution of a light microscope is limited by the wavelength of light. Visible light has a wavelength of around 400-700 nm, which means that objects smaller than this cannot be seen clearly. Many organelles are smaller than this, such as ribosomes, which are around 20 nm in diameter.

In addition, the contrast of an object must be high enough in order to be seen with a light microscope. This means that the object must be significantly different in brightness or color from its surroundings. Many organelles are not very contrasty, such as mitochondria, which are often similar in color to the cytoplasm.

Finally, the thickness of an object can also affect its visibility under a light microscope. Objects that are too thick can absorb or scatter too much light, making them difficult to see. Many organelles are very thin, such as the endoplasmic reticulum, which is only a few nanometers thick.

For all of these reasons, some organelles cannot be seen clearly using a light microscope. However, there are other techniques that can be used to visualize these organelles, such as electron microscopy and fluorescence microscopy.