Why does a rough endoplasmic reticulum look rough under a microscope?

Hint: The Endoplasmic Reticulum (ER) is a network of small, tube-like structures which is present in the cytoplasm. It is a membrane bound organelle and is found only in eukaryotic cells. The endoplasmic reticulum is of two types on the basis of presence or absence of ribosomes. The ribosomes are non-membrane bound organelles that are involved in synthesis of proteins.

Complete answer
Two types of endoplasmic reticulum (ER) are- Smooth Endoplasmic Reticulum (SER) and Rough Endoplasmic Reticulum (RER).
When the endoplasmic reticulum does not bear ribosomes, it appears smooth under the microscope and hence, called Smooth Endoplasmic Reticulum. It is involved in the formation of lipids.
When the endoplasmic reticulum bears protein synthesizing structures known as ribosomes, it is called rough endoplasmic reticulum. The RER is so called because of the appearance of its outer surface. In the presence of ribosomes, the endoplasmic reticulum appears rough under the microscope and hence, it is called Rough Endoplasmic Reticulum. The RER is mainly involved in the synthesis of proteins. They are mostly found on the outer membrane of the nucleus. It is also found near Golgi bodies. Most of the proteins which are synthesized into RER are packaged and transported by Golgi bodies to the targeted cells or tissues. The presence of ribosomes on RER makes it different from other ER in both structure and function. The ribosomes help in the translation process i.e. formation of proteins from mRNA sequence. Thus, the protein formation begins in the cytoplasm of cell.

Note:
Ribosomes are granular structures present in both prokaryotes and eukaryotes. In prokaryotes, 70S ribosomes are present while in eukaryotes, 80S ribosomes are present. The ribosomes in both prokaryotes and eukaryotes are made up of two subunits. The ribosomes as dense particles were first discovered by George Palade.