Surface roughness Ra is a specification of which surface characteristic?

• A. The color of the surface.
• B. The hardness of the material.
• C. The texture of the surface.
• D. The thickness of the coating.

Answer: (C)

Ra describes the texture of a surface—the average roughness of the surface profile. It is the arithmetic mean of the absolute deviations of the surface from a reference line within a sampling length, giving a single value that shows how smooth or rough the finish is. Measured with a profilometer, Ra is typically expressed in micrometers (or microinches). This parameter affects how parts fit, wear, and look, so it’s a key surface finish specification. It doesn’t describe color, hardness, or coating thickness, which are different properties. A smaller Ra means a smoother surface; a larger Ra means a rougher texture.