In optics, what does the term 'normal' refer to when discussing refraction?

In optics, the term 'normal' specifically refers to a line that is perpendicular to the boundary where two different media meet, such as air and glass. This concept is crucial in the study of refraction, which is the bending of light as it passes from one medium into another with a different density. When light encounters this boundary, the angle of incidence (the angle at which the incoming light strikes the boundary) is measured with respect to the normal line, and the angle of refraction (the angle at which light continues in the second medium) is also measured from this perpendicular line.

Understanding the normal is essential for applying Snell's law, which relates the indices of refraction of the two media to the angles of incidence and refraction. This connection helps predict how much the light will bend, which is fundamental in optics design, including lenses and various optical instruments.

The other options describe unrelated concepts. The type of lens related to refraction focuses on how light is manipulated but does not define the term 'normal.' The speed of light in a vacuum is a constant relevant to the characteristics of light but not specifically about the concept of the normal in refraction. Lastly, a unit of measure for light intensity does not pertain to how