When a longitudinal wave is refracted at 90 degrees, the incident angle is called

When a longitudinal wave is refracted at 90 degrees, the incident angle is called

• A. The second critical angle
• B. The first critical angle
• C. The third critical angle
• D. The critical angle

Answer: (B)

The incident angle that produces a refracted ray along the boundary (90 degrees to the normal) is called the critical angle. For a given pair of media, there is only one such angle. At this angle, Snell’s law gives the refracted angle as 90°, since sin(90°) = 1, so the relationship is n1 sin θi = n2. If the incident angle is smaller than this, the wave refracts into the second medium at some angle. If it’s larger, the wave cannot refract and is totally internally reflected. Therefore, the correct term is simply the critical angle; there aren’t multiple “first,” “second,” or “third” critical angles for a single boundary.

The incident angle that produces a refracted ray along the boundary (90 degrees to the normal) is called the critical angle. For a given pair of media, there is only one such angle. At this angle, Snell’s law gives the refracted angle as 90°, since sin(90°) = 1, so the relationship is n1 sin θi = n2. If the incident angle is smaller than this, the wave refracts into the second medium at some angle. If it’s larger, the wave cannot refract and is totally internally reflected. Therefore, the correct term is simply the critical angle; there aren’t multiple “first,” “second,” or “third” critical angles for a single boundary.