In reflectance spectrophotometry with dry slides, light is typically projected at what angle relative to the detector?

• A. 0 degrees
• B. 90 degrees
• C. 180 degrees
• D. 45 degrees

Answer: (C)

In reflectance measurements, the detector is arranged to view light that has reflected straight back toward the light source. This backscatter geometry means the light path goes out, hits the surface, and returns along the same line to the detector, giving an angle of 180 degrees between the direction the light is traveling and the detector’s line of sight. This configuration emphasizes light that reflects off the surface rather than light that passes through the sample.

So, the typical setup projects light in one direction and the detector looks directly opposite, capturing back-reflected light at an angle of 180 degrees.