Question

When measuring a refractive index, what should you do if:

A) No dividing line between light and dark areas can be observed through the eye piece?

B) The dividing line can be seen, but it is fringed with color.

Answer 1

First ensure that the sample holder is clean and free from dirt or traces or previous samples leftover. For this use a clean lint free tissue and acetone to clean the sample adaptor. Also check whether the instrument is calibrated or not prior to the measurement.

(A) Sometime clear distinction cannot be obtained between the dark and light area and this can happen when very little sample has been added and it does not cover the prism and also when the light source is not sufficently bright enough. Check the suitability of the lamp used. Also sometimes when the reading is very close to the actual refractve index value, the view appears to be hazy without clear distinction. Slight adjustment of the knob can bring about clear division between the light and dark areas since the instrument is highly sensitive.

(B) This can happen if either the prism is not clean, or sample is highly colored or the sample is not evenly applied. Try to gently clean the pism and apply the sample to cover the entire sample area as a thin homogeneous coating.