In the context given, more optically dense would be interpreted as "higher
refractive index." It turns out that this typically does go hand-in-hand
with mass density; that is, glass with higher mass per unit volume also has
higher refractive index. That's why we make "lead crystal" by adding lead
salts to the glass to make it more mass-dense which also makes it have a
higher refractive index.
Optical density is also an old term for "absorbance." However, it is clear
that is not what is meant in the context of this question.
It would be best if optical density were not used because it has from time
to time been used to connote both absorbance and refractive index, and today
the words absorbance and refractive index are the preferred terms for these
properties. Thus, I would rephrase the statement as: light refract towards
the normal when it enters an medium with higher refractive index.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817