Refraction of Light Demo – Show students a bottle of mineral oil and a glass straw.  Ask them if they can see the clear objects.  Hold your finger over one end of the glass straw and slowly put the glass straw into the mineral oil (keep the other end covered with your finger).  With the glass straw in the mineral oil, ask if they can see the glass straw.  They will typically say yes.  Remove your finger from the end of the glass straw and allow the air column to be replaced by the mineral oil.  What they were seeing was the air column in the hollow glass straw not the actual glass straw.  The refractive index of the pyrex glass straw and the mineral oil are approximately the same so the light is not bent as it goes from the oil to the glass and you don’t see the glass straw.