Hi all, just wanna hear some insight for a question recently asked by one of my students which is puzzling me. The quesion is why the reflection of light from a vacuum flask surface always appears as a line. Say if you shine a beam of laser toward the flask surface and you will see a line beam from reflection, as seen in the picture below. I wonder if it is because of the special structure of the surface that scatters the light in the vertical direction and causes this effect. Hope you can shed some light on this question. Thank you!