The main canvas/viewport is only a very rough preview designed to be fast, but it is not accurate for many things including glass, translucency, reflections, and shadows. Be sure to turn on the render preview in design mode (default shortcut is \ )when finalizing your design before doing a final render in render mode. As you can see from this screenshot the render preview looks pretty different and is much closer to the actual final render. Previewing the render takes some CPU and battery though so we close it by default.
