This is a test of commonsense with single/multi-choices. Complete the next sentence:

How to calculate the refractive index in physics
Measure the width of the transparent object.
For example, a pane of window glass has a thickness that may be stated by the manufacturer, but which should be measured to ensure accurate calculation of the refractive index.
Send light from a point light source through the object.

OPT: 1. Light from an invisible window is visible through a transparent window. Calculate refractive index by finding the refractive index number (pi).. 2. Monochromatic point light sources are available through laboratory supply outlets. Place the point light source a known distance from the surface of the object.. 3. Suppose a window pane has a thickness of 180 degrees. However, as a general rule of thumb, you must send light that's smaller than 180 degrees through the object.. 4. To generate a refractive index, plug light into a fraction of their length and have the light propagate from the fraction to the center point. A chart, for example, would use a point light source, to represent the diameter of a space..
The answer is:
2.