Question

Compare the effects of light on the plain water and on the soapy water.

Answer 1

Surface tension is the answer. Soap contains surfactant whose purpose is to lower the tension. So, it wets the penny better rather than balling up, and the size ball that can be held together is smaller.

Answer 2

The main difference between the plain water and soapy water is their tension surface.

The surface tension of soapy water is less than plain water and H2O has strong polar bonds.

But when you mix the plain water with soap, the polar bonds will become week and thus lessening the surface tension.

So if you compare the lights on the plain and soapy water then you can find less tension and high light refection in plain water.