Molecules sometimes move across a plasma membrane from high concentration to low concentration by passing through channel proteins or with the help of transport proteins. If no energy is used by the cell during this process, the process is called:

Passive transport.

Explanation:

The cell membrane is known as semi-permeable in nature it does not allow all the molecules to pass through the membrane. Sometimes molecules need transport proteins to move across a plasma membrane. The movement of molecules without the use of energy across the cell membrane is known as passive transport.

Diffusion is known as the example of passive transport, in which molecules can move from high concentration to low concentration without any input of energy.

the index of refraction of the second medium is higher. i just took the test trust me !