Refraction occurs when light crosses the boundary between one material and another material. what is the primary cause for this refracting of light upon crossing a boundary?

They differ in the available amount and rate they are used and replaced.

Explanation:

Renewable resources are the ones which are present in abundant quantity and can be repeatedly used. These resources are replaced naturally. For example: wind, solar energy, biomass.

Non-Renewable resources are the resources which are not present in abundant quantity. It takes millions of years to replenish and hence there is zero rate of sustainable replenishment. For example coal, gas, oil.

Hence, we can correctly differentiate between the two by: available amount and rate they are used and replaced.