Final answer:
Source size affects the sharpness and definition of the penumbra. Smaller source sizes result in sharper, well-defined penumbra, while larger source sizes create softer, more diffused penumbra.
Step-by-step explanation:
When it comes to the effect of source size on penumbra, we need to understand that the penumbra is the region of partial shadow created by a source of light. In general, a smaller source size produces a sharper and more well-defined penumbra, while a larger source size tends to create a softer and more diffused penumbra.
To understand this concept better, let's consider an example of a light bulb. If the light bulb is smaller in size, the light rays that reach the edges of the object casting the shadow will be more focused, resulting in a narrower penumbra. On the other hand, if the light bulb is larger, the light rays will spread out more, causing a wider penumbra.
In summary, as the source size decreases, the penumbra becomes sharper and more defined, whereas a larger source size results in a softer and more diffused penumbra.