When you hold your cardboard square between the lights source and the wall you observe a shadow.
Does light penetrate a cardboard?
We know that light is a source of energy. Light travels in straight lines and this is called the rectilinear propagation of light. Having known that light travels along a straight path, we know that light would pass through the cardboard which is not too thick.
This cardboard is now able to scatter the light waves. We would then see a shadow at the wall. This shadow would have two regions; the region of complete darkness called umbra and the region of partial darkness called penumbra.
Thus, when you hold your cardboard square between the lights source and the wall you observe a shadow.
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