How come shadows get blurrier the farther it is from an object?



The contact point in which the shadow touches the object is way sharper than say 5 metres from the object that’s casting a shadow. How come?

In: Physics

There’s this little thing called diffraction in the way unfortunately. What it means is that the light does not go straight from the source. It can “bend” around edges a little which makes shadow edges blurry. The farther the object is from shadow the more light can get dispersed.

the size and shape of the light source is a contributor to this effect. with a very fine ‘point’ source, all of the light is traveling roughly in the same direction as it passes the edge of the object, casting a sharper shadow (hard), even over longer distances. with a larger bulb, or series of LEDs (and especially multiple light sources), light is arriving at the edge of an object with slightly more variation in direction. the effect of these variations is small at short range, but contributes to blurry (soft) shadows as distance increases.

imagine if you were to point 2 laser pointers in as close to the same direction as possible before turning them on. a wall 5 feet away would not show much separation between the two light points, however, pointing them at a barn 100 feet away would yield a much wider separation.

Because the sun is not a point source; it has a visible disc. That means that the light from the sun doesn’t come from exactly one direction but from a small range of directions. So shadows due to the sun have blurry edges to match the apparent size of the sun, which as about half a degree across. The farther the shadow from the object that’s casting it, the larger the burred region caused by the half-a-degree angle. The same effect works for other lights that cast shadows too. If you want sharp shadows, use a tiny light source; you can use a lens instead, like a projector, but then you have to focus.