Here's why:

* Light is a wave: Light travels as an electromagnetic wave. Waves don't have a specific length in the way we think of objects.

* Light travels at a speed: The speed of light is constant in a vacuum (approximately 299,792,458 meters per second). We can measure how long it takes light to travel a certain distance, but not the length of the light itself.

Perhaps you were trying to ask:

* How long does it take light to travel across Jupiter? We could calculate that based on the diameter of Jupiter.

* What is the diameter of Jupiter? This is a measurable quantity and the answer is approximately 142,984 kilometers.

Let me know if you had a different question in mind.