In the absence of external forces, every particle continues in its state of rest or straight and uniform movement with respect to an inertial or Galilean reference system.

summarize this phrase from galileo galilei in your own words

Provided there is no external force applied to an object at rest, or in motion, the object will continue resting, or in motion respectively. For example, when you slide a glass across the table, it continues sliding forever if there is no force applied. However, it stops sliding because of a force called friction. Similarly, if you leave a book in a shelf, it will stay there forever unless you apply external force like pushing, lifting, and pulling.

Explanation:

c. newton's third law of motion

wavelength divides in half and speed stays the same.