First, let's look at the rays. The ray is determined by the endpoint and direction. We can imagine a ray as an infinitely extending line segment with a fixed starting point and no ending point. So we can't use the traditional concept of distance or length to measure rays. However, we can use the concept of vector to describe the direction and size of light. A vector is a quantity with magnitude and direction, which can be represented by an arrow. The vector of a ray is an arrow, starting from the endpoint and pointing to infinity along the direction of the ray. The size (length) of this vector is the "length" of the light.
Next, let's look at the straight line. A straight line is made up of countless points, which are on the same borderless line. Similarly, we can't measure a straight line with the traditional concept of distance or length. However, we can use the concept of vector to describe the direction and size of a straight line. The vector of a straight line is an arrow that starts from a point on the straight line and points to infinity along the straight line. The size (length) of this vector is the "length" of a straight line.
Generally speaking, the length of light and straight line is infinite, and we can't directly measure it with the traditional concept of distance or length. Instead, we use the concept of vectors to describe their direction and size. This description is more in line with the essential characteristics of rays and straight lines, because they are infinitely extended.