To understand the concept of a step index fiber, let's explore the structure and characteristics of optical fibers used in telecommunication. Optical fibers are thin strands of glass or plastic that transmit light from one end to the other. There are two main types of optical fibers: step index fibers and graded index fibers.

A step index fiber consists of:

• Core: It has a constant refractive index denoted by n_1.
• Cladding: It surrounds the core and has a slightly lower refractive index denoted by n_2.

The core and cladding are carefully chosen so that the refractive index difference between them causes the light to undergo total internal reflection, which is critical in effectively guiding the light through the fiber.

This matches the first option: "Core with constant index of refraction n_1 and cladding with slight lower index of refraction n_2."

Let's analyze why the other options are incorrect:

• Option 2: "n_1 is varying and n_2 is higher than n_1" - This is incorrect because in a step index fiber, n_1 is constant, and n_2 is lower, not higher.
• Option 3: "n_1 and n_2 are same" - This is incorrect as the light wouldn't be guided; total internal reflection requires a difference between n_1 and n_2.
• Option 4: "\frac{n_1}{n_2} = 1" - Similar to the previous point, this implies n_1 equals n_2, which is not the case for step index fibers.

In conclusion, the correct answer is that a step index fiber has a "Core with constant index of refraction n_1 and cladding with slight lower index of refraction n_2."