Assertion: In optical fibre, the diameter of the core is kept small.
Reason: This smaller diameter of the core ensures that the fibre should have incident angle more than the critical angle required for total internal reflection.

Explanation:
1. **Small Core Diameter:** In optical fiber communication, the core is the central part through which light is transmitted. Keeping the diameter of the core small is important because it helps maintain a higher critical angle for total internal reflection.

2. **Critical Angle and Total Internal Reflection:** Total internal reflection is a phenomenon where light traveling within a medium (in this case, the core of the optical fiber) is reflected back into the same medium when it encounters the boundary with another medium (typically the cladding). The critical angle is the angle of incidence beyond which total internal reflection occurs.

3. **Incident Angle:** The incident angle is the angle at which light enters the core of the optical fiber. By keeping the core diameter small, you ensure that light entering the core is more likely to have an incident angle greater than the critical angle, promoting total internal reflection.

4. **Efficient Light Transmission:** Total internal reflection is crucial for efficient light transmission within the optical fiber. If the incident angle is less than the critical angle, light may escape through the boundary between the core and cladding, leading to signal loss.

Therefore, the smaller diameter of the core in optical fibers is indeed maintained to ensure that the incident angle of light is more than the critical angle required for total internal reflection, facilitating efficient light transmission through the fiber.