At the very end of a long solenoid, the magnetic field along the axis is

1. The on-axis field of a finite solenoid involves angles subtended at its two ends. 2. Deep inside (both ends far), the formula gives B = μ₀nI. 3. At an end face, one angle is 90° while the other approaches 0°. 4. The field at the end of a long solenoid is therefore B = μ₀nI/2, half the interior value. _Source: syllabus Physics Ch 4 "Moving Charges and Magnetism", §4.8.1_