import numpy as np
import matplotlib.pyplot as plt

r = 1.0

R = np.linspace(0.001, 10, 500)

eta = R / (R + r)

plt.plot(R, eta)
plt.xlabel("R")
plt.ylabel("η(R)")
plt.grid(True)

plt.savefig("4.png", dpi=300, bbox_inches="tight")
plt.show()