The focal length of the lens of a normal human eye is about

The focal length of a normal human eye is approximately 1.7 cm to 2.5 cm. In a relaxed state, the eye focuses light from infinity onto the retina, which is located about 1.7 cm to 2.4 cm behind the lens system. Specifically, the cornea acts as a primary converging lens with a focal length of about 2.3 cm, while the crystalline lens provides fine adjustment with a focal length of approximately 6.4 cm. Together, they function as a single lens system with an effective focal length of roughly 22 mm (2.2 cm) in the eye's internal medium, or about 17 mm (1.7 cm) when calculated as an equivalent in air. Option 1 (25 cm) refers to the 'least distance of distinct vision' or near point, not the focal length. Therefore, the focal length falls within the 2-5 cm range.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > 10.1.1 Power of Accommodation > p. 162