Explanation:

Root hairs are the outgrowths of epidermal cells present in the roots of plants. These root hairs absorb water and minerals from the soil. The absorption of water by root hairs depends on the concentration of solutes in the soil and the root cells.

The concentration of solutes in the soil and the root cells is expressed in terms of tonicity. Tonicity is the measure of the osmotic pressure gradient between two solutions separated by a semipermeable membrane. Based on tonicity, the external medium can be classified into three types:

1. Hypotonic
2. Hypertonic
3. Isotonic

Hypotonic: A solution is said to be hypotonic when the concentration of solutes in the external medium is lower than the concentration of solutes in the cell. In a hypotonic medium, water moves from the external medium into the cell, and the cell swells up. In plants, when the external medium is hypotonic, water is absorbed by root hairs through osmosis.

Hypertonic: A solution is said to be hypertonic when the concentration of solutes in the external medium is higher than the concentration of solutes in the cell. In a hypertonic medium, water moves from the cell into the external medium, and the cell shrinks. When the external medium is hypertonic, water is not absorbed by root hairs.

Isotonic: A solution is said to be isotonic when the concentration of solutes in the external medium is equal to the concentration of solutes in the cell. In an isotonic medium, there is no net movement of water across the semipermeable membrane.

So, in conclusion, water will be absorbed by root hair when the external medium is hypotonic, which means the concentration of solutes in the soil is lower than the concentration of solutes in the root cell.