What does D/L & dl represent.

Answer

### Step-by-Step Solution

1. **Understanding D/L and d/l Notation**:
- The capital letters D and L are used in the nomenclature of stereoisomers, specifically to denote the configuration of certain molecules, particularly sugars and amino acids.
- The small letters d and l refer to the optical activity of these isomers, indicating the direction in which they rotate plane-polarized light.

2. **Capital D and L**:
- The designation of D or L is based on the position of the hydroxyl (-OH) group in relation to the chiral carbon furthest from the carbonyl group (C=O) in sugars.
- If the hydroxyl group is on the right side of the chiral carbon, the compound is designated as D (from the Latin "dexter," meaning right).
- If the hydroxyl group is on the left side, it is designated as L (from the Latin "laevus," meaning left).

3. **Small d and l**:
- The small letters d and l refer to the optical rotation of the isomers.
- A compound that rotates plane-polarized light to the right (clockwise) is designated as d (positive rotation).
- A compound that rotates plane-polarized light to the left (counterclockwise) is designated as l (negative rotation).

4. **Relative Nature of D/L and d/l**:
- It is important to note that D and L are relative terms. You cannot refer to a single isomer as D or L without a counterpart.
- Similarly, d and l also require a comparison with another isomer to establish their optical activity.

5. **Example**:
- For instance, in a sugar molecule, if we have two isomers, one with the hydroxyl group on the right (D) and one with it on the left (L), we can then refer to their optical activities as d (if it rotates light to the right) and l (if it rotates light to the left).