At a given temperature and for a given wavelength of plane polarized light, the fundamental property of an optically light, the fundamental property of an optically active substance is. both magnitude and direction of specificrotation. Neither magnitude nor direction of specific rotation.

Introduction:
An optically active substance is a substance that rotates the plane of polarized light as it passes through it. This phenomenon is known as optical activity and is a fundamental property of certain compounds. The specific rotation is a measure of the degree of rotation of polarized light by a given substance. It is defined as the angle through which the plane of polarized light is rotated per unit length of the sample.

Magnitude and Direction of Specific Rotation:
The specific rotation of an optically active substance depends on various factors including the concentration of the substance, the path length of the sample, and the temperature at which the measurement is made. However, for a given temperature and a given wavelength of light, the specific rotation of an optically active substance is a constant value.

The magnitude of specific rotation refers to the numerical value of the angle of rotation. It is a quantitative measure of the extent of rotation of the plane of polarized light. The magnitude of specific rotation is a characteristic property of a substance and can be used to identify and distinguish different compounds.

The direction of specific rotation refers to the clockwise or counterclockwise rotation of the plane of polarized light. The direction of rotation is determined by the arrangement of atoms or groups of atoms in the molecule of the substance. If the rotation is clockwise, it is labeled as (+) or dextrorotatory, while if it is counterclockwise, it is labeled as (-) or levorotatory.

Both Magnitude and Direction:
The fundamental property of an optically active substance is both the magnitude and direction of specific rotation. The magnitude provides information about the extent of rotation, while the direction indicates the stereochemistry or arrangement of atoms in the molecule.

The magnitude and direction of specific rotation are essential for characterizing optically active substances and studying their behavior in various applications such as pharmaceuticals, food chemistry, and chemical synthesis. By analyzing the specific rotation, scientists can determine the identity, purity, and concentration of optically active compounds.

Conclusion:
In summary, the fundamental property of an optically active substance is both the magnitude and direction of specific rotation. The magnitude provides quantitative information about the extent of rotation, while the direction indicates the stereochemistry or arrangement of atoms in the molecule. Understanding the specific rotation of optically active substances is crucial for various applications in chemistry and related fields.