The correct answer is No unit. Explanation & Key Takeaways: • The refractive index is a ratio of the speed of light in a vacuum to the speed of light in the given medium. It is expressed as: Refractive Index (n) = Speed of Light in Vacuum / Speed of Light in Medium. • Since it is a ratio of two quantities with the same units (speed), the resulting value is dimensionless, meaning it has no unit. • In the given scenario, light travels from a denser medium A to a rarer medium B. The refractive index of medium B with respect to medium A is represented as: n_(BA) = Speed of Light in Medium A / Speed of Light in Medium B. • The refractive index simply compares how much light slows down in one medium compared to another and does not involve any physical units. • Hence, the refractive index of medium B with respect to medium A is unitless. Related Concepts: • Refractive Index: • The refractive index is a fundamental property of transparent materials and determines the bending of light (refraction) when passing from one medium to another. • A higher refractive index indicates that light travels slower in the medium, while a lower refractive index means light travels faster. • Example: The refractive index of water is approximately 1.33, meaning light travels 1.33 times slower in water compared to a vacuum. • Snell's Law: • Snell's Law governs the relationship between the angles of incidence and refraction when light passes between two media with different refractive indices. • It is expressed as: n_(1)sinθ_(1) = n_(2)sinθ_(2), where n_(1) and n_(2) are the refractive indices of the two media, and θ_(1) and θ_(2) are the angles of incidence and refraction, respectively. • Relative Refractive Index: • The relative refractive index compares the refractive indices of two media, such as medium A and medium B, and is given by n_(BA) = n_(B)/n_(A). • This value helps determine the direction and extent of light bending when transitioning between the two media. • Applications of Refractive Index: • Used in designing lenses for optical instruments like cameras, microscopes, and telescopes. • Helps in identifying materials based on their optical properties, such as gemstones or liquids. • Plays a crucial role in fiber optics and telecommunications for efficient signal transmission. Important Notes: • Absolute Refractive Index: This is the refractive index of a medium with respect to a vacuum and is always greater than or equal to 1. • Critical Angle: The angle of incidence in the denser medium at which the angle of refraction becomes 90° in the rarer medium. • Total Internal Reflection: When light traveling from a denser to a rarer medium hits the interface at an angle greater than the critical angle, it gets completely reflected back into the denser medium. • Optical Density: A medium with a higher refractive index is considered optically denser than one with a lower refractive index, irrespective of its physical density.