MP Board · Class 10 · Science · Light – Reflection and RefractionDefine refraction of light. State the two laws of refraction of light. Also, define the refractive index of a medium and explain its relation with the speed of light in vacuum.

Definition of Refraction\nWhen a ray of light travels obliquely from one transparent medium into another, the direction of its propagation changes at the interface of the two media. This phenomenon is called the refraction of light. Refraction occurs because the speed of light changes as it enters from one medium to another.

Laws of Refraction\nAccording to Snell and experimental observations, the refraction of light obeys two fundamental laws:

• First Law: The incident ray, the refracted ray, and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
• Second Law (Snell's Law): The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for the light of a given color and for the given pair of media. Mathematically, $\frac{\sin i}{\sin r} = \text{constant} = n_{21}$, where $n_{21}$ is the refractive index of the second medium with respect to the first medium.

Refractive Index\nThe refractive index is a dimensionless quantity that expresses how much the speed of light is reduced inside a medium compared to vacuum.

• Formula: The refractive index of a medium ($n$) is defined as the ratio of the speed of light in vacuum ($c$) to the speed of light in that medium ($v$).
• Equation: $n = \frac{c}{v}$
• Explanation: Since the speed of light in vacuum is the maximum possible speed ($c \approx 3 \times 10^8 \text{ m/s}$), the value of refractive index for any optical medium is always greater than 1. A higher refractive index indicates that light travels slower in that medium and undergoes greater bending.