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

Introduction to Refraction\nRefraction of light is the phenomenon of the change in the direction of path of light when it passes obliquely from one transparent medium to another transparent medium. This bending of light happens because the speed of light is different in different media. When light travels from a rarer medium (like air) to a denser medium (like water or glass), its speed decreases, and it bends towards the normal. Conversely, when it travels from a denser medium to a rarer medium, its speed increases, and it bends away from the normal.

Laws of Refraction\nThere are two fundamental laws of refraction of light, also known as Snell's laws:

1. 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.
2. Second Law (Snell's Law): The ratio of the sine of the angle of incidence ($i$) to the sine of the angle of refraction ($r$) is a constant for the light of a given color and for the given pair of media. Mathematically, it is expressed as: $$\frac{\sin i}{\sin r} = \text{constant} = n_{21}$$ Here, $n_{21}$ is called the refractive index of the second medium with respect to the first medium.

Refractive Index

1. Definition in terms of Speed: The refractive index of a medium can be expressed in terms of the speed of light in different media. If $c$ is the speed of light in vacuum (or air) and $v$ is the speed of light in a given medium, then the refractive index $n$ of the medium is given by: $$n = \frac{c}{v}$$
2. Significance: The refractive index is a scalar quantity that indicates how much the optical density of a medium is. A higher refractive index means the light slows down more significantly in that medium, and the bending (refraction) will be more pronounced. For example, the refractive index of glass is about 1.5, and that of water is 1.33. This indicates that light travels slower in glass than in water, and glass is optically denser than water. Refractive index helps in designing lenses, prisms, and various optical instruments used in daily life.