■ Refraction of Light – Study Notes
1. Introduction
•Light travels in a straight line in a homogeneous medium.
•When light passes into a medium with a different optical density, its speed changes, causing
bending of the ray.
•This bending of light is called refraction.
2. Causes of Refraction
•Light slows down or speeds up depending on the optical density of the medium.
•Optically denser medium: light slows down.
•Optically rarer medium: light speeds up.
•Change in speed causes change in direction, governed by Snell’s Law.
3. Important Terms
•Incident Ray: Ray striking the boundary between two media.
•Refracted Ray: Ray that passes into the second medium and bends.
•Normal: Perpendicular to the boundary at the point of incidence.
•Angle of Incidence (i): Angle between incident ray and normal.
•Angle of Refraction (r): Angle between refracted ray and normal.
•Optical Medium: Any transparent material through which light can pass.
4. Laws of Refraction (Snell’s Laws)
•First Law: The incident ray, refracted ray, and the normal all lie in the same plane.
•Second Law (Snell’s Law): sin(i) / sin(r) = constant = n, where n is the refractive index of the
second medium relative to the first.
5. Refractive Index
•Refractive Index (n): A measure of how much the speed of light is reduced in a medium.
•n = Speed of light in vacuum / Speed of light in medium.
•Relative Refractive Index: n21 = v1/v2, where v1 and v2 are speeds of light in medium 1 and 2.
•Example: For air to water, n ≈ 1.33.
6. Refraction Through a Prism
•A prism bends light twice: at entry and at exit.
•Deviation (δ): Angle between incident and emerging rays.
•Angle of minimum deviation (δm): When light passes symmetrically through the prism.
•Refractive index of prism: n = sin((A + δm)/2) / sin(A/2), where A is apex angle.
7. Refraction Through Lenses
•Convex (Converging) Lens: Bends parallel rays to meet at a focus.
•Concave (Diverging) Lens: Bends rays outward, appearing to diverge from a focus.
•Lens Formula: 1/f = 1/v - 1/u, where f = focal length, v = image distance, u = object distance.
•Magnification (M): M = hi/ho = v/u.
8. Phenomena Related to Refraction
•Apparent Depth: Objects in water appear shallower. Apparent depth = Real depth / n.
•Mirage: Optical illusion caused by refraction in air layers with different temperatures.
•Total Internal Reflection (TIR): Occurs when light moves from denser to rarer medium and
angle of incidence > critical angle.
•Critical angle relation: sin(θc) = n2/n1.
•Applications: Optical fibers, periscopes, binoculars.
9. Practical Applications of Refraction
•Lenses in eyeglasses, cameras, microscopes, and telescopes.
•Optical fibers in communication.
•Correcting vision defects such as myopia and hypermetropia.
•Prism spectrometers for studying light spectrum.
10. Key Points to Remember
•Refraction is bending of light due to change in speed when moving between media.
•Snell’s Law defines relationship between angles of incidence and refraction.
•Refractive index measures how much light slows in a medium.
•Lenses and prisms use refraction for image formation and deviation.
•TIR is the basis for optical fibers and many optical devices.
