Chapter 9: Light – Reflection and Refraction
Reflection of Light and Spherical Mirrors
Light travels in straight lines and reflects off surfaces according to two main laws: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray, and normal all lie in the same plane. Plane mirrors form virtual, erect, laterally inverted images of the same size. Curved mirrors, or spherical mirrors, are either concave (inward-curved) or convex (outward-curved). Concave mirrors can form real and inverted images or virtual and erect images, depending on object distance. Convex mirrors always form virtual, erect, and diminished images. The mirror formula (1/f = 1/v + 1/u) and magnification formula (m = –v/u) help in numerical calculations.
Refraction of Light and Refractive Index
Refraction occurs when light passes from one transparent medium to another, bending due to a change in speed. This causes phenomena like objects appearing displaced in water. The laws of refraction include Snell’s Law: sin(i)/sin(r) = constant. This constant is the refractive index, which also equals the ratio of the speed of light in the two media. A medium with a higher refractive index is optically denser. In rectangular slabs, the emergent ray is parallel to the incident ray but shifted sideways due to refraction at parallel surfaces.
Lenses: Types, Image Formation, and Power
Lenses are transparent materials with at least one curved surface. Convex lenses (converging) focus light rays, while concave lenses (diverging) spread them out. Convex lenses can form real, inverted images or virtual, erect ones depending on the object’s position. Concave lenses always produce virtual, erect, and diminished images. The lens formula (1/f = 1/v – 1/u) and magnification formula (m = v/u) are used for image calculations. Power of a lens (P = 1/f in metres) indicates its ability to bend light and is expressed in dioptres. Convex lenses have positive power, and concave lenses have negative power.