Light and the electromagnetic spectrum Study Guide

This core practical investigates how the nature of a surface affects two key processes related to thermal radiation: emission (how much thermal energy a surface radiates) and absorption (how much thermal energy a surface takes in). Different surfaces emit and absorb thermal radiation at different rates. For instance, dull, dark surfaces are generally good emitters and good absorbers of thermal radiation, while shiny, light surfaces are poor emitters and poor absorbers, instead reflecting more thermal radiation. Understanding these properties is crucial for applications such as insulating buildings (using reflective foils) or designing solar water heaters (using dark, matt surfaces). The practical typically involves comparing the rate of temperature change of water in tins with different surface finishes, or comparing the temperature increase of different surfaces exposed to a heat source.

The power of a lens is a measure of its ability to converge or diverge light. A more powerful lens bends light rays more strongly. This property is directly related to the lens's focal length: a shorter focal length indicates a more powerful lens. Mathematically, power (P) is the reciprocal of the focal length (f), expressed as P = 1/f. The unit for focal length must be in metres to obtain power in dioptres (D). Convex lenses, which are thicker in the middle, converge light and have positive power and a real focal point. Concave lenses, which are thinner in the middle, diverge light and have negative power and a virtual focal point. The curvature of the lens surfaces determines how much the light is bent, influencing both its focal length and thus its power.