Why does oil rise up through the wick in a lamp?

Oil rises up through the wick in a lamp because of capillary action, a physical phenomenon that occurs when a liquid moves through narrow spaces without any external force, and sometimes even against gravity. This action happens due to the combination of two molecular forces — cohesion and adhesion. Cohesion refers to the attractive force between molecules of the same substance (oil molecules attracting each other), while adhesion is the attractive force between different substances (oil molecules and the wick’s fibers).

In a lamp, the wick is made of many fine cotton threads that contain tiny gaps or capillaries. When the bottom end of the wick is placed in oil, the adhesive force between the oil molecules and the wick’s fibers pulls the oil upward into these narrow pores. At the same time, the cohesive force within the oil helps to pull more oil upward along the thread, forming a continuous column of liquid.

The balance between these two forces allows the oil to rise steadily through the wick until it reaches the top, where it is vaporized by the flame. The vaporized oil then burns, producing light and heat. This process continues as long as the oil reservoir contains fuel and the wick remains intact.

Capillary action is not limited to lamps; it is also seen in many natural and scientific processes. For example, plants use capillary action to draw water and nutrients from the soil through their roots and stems. In laboratory experiments, capillary tubes demonstrate this phenomenon clearly by showing how liquid levels rise differently in tubes of varying diameters — the narrower the tube, the higher the liquid rises.

This concept illustrates the relationship between surface tension, molecular forces, and fluid movement, making capillary action an important topic in Physics and Everyday Science.