Thermal lensing is a photo thermal effect which is induced by temperature gradients and results when energy from a laser beam passing through a sample is absorbed, causing heating of the sample along the beam path. As a result of temperature difference, the temperature is varied causing a change in refractive index. A thermal lens in created as a result of change in refractive index. The effect, termed thermal lensing, arises when a high-intensity beam of light interacts with an absorbing medium and highly localized heating occurs.