It's the unit of intensity
melting point, temperature at the solid and liquid forms of a pure substance can exist in equilibrium. as heat is applied to a solid, its temperature will increase until the melting point is reached. more heat then will convert the solid into a liquid with no temperature change. when all the solid has melted, additional heat will raise the temperature of the liquid.
the image is at the distance of 4.28 cm from the mirror.