To calculate the surface tension of jelly, the equation Y = F/L can be applied, where F is the force measured by a weighting spring and L is the length of the slide in contact with the jelly. When a microscope slide is dipped into the jelly and pulled out, the force reading on the spring increases until the slide detaches, indicating the force due to surface tension. It's important to note that since the slide has two surfaces, the effective length for the calculation should be doubled, leading to the modified equation Y = F/2L. The length L refers specifically to the portion of the slide that is in contact with the jelly, not the distance from the spring to the slide. This method provides a practical approach to measuring the surface tension of jelly.