Focus the sun rays on paper and measure the distance of focus from lens it will be the focal length of lens.
covered the one face of lens then refraction will not occur

The Brainliest Answer!
By keeping the lens at place where sun rays r coming. keep the lens above a sheet of paper. on the paper where the image of sun formed is sharp, measure the distance b/w the lens and the image formed. that will b the focal length.

no refraction occurs when :
1.light is incident normally on a boundary
2. when the refractive indices of 2 media are equal
1 5 1