[" Four modifications are suggested in the lens formula to "],[" include the effect of the thickness "t" of the lens.Which "],[" one is likely to be correct? "],[[" (a) "(1)/(v)-(1)/(u)=(t)/(uf)," (b) "(t)/(v^(2))-(1)/(u)=(1)/(f)],[" (c) "(1)/(v-t)-(1)/(u+t)=(1)/(f)," (d) "(1)/(v)-(1)/(u)+(t)/(uv)=(t)/(f)]]