Curvature of the Iso-quant:

An iso-qunat is convex to the origin. This is so because as more and more units labour are employed, the producer would prefer to give up less and less of the other input to produce the same amount of output. This is shown in the following Figure:  

For a rise in L from L₁ to L₂, the producer gives up K₁ amount of K. For the same output level  q, as L increases from L₃ to L4, she gives up K₂ amount of K. As K₂ < K₁, it implies that for more units of L, the producer is willing to give up less of K. A convex iso-quant implies a diminishing MRTS_LK. As   MRTS_LK = MP_i/MP_k, a diminishing MRTS means as L increases, L MP  decreases and MPK increases.