The given graph shows the variation of photo-electric current (I) versus applied voltage (V) for two different photosensitive materials and for two different intensities of the incident radiation. Identify the pairs of curves that correspond to different materials but same intensity of incident radiation. 
The pair of curves (1, 3) and (2, 4) corresponds to different materials but having same intensity of incident radiation. The value of stopping potential for the curves (1, 2) and (3, 4) is same implying that they correspond to similar materials.
