The student's observation can be explained by the concept of absorption and reflection of light. Black surfaces tend to absorb more light and heat compared to white surfaces, which reflect a significant amount of light.
In the sunny spot, the black paper absorbs more sunlight and converts it into thermal energy, causing it to feel warmer after 30 minutes. In contrast, the white paper reflects much of the sunlight, absorbing less heat, and therefore feels cooler. This fundamental principle of thermal radiation and light absorption accounts for the difference in temperature between the two pieces of paper after the exposure to sunlight.