Many plants obtain energy from sunlight using a green pigment called chlorophyll. Other plant pigments can also absorb sunlight, for example the red pigment in raspberries. The photograph shows a solar cell that uses a pigment from raspberries to absorb light in order to generate an e. m.f. When measured with a high resistance voltmeter, the e. m.f. is found to be 400 mV. When the solar cell is connected across a 4700 ⦠resistor, the potential difference across the resistor is 18 mV. Calculate the internal resistance of the solar cell.