Final answer:
Increasing the wavelength of a photon by a factor of two halves its energy. The frequency of the photon will decrease as the wavelength and frequency are inversely related. Doubling the voltage across a fixed resistance will also double the current.
Step-by-step explanation:
If a photon's wavelength is increased by a factor of two, its energy decreases. This is because the energy of a photon is inversely proportional to the wavelength, meaning as the wavelength of a photon increases, its energy decreases accordingly. The correct answer to the question, 'If a photon's wavelength is increased by a factor of two, what happens to its energy?' is (a) It is halved.
Concerning the frequency of the photon when its wavelength is doubled, the frequency (d) will decrease. This is because the wavelength and frequency of a photon are inversely related to the speed of light, so when the wavelength increases, the frequency must decrease to maintain the constant speed of light.
When the voltage across a fixed resistance is doubled, (a) the current doubles as well. This is due to Ohm's Law (V = IR), which states that the current through a resistor is directly proportional to the voltage across it and inversely proportional to its resistance.