Photoelectric Effect

We are given that photons of wavelength 660 nm are emitted per second from a lamp. We need to find the wattage of the lamp, which is the total power emitted by the lamp.
Step 1: Energy of Each Photon The energy of a single photon can be found using the formula: where: - is Planck's constant ( ), - is the frequency of the photon. The frequency can be related to the wavelength using the formula: where: - is the speed of light ( ), - is the wavelength of the photon ( ). Substitute the values for and : Now, calculate the energy of a single photon:
Step 2: Total Energy Emitted per Second The total energy emitted per second (which is the power of the lamp) is the energy of a single photon multiplied by the number of photons emitted per second. Since photons are emitted per second, the total energy is: Thus, the power (wattage) of the lamp is:

Concept: Power = energy emitted per second. Photon energy: Step 1: {\color{red}Substitute values.} Step 2: {\color{red}Total power.}

Concept: Energy levels of hydrogen: Step 1: {\color{red}Energy difference (n=2 to n=1).} Step 2: {\color{red}Photoelectric equation.} Step 3: {\color{red}Stopping potential.} Closest intended answer ⇒ 2 V (exam approximation).