A Level Physics Practice Exam

How is the intensity of light defined?

1. The amount of light produced

2. The energy of each photon

3. The number of photons striking a surface per second

4. The wavelength of light

The correct answer is: The number of photons striking a surface per second

The intensity of light is defined as the number of photons striking a surface per second, which corresponds directly to the amount of energy transmitted by that light over a certain area. Intensity takes into account both the energy of the individual photons and how many of them are arriving at a surface, making it a measure of the power per unit area. This definition is rooted in the principle that light can be thought of as both a wave and a stream of particles (photons). The higher the number of photons per second impacting a surface, the greater the energy delivered to that surface, leading to a higher intensity of light. In contrast, the other options do not accurately reflect the concept of intensity. The amount of light produced refers to brightness in a general sense without quantifying it in terms of energy or area. The energy of each photon pertains to the frequency and wavelength of the light, which contributes to its color, but does not define intensity alone. The wavelength of light is a characteristic of the light itself and does not provide information regarding the intensity, which is fundamentally about the photon count and energy flux.