a very bright source of red light has much more energy than a dim source of blue light, but the red light has no effect in ejecting electrons from a certain photosensitive surface. why is this so?
you used "bright" and "dim" This means the rate of photon emission, not individual photon energy. Blue photons are individually much more energeticthan red photons.
The energy of light is determined by its frequency or color. Blue light has a higher frequency and therefore carries more energy than red light. However, the ability to eject electrons from a photosensitive surface depends on the photon's energy exceeding the work function of that material.
The work function is the minimum amount of energy required for an electron to overcome the attractive forces holding it within the material. The work function varies depending on the specific material's properties.
In the scenario you described, the red light, despite being a brighter source, does not have enough energy per photon to exceed the work function of the photosensitive surface. This means that even though the red light carries less energy, it cannot overcome the work function and eject electrons from the surface.
On the other hand, the blue light, with its higher energy per photon, has the potential to exceed the work function, allowing electrons to be ejected from the photosensitive surface. Therefore, the energy of light alone is not the sole determinant of whether electrons will be ejected, but rather the comparison between the energy of the light and the work function of the material.
This observation can be explained by understanding the concept of the photoelectric effect. The photoelectric effect states that electrons can be ejected from a material when it is exposed to light of sufficient energy. The energy of the light is determined by its frequency or color.
In the scenario you've described, the red light, despite being very bright, does not have enough energy to eject electrons from the photosensitive surface. On the other hand, the dim blue light has higher energy and can cause ejection of electrons.
To understand this further, you need to know that different colors of light correspond to different wavelengths and frequencies. Blue light has a shorter wavelength and higher frequency compared to red light. The energy of a photon (a particle of light) is directly proportional to its frequency: higher frequency means higher energy.
When a photon with sufficient energy strikes the photosensitive surface, it transfers its energy to the electrons in the material. If the energy of the photon is high enough, it can overcome the binding forces holding the electrons in the material, and the electrons will be ejected, resulting in the photoelectric effect.
In the case of the red light, although it may be very bright, its wavelength is longer, and thus its energy is lower. This lower energy is insufficient to eject electrons from the photosensitive surface, regardless of the light's brightness.
In summary, the red light, despite being bright, does not have enough energy per photon to cause the photoelectric effect and eject electrons from the photosensitive surface. The blue light, even if dimmer, has a higher energy per photon due to its shorter wavelength, making it capable of ejecting the electrons.