Chapter 14, Problem 19RQ

To determine

The reason for the impartment of more energy by a feeble beam of violet light than a bright beam of red light.

Answer to Problem 19RQ

A feeble beam of violet light imparts more energy than a bright beam of red light because of the greater frequency of violet light than red light.

Explanation of Solution

According to Einstein, electrons are ejected from a metal surface by shining light on it only if the light has a minimum frequency. The energy of the photons of light is directly proportional to the frequency of light.

The frequency of light is greater than that of red light. The frequency of red light is small enough to cause the ejection of electrons from a metal surface. More electrons are emitted by a bright light only if the frequency of light is greater than the minimum frequency necessary for electron emission.

Since violet light has a high frequency, the photon's energy will also be high and thus, even a feeble light beam can cause emission of electrons. Due to low frequency, even bright red light cannot impart much energy.

Conclusion:

Therefore, a feeble beam of violet light imparts more energy than a bright beam of red light because of the greater frequency of violet light than red light.