Modern Physics and Electronics · 8 min read
Photoelectric effect
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Introduction to Photoelectric Effect
Have you ever noticed how a solar calculator starts working the moment you take it out into the sun? Or how some street lights automatically turn on once the sun goes down? This happens because of a special relationship between light and electricity.
Imagine throwing a stone into a bowl of garri; the stone will hit the garri and make some grains fly out. In Physics, light behaves like that stone. When light hits certain surfaces, it kicks out tiny particles called electrons.
Meaning of Photoelectric Effect
The Photoelectric effect is simply the process where light energy hits a metal surface and causes electrons to be released from that metal. It shows that light carries energy in small packets.
In simple terms, it is when light 'knocks' electrons out of a metal surface.
Threshold Frequency and Work Function
Not just any light can kick out electrons. If you use a weak light (like a red bulb), nothing might happen. You need light with enough 'power' or frequency. This minimum frequency is like the 'entry fee' needed to get the electrons out.
Think of the work function as the price of a ticket to enter a cinema; if you don't have up to that amount, you cannot enter.
Einstein Photoelectric Equation
Albert Einstein explained that the energy of the incoming light is used for two things: first, to pay the 'entry fee' (work function), and second, the leftover energy gives the electron speed to move away (kinetic energy).
The formula is: E = W + K.E, which can be written as hf = hf0 + 1/2mv2.
In this formula, 'h' is Planck's constant, 'f' is the frequency of light, and 'f0' is the threshold frequency.
Worked Example
Calculate the maximum kinetic energy of photoelectrons emitted from a metal whose work function is 3.2 x 10-19 Joules, when light of energy 5.0 x 10-19 Joules falls on it.
1. Write down the given values: Incident Energy (E) = 5.0 x 10-19 J, Work Function (W) = 3.2 x 10-19 J.
2. Use the formula: E = W + K.E.
3. Rearrange to find K.E: K.E = E - W.
4. Substitute the values: K.E = (5.0 x 10-19) - (3.2 x 10-19) = 1.8 x 10-19 Joules.
Key points
- •Light behaves like a particle (photon) in this effect.
- •Electrons are only emitted if the frequency is above the threshold frequency.
- •Increasing light intensity (brightness) only increases the number of electrons, not their speed.
- •The kinetic energy of electrons depends only on the frequency of the light.
- •The work function is a property of the specific metal used.