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Investigating the Photo-Electric Effect
When a metal is exposed to electro-magnetic radiation, light, UV, x-rays etc electrons can be ejected from the metal surface.  
This process is called the photo-electric effect.  
The intensity of light at a distance, r from a point (small) light source is inversely proportional to r2.  
Consider a photo-electric-cell illuminated by light from a point source, as shown below.  
   
 
   
If the photo-current, Ip, is measured with the light source at different distances, r, we find Ip is inversely proportional to r2.  
We therefore conclude that the photo-current (number of electrons emitted per unit time) is directly proportional to intensity of light.  
   
Relation Between Photo-Electric Emission and Colour of Light  
In the wave model of electro-magnetic radiation, different colours of light correspond to different wavelengths (or frequencies).  
 
In the diagram above, the arrow representing the current, Ip, is drawn in the sense of electron current (not conventional current).  
   
The coloured filter allows a narrow range of wavelengths to pass through it.  
A variable voltage supply is connected in opposition to the photo-electric cell.  
   
With a coloured filter is put in place, the voltage of the supply is gradually increased until Ip is reduced to zero.  
The voltage required to reduce the photo-current to zero is called the stopping voltage, Vs.  
   
Knowing this voltage allows us to calculate the kinetic energy (Kmax) with which the most energetic electrons were emitted from the photo-emissive surface.  
   
It is found that:  
 Vs increases as the wavelength of the light decreases  
There exists a maximum wavelength for photo-emission to occur (in a given metal).  
For wavelengths longer than this threshold, no photo-emission occurs no matter how intense the radiation.  
   
These observations are summarized in the laws of photo-electric emission which are traditionally stated in terms of frequency of radiation (remember f α 1/λ).  
   
1. The number of electrons emitted per second is directly proportional to the intensity of the radiation.
2. The maximum kinetic energy of the electrons emitted increases with the frequency of the radiation.
3. There is a minimum frequency (the threshold frequency, fo) below which no emission occurs.
 
   
A graph of Kmax of photo-electrons against frequency of radiation has the following form.  
 
See also Einstein's Photo-Electric Equation.  
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