Placement of Pins

Your physics teacher will insist that optical pins inserted into a cork board are inserted far apart. Why is this important?

1. Image these are the 2 holes in your paper from the 2 optical pins you have aligned.
2. A perfect line through the centres of the 2 pinholes would look like this.
3. As the pinholes are fairly large we are unable to accurately draw a line through their centres. The 2 lines here show the range of angles that you could get when drawing a line through the 2 pinholes – you can see there is quite a bit of uncertainty in its angle.
4. If the 2 pinholes had been drawn further apart then the uncertainty in the angle of the line to be drawn through them is considerably reduced.
   Thus the pinholes should be made as far apart as you can make them.
   In practice we will expect to see pins inserted AT LEAST 5.0 cm APART. But you should be trying to get the points much further than this if possible.
5. Another option available to us is to insert the pins less deeply into the cork board. This results in much smaller pinholes being made in the paper.
6. Such pinholes will have less uncertainty in the angle of lines that can be drawn through them.
7. This is the best option – small holes placed a far apart.

pins shown pushed fully (left) v.s lightly (right)  into cork board

the size of the pinholes in the paper are visibly different

 

Note in the top photo here one of the pins is clearly bent. A good experimenter would exchange this pin for a straight one to enable them to align the pins more easily.

 

 

Practical Skills