Refracting Light With LASERs

LASERs make almost anything better, and that includes refracting light in physics lessons.  Here's a guide to refracting LASER light, and some calculations you might need to do afterwards.

As a beam of light enters an optically more dense medium it slows down.  If a beam of light is traveling diagonally, some parts will slow down before others. This causes the light ray to bend towards the normal (the normal is a line drawn perpendicular to the surface where the beam of light hits).  The opposite happens when a light ray leave a more dense medium.

Your will need:
  - A laser (and plug if necessary)
  - A protractor
  - A ruler
  - A calculator
  - A pencil
  - A perspex block
  - A peice of paper
and, most importantly:
  - A really cool sticker!

Place the block on the paper and draw all the way around it with the pencil.  This rectangle (unless you have a differently shaped perspex block) will serve as a guide to show you where the block was when it is removed for measuring the angles.

This is probably my favourite step:
Plug in and turn on the LASER!

(Seriously though, DO NOT SHINE THE LASER AT OTHER PEOPLE)

Place the perspex block and paper in the path of the laser, making sure that the block is still aligned with the rectangle you drew.  It should look something like the picture.

When trying to draw out the line formed by the LASER, you can't simply trace it with your pencil, because that wouldn't be a properly straight line so it would be difficult to measure the angles for calculations, and placing a ruler down would break the laser beam and you wouldn't know where it is.

To mark out the beam, you take your pencil (make sure it is sharpened) and mark one dot at one end of the line, one near the middle, and one at the other end of the line.  Do this for the beams either side of the block, being careful that you don't move it.

Turn off and tidy away the LASER.

On either side of the block, join up the dots, to make the line (by the way the reason we drew three dots instead of two, when you only need two dots to make a line, is that the third one can act as a check to see if the other two make sense, e.g. if all three don't line up, its best if you start again).

Then we join the points where the lines meet the edge of the rectangles, and this shows where the LASER beam was as it traveled through the block.

Image three shows how it should look after you have completed this step.

Use a ruler to draw a dashed line, called the normal line, at 90 degrees to the block's surface.

Draw the angles i and r like they are in the picture - between the light ray and the "normal" line.
Measure this with a protractor, and note this down.
I got  i = 41 degrees and  r = 25 degrees

The formula for finding the refractive index (usually called n) is:

        sin  i
n = --------
        sin  r

plugging in the values we got for i and r we get

n = 1.55 (2dp)

Of course, in an exam you might not always get told to find n, but instead be given i , n and asked to find r, so it is useful to have seen other versions of the formula:

i = arcsin( n x sin( r ) )
 
and
             
r = arcsin( sin( i ) / n )
                    
(arcsin is the inverse sin function, or sin to the -1 on your calculator)

Good Luck!