Introduction: Measuring Laser Wavelengths

About: By day I am an electronic engineer for a certification and approvals company, by night I am an avid technology hobbyist and DIY'er. I enjoy learning how things work, and sharing my learning experiences with ot…

Hi All, welcome to another instructable! This time I wanted to make a really easy instructable you can do as an evening or weekend project. As part of my ongoing learning into spectrophotometry I have been experimenting with diffraction gratings and monochromators, and stumbled upon "Young's double slit experiment". This is a fascinating observation about how light travels (in waves) and reveals the effect of diffraction for different wavelengths of light.

I decided to try and replicate the experiment to find out for myself how it worked with some laser pointers, and see if I could make the experiment work.

Step 1: Prerequisites and Safety!

Lasers are really cool, but a warning before we continue! Looking into a laser or a strong collimated beam can blind you. Where possible I would recommend the use of colour filtered safety glasses to prevent stray beams damaging your eyes.

Laser pointers are often sold as "cat toys" and granted I love to tease my cat with this, but I found the green one very strong (almost too bright to look at). They also profess to be less than 5 mW of power but I found a great disparity between the intensities of each colour (I may make an optical power meter to measure this in a separate instructable?). I doubt the label matches with reality, which we will soon discover when we measure the wavelengths.

I bought the following materials for the experiment:

  • x3 laser pointers (red, green, blue)
  • A retort stand
  • A diffraction grating slide (500 lines per mm)
  • Paper and pens
  • Bulldog grips
  • Measuring ruler
  • Safety glasses

Step 2: Equipment Setup

The stand should be setup so that the laser pointer is aimed down towards the diffraction grating. The laser will pass through the grating and be projected onto a piece of paper at the bottom (the screen). To set this up follow these simple steps:

  1. Place a piece of paper at the bottom of the stand to make a screen
  2. Place the lower arm of the retort stand about 10 cm above the stand
  3. Attach the diffraction grating to the lower arm and secure it with a bulldog grip
  4. Place the upper arm above the diffraction grating (the distance above the grating doesn't matter)
  5. Attach the laser to the upper arm so that it is aimed so the beam passes through the diffraction grating
  6. Put your safety gear and, and then your ready to shoot some lasers!

Step 3: Experiment

To find the wavelength of the laser you need to measure the fringe separation. To do this follow this method:

  1. When the lasers hit the paper (screen) write down with a pen where the light spots occur (these are known as finges). Make sure you write down the middle one and the ones on both sides.
  2. Repeat step 1 for each colour, marking the fringes on the paper
  3. Once you have done this for all lasers, measure the distance between the middle fringe and the 1st fringe next to it (this is known as the 1st order fringe).

(You'll notice that there is a discrepancy between the picture and what I have recorded in my results later. This is because I did this a few times to determine uncertainty in the measurement).

But how does this relate to wavelength? The equation is lambda = (a * x) / d, where 'lambda' is the wavelength in meters, 'a' is the distance between the slits in the diffraction grating, 'x' is the fringe separation, and 'd' is the distance between the screen and the grating. All of this is available for you to substitute into the equation to give you the wavelength.

But you might ask "how do I know what 'a' is?". Well, if we know the grating has 500 'lines' per mm, that means there are 500,000 lines per m. If we divide 1m by 500,000 lines, we get the distance between them which is 2 µm. Together with x and d we can now calculate wavelength.

Remember that all these distances are in meters. Wavelength is usually expressed nano meters (10^-9 m) so you will need to consider if you want to convert your answer to nano-meters or simply express is a something times 10^-9.

Step 4: Results

I repeated this experiment for this instructable to produce the graph above. In the table you can see two rows (min and max). These are maximum and minimum wavelengths which are indicated on the lasers themselves, so I knew approximately what the wavelength should be to see if I got the right answer.

Looking at the calculations, my measurements do not lie within the maximum and minimum bounds but they are at least consistent. The difference between the measured and expected was between 4% and 10%. I did not do a full uncertainty measurement but it is obvious there will be uncertainty introduced by the measurement techniques (i.e. measuring the distance to the screen not being perfectly perpendicular etc). Even with some unaccounted error I believe this is a fair representation of the actual wavelengths and perfectly demonstrates the double slit experiment.

If you are interested to see the full set of results I have attached the excel file you can use to perform your own measurements. I am now in the process of playing with collimating lenses and reflectors, let me know if you would be interested in an instructable on this, and let me know what you thought about this quick instructable in the comments.

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