In this experiment we'll measure the laser's wavelength using household materials:

• Clothespins
• Small piece of cloth
• Straightpin
• Meter tape
• Calculator
• Laser pointer

Find a spot where you can put down the laser such that it shines on a wall 6-8 feet or more away.

One clothespin is used to turn on the laser and keep it on, the other can be split to make 2 wedges to roughly level the laser

First we'll find the thread count of our cloth 'grating', by counting how many threads per cm are in the cloth:
Lay the meter tape on the table, cm side up, untape the cardboard disk from the plastic cap and lay it cloth down on the meter tape.

Count the threads in one cm. You need good light (maybe the flashlight helps), sharp eyes and the straightpin (which you can find stuck in the side of the black disk). Plus patience. Count a few times, and decide how many threads you count in one cm.

Calculate the thread spacing

threadspacing = 1 cm / [your count]

(I counted 26 threads per cm, so my threadspacing = 0.038 cm)

Tape the disk+cloth back onto the plastic ring, and place it in front of the laser.

With the meter tape, measure the distance from the cloth to the wall. We'll call this distance

Wall_distance (make sure it's in cm)

Now we turn on the laser, and look closely at the pattern on the wall. (maybe you'll have to put a book under the laser so it can shine through the cloth).

You want to measure the distance between adjacent dots. I lined up the 0 of the meter tape with one of the dots on the left, and taped it to the wall. The 14th dot over to the right, under the other arrow, is at 4.6 cm. So in my case, the spacing between dots is 4.6 / 14 = 0.33 cm

Find your

dot_spacing

Now look at the diagram. In the top diagram [1] you can see 2 rays coming through adjacent openings between threads. They hit the far wall at the brightest center dot position.
In these diagrams,

   D is the distance from the cloth to the wall
   d is the distance on the wall between 2 adjacent bright dots
   spacing is the thread spacing in the cloth
   lambda is the wavelength of the laser pointer

In the second drawing [2] 2 rays constructively interfere to make the next dot on the wall. The triangle is tipped down, and the gap that opens up on the top left is one wavelength (lambda) wide.

We can now draw 2 skinny similar triangles [3]. Triangle (A) has short side d and long sides D. Small triangle (B) has small side lambda and long sides spacing.

We can now write lambda/spacing = d/D, and therefore lambda = d×spacing/D

Filling in the numbers, I get lambda=568 nm. On the laser's label it says 532±10 nm, so I was off by only 7%. Good enough for a quick kitchen measurement!