Diffraction grating experiment

1. Nov 20, 2013

m_p_w

I did an experiment where a red laser went through a diffraction grating
The distance between the diffraction grating to the screen is 10cm (x)
Later I measure the distance from the central dot to the first dot (4.5cm) (y)
after this I measure the distance from the central dot to the second dot(13.1cm) (y)

I used this formula to calculate the wavelength of the red laser: d*tan(y/x) = m λ
where m = 0,1,2... and d = 1.67*10^-6 in this experiment.

The problem which I've got is that I got two different wavelengths for the same red laser
692.4nm and 765.6nm.
My question is this, should I add these two wavelengths and then divide it by two to get the "average" wavelength or should I just say in the experiment that the wavelength "765.6nm" is a random error; thus, we should not take it to the consideration?

2. Nov 20, 2013

m_p_w

any one please, if this is a stupid question then please tell me

3. Nov 20, 2013

ehild

The equation you quoted is wrong. The correct equation is d sinθ = mλ.

What you did was to replace sinθ by tanθ=y/x, so you calculated the wavelength from the formula d y/x = mλ. But sinθ can be approximated by tanθ only for small angles, that is, when y<<x. That was not valid in your experiment. Find θ=arctan(y/x), and use the formula d sinθ = mλ to get the wavelength.

ehild

4. Nov 20, 2013

m_p_w

thank you so much

5. Nov 20, 2013

ehild

You are welcome.

ehild