# Diffraction & Interference problems

1. Apr 22, 2004

### mustang

Problem 15.
By attaching a diffrection-grating spectroscope to an astronomical telescope, one can measure the spectral lines from a star and determine the star's chemicalcomposition. Assume the grating has 3224 slits/cm. The wavelength of the star's light is wavelength 1 = 463.200nm.
find the angle at which the second-order spectral line for wavelength 1 occurs. Answer in units of degrees.
How do you start?

2. Apr 22, 2004

### Staff: Mentor

3. Apr 22, 2004

### mustang

Regards on problems

5.
Light falls on two slits spaced 0.374mm apart. The angle between the first dark fringe and the central maximum is 0.0589 degrees.
What is the wavelength of the light? Answer in units of nm.
Note: This is what i have done
d(sin thetha)=(m+1/2) * wavelength(or w)
(0.374e-3)(sin 0.0589)=(1+1/2)*w
256=w
is this right?

Problem 6. A sodium-vapor street lamp produces light that is nearly monochromatic. If the light shines on a wooden door in which there are two straight, parallel cracks, an interfernece pattern will form on a distant wall behind the door. The slits have a separation of 0.3740mm, and the second-order maximum occurs at an angle of 0.18046 degrees from the central maximum.
determine the wavelength of the light.
This is what I have done:
d(sin thetha)=(m+1/2) * wavelength(or w)
(0.3740e-3)(sin0.18046)=(2+1/2)*w
471 = w
Is this right?

4. Apr 22, 2004

### Staff: Mentor

No. The first dark fringe occurs for a path difference of 1/2λ, not (1 + 1/2)λ

No, same kind of problem. The second order maxima occurs for a path difference of 2λ.

You need to understand that the diffraction pattern goes like this: central max - dark fringe - 1st order maxima- dark fringe - 2nd order maxima.... etc. Each maxima corresponds to another integral wavelength of path difference.

5. Apr 22, 2004

### mustang

So for problem 5.

For problem 5. w=wavelength
(d)sin(thetha)=(m+1/2)*w
(0.374*10^-3)sin(0.0589)=1/2w
922 = w

6. Apr 22, 2004

### Staff: Mentor

I presume you mean: d sinθ = 1/2λ

7. Apr 22, 2004

### mustang

Whoops!

I rechecked my math and I found that w= does not equal 922 but 769.