Measurement of Slit Width

from the Diffraction of Laser Light

Introduction

In the previous lesson, you were introduced to the concept of diffraction.

Student Objectives

* Students will determine the width of various slits on a diffraction plate using laser light.

* Students will relate their results to X-ray crystallography used to determine the structure of DNA by Watson and Crick.

Materials

Diffraction plate and holder

LASER

White paper taped to book to use as a screen

Meter stick and ruler

The equation below is used to solve problems based on what type of diffraction?

Identify each of the above variables.

[[lambda]] x

d L

sin [[Theta]]

The equation below is used to solve problems based on what type of diffraction?

Identify each of the above variables.

[[lambda]] y

w L

Data

The LASER you will use in this lab is a helium-neon laser He-Ne. The wavelength of the laser you will use is 640 nm.

For each single slit pattern measure three different L's and y's

For each double slit pattern measure three different L's and x's

The table below will explain whether the pattern is single slit or double slit.

Pattern    No.     
          Slits    
   A        1      
   B        1      
   C        1      
   D        2      
   E        2      
   F        2      

Pattern    Measurement of    Measurement of y or   Calculated value of    
              L  (mm)              x (mm)          d or w  (mm)           
   A      L1                y1                                            
   A      L2                y2                                            
   A      L3                y3                                            
                                                                          
   B      L1                y1                                            
   B      L2                y2                                            
   B      L3                y3                                            
                                                                          
   C      L1                y1                                            
   C      L2                y2                                            
   C      L3                y3                                            
                                                                          
   D      L1                x1                                            
   D      L2                x2                                            
   D      L3                x3                                            
                                                                          
   E      L1                x1                                            
   E      L2                x2                                            
   E      L3                x3                                            
                                                                          
   F      L1                x1                                            
   F      L2                x2                                            
   F      L3                x3                                            

Calculations

1. Calculate the value of w for slit patterns A, B and C. Place your answer in the data table. (Show your work below)

2. Calculate the value of d for slit patterns D, E and F. Place your answer in the data table. (Show your work below)

3. Calculate the average w for slit patterns A, B and C. Put your answer in table below using +/- notation.

Pattern    Average value of w    
                  (mm)           
   A                             
   B                             
   C                             

4. Calculate the average d for slit patterns D, E and F. Put your answer in table below using +/- notation.

Pattern    Average value of d    
                  (mm)           
   D                             
   E                             
   F                             

Further Invesigation

A diffraction grating is a clear piece of glass or plastic with many parallel lines scratched in it. It operates on the same principle as double slit diffraction, and all of the formulas still apply.

Use the procedure in this lab to determine the spacing of lines in a diffraction grating your teacher will supply. Make up your own data table.

Conclusion

Describe in a paragraph or two, how LASER light can be used to determine the distance between two slits.