Answer:
The width is
![w_c = 0.00252 \ m](https://img.qammunity.org/2021/formulas/physics/college/h5pbsfsyq9ci4te9ltdvew0wemn61c1nqf.png)
Step-by-step explanation:
From the question we are told that
The width of the single slit is
![a = 1.5 \ mm = 1.5 *10^(-3) \ m](https://img.qammunity.org/2021/formulas/physics/college/wqsla0r1faqfapjs849hqyqt8r8z7goo4q.png)
The wavelength is
![\lambda = 420 *10^(-9) \ m](https://img.qammunity.org/2021/formulas/physics/college/3dlxrh08l87wzw1l6vrp3acmij38nzq1p9.png)
The distance of the screen is
![D = 4.5 \ m](https://img.qammunity.org/2021/formulas/physics/college/qtxeem9nhchdybsge8oo55vbudz830c5hp.png)
Generally the width of the central maximum is
![w_c = 2 * y](https://img.qammunity.org/2021/formulas/physics/college/bsbkjguymvkjpia0qzo46ebv8l3ctbtsel.png)
where y is the width of the first maxima which is mathematically represented as
![y = (\lambda * D)/(a)](https://img.qammunity.org/2021/formulas/physics/college/okiqh5sdpz6q48wy92tjfums8l4c9z85o1.png)
=>
![y = ( 420 *10^(-9) * 4.5)/( 1.5*10^(-3))](https://img.qammunity.org/2021/formulas/physics/college/effgi7wimwbabg8hbm34brox69hdyimqn2.png)
=>
![y = 0.00126 \ m](https://img.qammunity.org/2021/formulas/physics/college/a652yha0jci15ov6al4taq55sq51tbw4xm.png)
So
![w_c = 2 *0.00126](https://img.qammunity.org/2021/formulas/physics/college/y15iavrv1mm0ffpt1pfh0krbl1tzz98tcy.png)
![w_c = 0.00252 \ m](https://img.qammunity.org/2021/formulas/physics/college/h5pbsfsyq9ci4te9ltdvew0wemn61c1nqf.png)