An infinite diffraction grating has rulings of 800 lines per mm. The monochromat
ID: 1509239 • Letter: A
Question
An infinite diffraction grating has rulings of 800 lines per mm. The monochromatic, 400 nm wavelength, light leaving the grating is incident on a convex focusing lens with focal length f = 5 cm placed a distance u = 9 cm from the grating as shown in the figure below. A magnified image of the grating is formed on a screen at a distance v from the lens. What is the separation between the diffraction fringes formed between the screen and lens? What happens as the diameter of the lens is increased? (Explain)Explanation / Answer
lets calculate it simply
the lens magnifies the diffraction grating and produces an image.
lets fid that out.
for a convex lens,
f= 5 cm
u= 9 cm
v= ?
we use les formula,
1/f = 1/u + 1/v
=> 1/5 = 1/9 + 1/v
=> 1/v = 4/45
=>v= 11.25 cm
now,magnificaion= - v/u = -1.25
so,now ,
l length of grating will be 1.25 l
800 lines pem mm means gap = 0.001 /800 = 1.25x10-6 m
so grating space will be 1.25* 1.25x10-6 m= 1.5625 x10-6 m