Question

The position of the direct image obtained at O${}_3$, when a monochromatic beam of light is passed through a plane transmission grating at normal incidence as shown in Fig. The diffracted images A, B and c correspond to the first, second and third order diffraction. when the source is replaced by another source of shorter wave-length

• A. all the four will shift in the direction C to O
• B. all the four will shift in the direction O to C
• C. the images C, B and A will shift towards O
• D. the images C,B and A will shift away from O

Answer 1

Answer: (C)

the images C, B and A will shift towards O

For a diffraction grating, with distance 'd' between adjacent slits, and ${\theta }_n$ the angle between normal to the grating and the diffracted wavelength for nth order,

$d\sin {\theta }_n=n\lambda$

If $\lambda$ is reduced, $\sin {\theta }_n$ becomes smaller. Thus the angle of diffraction for all order reduces, hence the pattern shrinks and images move towards the normal, not diffracted beam.

Hence the images shift towards O.