Question

Which combination of wavelength and lines/meter will cause the first bright fringe to be closest to straight ahead on a screen in front of a diffraction grating?

• A. wavelength - lines/meter, $400nm-100,000$
• B. wavelength - lines/meter, $500nm-300,000$
• C. wavelength - lines/meter, $600nm-100,000$
• D. wavelength - lines/meter, $700nm-200,000$
• E. wavelength - lines/meter, $500nm-100,000$

Answer 1

The correct option is A wavelength - lines/meter, $400nm-100,000$

The relationship between the wavelength of light $\lambda$ and angle $\theta$ is $d\sin \theta =n\lambda$, where d is the spacing of adjacent lines on the grating, and n is the order number.

For first bright fringe to be closest to straight ahead on a screen in front of a diffraction grating, $\frac{\lambda }{d}=\lambda N$ should be minimum.

Here only wavelength 400nm and 100,000 lines/meter satisfy this condition.