(a) Explain what is meant by the term superposition.

(b) In a double-slit experiment, yellow light of wavelength $590 \mathrm{nm}$ from a sodium discharge tube is used. A student sets up a screen $1.8 \mathrm{m}$ from the double-slit. The distance between $12$ bright fringes is measured to be $16.8 \mathrm{mm}$.

Calculate the separation of the slits.

(c) Describe the effect of:

(i) Using slits of narrower width, but with the same separation

(c) Describe the effect of:

(ii) Using slits with a smaller separation, but of the same width.

(a) A laser light is described as producing light that is both highly coherent and highly monochromatic.

Explain what is meant by the terms coherent and monochromatic.

(b) This diagram shows the experimental setup (left) used to analyse the spectrum of a sodium discharge lamp with a diffraction grating with $500$ lines ${\mathrm{mm}}^{-1}$, and the spectral lines observed (right) in the developed photographic film.

(i) Explain why two spectra are observed.

(b) This diagram shows the experimental setup (left) used to analyse the spectrum of a sodium discharge lamp with a diffraction grating with $500$ lines ${\mathrm{mm}}^{-1}$, and the spectral lines observed (right) in the developed photographic film.

(ii) Describe two differences between these two spectra.

(b) This diagram shows the experimental setup (left) used to analyse the spectrum of a sodium discharge lamp with a diffraction grating with $500$ lines ${\mathrm{mm}}^{-1}$, and the spectral lines observed (right) in the developed photographic film.

(iii) The green maximum near end $\mathrm{A}$ is at an angle $\mathrm{\theta }$ of $19.5°$. Calculate the wavelength of the green light.

(b) This diagram shows the experimental setup (left) used to analyse the spectrum of a sodium discharge lamp with a diffraction grating with $500$ lines ${\mathrm{mm}}^{-1}$, and the spectral lines observed (right) in the developed photographic film.

(iv) Calculate the angle produced by the second green line.