A laser produces visible light at a single frequency in vacuum. The light falls normally on a double slit and then creates a pattern of bright and dark fringes on a screen, as shown in Fig. 4.1. The slit separation is $1.0 \times 10^{-3}\,\text{m}$. The distance from the slits to the screen is $4.8\,\text{m}$. The separation between the centres of neighbouring dark fringes on the screen is $3.3\,\text{mm}$.
(a)[3]
Explain how the pattern of bright and dark fringes is formed.
(b)[4]
Calculate the frequency of the light emitted by the laser.
(c)[1]
The double slit is taken away. A second laser is positioned next to the first laser. The second laser emits visible light of a different frequency from the first laser. The beams from the two lasers overlap on the screen. Explain why a steady pattern of bright and dark fringes is not formed on the screen.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “light spreads out / diffracts at the slit(s)” …