(a)
For the correct answer:
$15\text{ Hz}$
Using the wave equation $v = f\lambda$, where $v$ is the wave speed and $\lambda$ is the wavelength. Rearranging the formula to solve for frequency gives $f = \frac{v}{\lambda}$. Substituting the given values, we calculate $f = \frac{18\text{ cm/s}}{1.2\text{ cm}} = 15\text{ Hz}$.
(b)(i)
For the correct answer:
Refraction
As the ray of red light passes from air into the glass prism, and then from the glass back into the air, it changes speed due to the change in optical density between the two mediums. This change in speed causes the light ray to change direction and bend at the boundaries, a process known as refraction.
(b)(ii)
For the correct answer:
Dispersion shown at the air-glass boundary with rays continuing to the screen. Red must be labeled at the top and violet/purple at the bottom.
To accurately show dispersion, the drawing should feature the single ray of white light splitting into multiple rays (forming a diverging beam) starting at the first boundary and widening further at the second. The top ray hitting the screen must be labeled red, because red light has the lowest frequency and is refracted the least. The bottom ray must be labeled violet, as it has the highest frequency and is refracted the most.
