1 · $v=f\lambda$
Ripples on a pond
Frequency $2.5\,\text{Hz}$, wavelength $12\,\text{cm}$. Find the speed.
$\lambda = 0.12\,\text{m}$, $f=2.5\,\text{Hz}$.
$$v = f\lambda = 2.5 \times 0.12 = 0.30\,\text{m/s}$$2 · $T=1/f$
Period of a radio wave
$f = 200\,\text{kHz}$. $f=2.00\times 10^5\,\text{Hz}$.
$$T = 1/f = 5.0 \times 10^{-6}\,\text{s}$$3 · EM wave
Microwave oven frequency
$\lambda = 12\,\text{cm}$, $v=3.0\times 10^8\,\text{m/s}$.
$$f = v/\lambda = 3.0\times 10^8 / 0.12 = 2.5 \times 10^9\,\text{Hz}$$4 · Doppler explain
Boat moving towards the shore
The boat (source) moves towards the observer [1]. Wavefronts in front are closer together so $\lambda$ decreases [1]. Observed frequency increases [1].
5 · $n=\sin i/\sin r$
Glass block
$i=42^\circ$, $r=26^\circ$. Calculator in degrees.
$$n = \sin 42^\circ / \sin 26^\circ = 0.6694 / 0.4384 = 1.53$$6 · $\sin c=1/n$
Critical angle of perspex
$n=1.49$.
$$\sin c = 1/1.49 = 0.671 \qquad c = \sin^{-1}(0.671) = 42.2^\circ$$7 · TIR conditions
Will this ray suffer TIR?
Ray in glass ($n=1.50$, $c=41.8^\circ$) hits glass–air at $i=50^\circ$. Yes: denser → rarer, and $50^\circ > 41.8^\circ$. The ray reflects with $r=50^\circ$.
8 · Explain (marked)
How an optical fibre transmits a pulse
Light enters the fibre and hits the boundary at $i > c$ [1]. Conditions for TIR are met (glass to rarer medium) [1]. The pulse is totally internally reflected along the fibre [1]. Information is carried as a light signal without the fibre needing to be straight [1].
“The fibre reflects the light” with no $i>c$ and no denser-to-rarer is 1 mark at most.
Then sit the five quizzes and the three papers.