IB Physics SL C.3.2 Snell's Law

Practise applying Snell's law with angles measured from the normal, selecting the incident and refracted indices correctly and predicting bending direction from relative wave…

[Maximum number: 6]

Monochromatic light enters the base of a plastic beaker that contains water with an oil layer floating on it. A student draws a diagram to show the directions the light takes as it passes through the layers. The student's diagram has one error at position A and one error at position B. The refractive indices of the materials are shown on the diagram.

Figure for Question C.3.2—Snell’s law question 1 — IB Physics SL

The light is refracted at an angle of $32^{\circ}$ when it enters the plastic layer as shown.

(a)

Identify, with a reason, the error in the student's diagram for

[ 4 ]

(i)

light crossing the plastic-water interface (position A).

[ 2 ]

the angle of refraction ought to be greater than the angle of incidence OR the ray should
refract away from the normal
because ray goes from high refractive index/<<optically>> more dense/slower medium to
low refractive index/optically less dense/faster medium

Marking guidance:

Do not allow use of e.g $n_{1}$
unless medium one is
described e.g. $n_{\text {air }}$

(ii)

light at the water-oil interface (position B).

[ 2 ]

(b)

Calculate the angle of incidence at the air-plastic interface.

[ 2 ]

$\begin{aligned} \text { Use of Snell's Law }<\frac{\sin i}{\sin r}=\frac{1.60}{1} \gg i=\sin ^{-1}<1.60 \times \sin 32^{\circ}>=58<^{\circ} \gg \end{aligned}$

'Use of' requires a substitution NOT just a statement of a formula

Accept 1.0 rad (unit must be included to show a deliberate attempt to use rad rather than a calculator mistake)

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C.3.2—Snell’s law question 1

Question (a)

Question (i)

Question (ii)

Question (b)