IB Physics SL C 3 Wave Phenomena Questions

Use wavefront and ray diagrams to trace boundary behaviour, combine coherent waves, and calculate fringe positions in Young double-slit setups.

Exam points

Apply Snell’s law and n = c/v to infer refraction angles, wave speed or wavelength across media.
Use the critical-angle condition to decide whether a wave undergoes total internal reflection.
Sketch or interpret wavefronts and rays for reflection, refraction and transmission at boundaries.
Explain why longer wavelengths diffract more, or larger apertures reduce spreading in telescope contexts.
Apply superposition to sketch the resultant displacement when wave pulses overlap.
Explain why coherent sources need a constant phase difference to produce stable interference fringes.
Use path difference to predict bright or dark fringes: nλ for maxima and (n + ½)λ for minima.
Apply s = λD/d to calculate Young fringe spacing or infer wavelength, screen distance or slit separation.

Covered syllabus points

C.3.1—Wavefronts and rays C.3.2—Snell’s law C.3.3—Refractive index C.3.4—Total internal reflection C.3.5—Superposition C.3.6—Coherence C.3.7—Path difference C.3.8—Two-source interference C.3.9—Young double-slit equation C.3.10—Reflection, refraction and transmission at boundaries C.3.11—Diffraction around bodies and through apertures

[Maximum number: 3]

A group of students is investigating refraction in a semi-circular glass block.

Light from a ray box enters the curved side of the block. The light passes through the block and leaves, refracted, at P .

(a)

Outline how the students can ensure that the light is not deflected at the curved surface.

[ 1 ]

measure P to be at the centre of the flat edge/side so that incident rays are aimed at P /go through radial lines

OR

check that the incident angle is $0^{\circ}$ at every point of incidence

[1]

(b)

They plot a graph of the variation with the sine of $\theta_{i}$ of the sine of $\theta_{r}$ .
They add uncertainty bars for $\sin \theta_{r}$ for the first and last data point and draw the best-fit line.

[ 2 ]

(i)

Determine the value of the refractive index of the glass with its absolute uncertainty.

[ 2 ]

States gradient gives the value of the refractive index

OR

n=1.5 $n=1.5 \pm 0.2$

MP1 can be shown as an equation.

Candidates may calculate the uncertainty by using the gradient of the line found in cii) or finding the average of the max and min lines of best fit.

Look for working leading to $0.1 \leq \Delta n \leq 0.2$ .

[Maximum number: 8]

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.

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)

(c)

Calculate the critical angle for the plastic-water interface.

[ 2 ]

$\begin{aligned} \frac{\sin i}{\sin r}=\frac{1.33}{1.60} \text { and } \sin r=1 i=\lessdot \sin ^{-1} 0.831 »=56<^{\circ} \gg \end{aligned}$

Accept 0.98 rad (unit required)

All question bank results loaded

Covered syllabus points

Question 1

Question (a)

Question (b)

Question (i)

Question 2

Question (a)

Question (i)

Question (ii)

Question (b)

Question (c)