Wave Phenomena

Introduction

Wave phenomena are the behaviours that reveal the wave nature of light, sound, and other disturbances: reflection, refraction, diffraction, and interference. The guide expects students to move between ray diagrams, wavefronts, path differences, and quantitative interference equations.

Guide Focus

• Analyse reflection, refraction, transmission, and diffraction at boundaries and apertures.
• Use Snell’s law, critical angle, and total internal reflection.
• Apply superposition, path difference, Young’s double slit, and HL diffraction grating ideas.

Key Concepts

1. Wavefronts and rays

In two and three dimensions, wavefronts show points in phase, while rays show the direction of energy transfer and are perpendicular to wavefronts.

2. Refraction and Snell’s law

Refraction occurs when wave speed changes at a boundary. Snell’s law may be written n1 / n2 = sin(theta2) / sin(theta1) = v2 / v1. Critical angle and total internal reflection occur when light attempts to move from higher refractive index to lower refractive index.

3. Diffraction

Diffraction is spreading around obstacles or through apertures. It is strongest when aperture size is comparable to wavelength. At HL, single-slit diffraction uses theta = lambda / b for the first minimum.

4. Interference and coherence

Superposition occurs when waves overlap. Constructive interference requires path difference n lambda, while destructive interference requires (n + 1/2)lambda. Young’s double slit fringe spacing is s = lambda D / d.

5. Multiple slits and gratings

At HL, diffraction gratings produce sharp maxima described by n lambda = d sin(theta), where d is slit spacing.

Common Mistakes

• Measuring angles from the surface instead of the normal.
• Using slit width where slit separation is required.
• Forgetting that coherent sources need a constant phase relationship.

Exam Tips

• Draw the normal first in refraction questions.
• For interference, translate bright/dark into constructive/destructive before calculating.
• Check that path difference and wavelength use the same units.

Practice Questions

Question 1 (Multiple Choice)

Two coherent waves arrive at a point with path difference 3.5 lambda. The result is:

A. Destructive interference. B. Constructive interference. C. Total internal reflection. D. No superposition.

Question 2 (Structured Paper 2 Style)

A double-slit experiment uses light of wavelength 6.0 x 10^-7 m. The slit separation is 0.30 mm and the screen is 2.0 m away.

(a) Calculate the fringe spacing. [2 marks]

(b) State one condition required for a stable pattern. [1 mark]