4. Wave phenomena. Flashcards
Harmonic oscillation and waves. Interference. Electromagnetic waves, diffraction, polarization.
Define electromagnetic wave and give its 3 **properties **derived from a formula
Electromagnetic wave:
- EM (perpendicular electric + magnetic components) disturbance propogating through space → no elastic medium required
- transverse wave
Properties: c = λ * f
- c = velocity
- λ = wavelength
- f = frequency
Give 3 examples for wave phenomena
Wave phenomena:
- diffraction
- interference
- polarization
Classify waves according to their source, propagation dimension, and the relative direction of oscillation and propagation and give examples
Source:
- mechanical: elastic deformation propagating through elastic medium
- EM: EM disturbance propagating through space
Propagation dimension:
- One-dimensional (rope)
- Surface waves (pond)
- Spatial waves (sound)
Relative direction of oscillation and propagation:
- longitudinal (sound)
- transverse (radiation)
Define **oscillation **(= vibration)
A phenomenon where quantity varies in time about an equilibrium value.
Define harmonic oscillation and give the formula to calculate the displacement x
Harmonic oscillation:
oscillation that can be described as a sine wave
The restoring force is proportional to the displacement and directed toward equilibrium.
Characterized by the eigenfrequency
Displacement: x = R * sin φ
- R = amplitude
- φ = ω * t = angular velocity * time
Define **eigenfrequency **(=natural frequency). What does depend on?
Eigenfrequency:
frequency at which a free osciallatory system oscillates. Independent of the displacement, its value is only defined by the characteristics of the system.
Distinguish between free damped oscillation, and free undamped osscillation
Undamped free oscillation:
oscillation system not affected by any external forces (e.g. friction, gravity), amplitude of the oscillation will be constant for an infinite time.
BUT: only in theory, always be external forces acting upon an oscillating system
Damped free oscillation:
amplitude (E) of the oscillation will gradually diminish over time due to external forces acting upon the system. The stronger the external force, the stronger the damping and the amplitude will diminish more quickly.
Explain critical damped oscillation
Critical damped oscillation:
external force is so strong that oscillation dissipates without even crossing the equilibrium point
What is resonance?
Resonance:
If driving frequency is almost identical to the eigenfrequency of the oscillatory system, the amplitude of the driven oscillation will be very high (infinite)
Classify interference and describe how to predict the amplitude of the newly formed wave. Explain the principle of superposition.
Interference:
When two propagating waves meet
- destructive interference
- constructive intereference
The amplitude of the newly formed wave is dependent on the phase shift of both waves, thus if φ=π cancellation, and in case of φ=0 amplification takes place.
Principle of superposition:
Depending on the distance of both waves **interference patterns **can be observed (interefence maxima/minima)
What is the difference between unpolarized and polarized light? Classify polarized light and explain how it can be obtained
Unpolarized light:
light propagating field vector in all directions
Linear polarization:
- oscillation in a **preferred direction **(only 1 plane)
- only transverse waves
- Can be done by a polarizing filter
Circular polarization:
combination of 2 perpendicular linearly polarized waves of the same wavelength, shifted by a quarter wavelength (λ/4), propagating along the same axis
direction may be either:
- positive (right-handed rotation) valze
- negative (left-handed rotation) value
Define diffraction, explain the related **Huygen’s Fresnel principle **and its relation to interference patterns
Diffraction:
occurs when a wave encounters an obstacle or a slit
→ Principle maximum, first order maximum, etc.
- low refr. index → high angle between maxima
Huygens-Fresnel principle:
Every point of a wavefront acts as a source for further waves (all with identical phases)
Interference pattern:
= result of Huygen’s Fresnel principle
new waves will meet later at different locations in different phases
- → constructive interferences (amplification)
- → destructive interferences (cancelation)
Define anisotropy
Anisotropy:
refractive index n is orientation dependent
→ in different directions within the sample, light propagates with different velocities
