Waves

PMT DETAILED NOTES

What is a progressive wave?

A wave that transfers energy without transferring matter; particles oscillate about a fixed position.

What is amplitude?

Maximum displacement from equilibrium (m).

What is frequency (f)?

Number of oscillations per second (Hz).

What is wavelength (λ)?

Distance between identical points on a wave (m).

What is wave speed (c)?

Distance travelled per second (m/s).

What is the period (T)?

Time for one oscillation (s).

What is phase?

Position in a wave cycle (degrees/radians).

What is phase difference?

How much one point lags behind another.

When are two points in phase?

Same displacement and velocity; phase difference = multiple of 360° (2π).

When are two points out of phase?

Odd multiples of 180° (π).

What is the wave equation?

c = fλ

What is the frequency-period relationship?

f = 1/T

What is a transverse wave?

Oscillations perpendicular to direction of energy transfer.

Give an example of a transverse wave.

Electromagnetic waves.

What is a longitudinal wave?

Oscillations parallel to direction of energy transfer.

What are longitudinal waves made of?

Compressions and rarefactions.

Can longitudinal waves travel in a vacuum?

No.

What is polarisation?

Oscillation in one plane only.

Which waves can be polarised?

Only transverse waves.

Application of polarisation?

Polaroid sunglasses reduce glare.

What is superposition?

Resultant displacement = sum of individual displacements.

What is constructive interference?

Waves combine in phase to give larger amplitude.

What is destructive interference?

Waves out of phase reduce or cancel amplitude.

What is a stationary wave?

Formed by two identical waves travelling in opposite directions.

Do stationary waves transfer energy?

No.

What are nodes?

Points of zero displacement.

What are antinodes?

Points of maximum displacement.

Distance between adjacent nodes?

λ/2

What is the first harmonic?

Lowest frequency; 2 nodes and 1 antinode.

How do harmonics relate to frequency?

fn = n × f₁

What is path difference?

Difference in distance travelled by two waves.

What is a coherent source?

Same frequency, wavelength, constant phase difference.

Condition for constructive interference?

Path difference = nλ

Condition for destructive interference?

Path difference = (n + ½)λ

Young’s double slit formula?

w = (λD)/s

What happens when using white light?

Central white fringe with coloured fringes (violet closest).

What is diffraction?

Spreading of waves through a gap or around an obstacle.

When is diffraction greatest?

When gap size is similar to wavelength.

Effect of increasing slit width?

Less diffraction; narrower central maximum.

Effect of increasing wavelength?

More diffraction; wider central maximum.

What is a diffraction grating?

Many closely spaced slits.

Diffraction grating equation?

d sinθ = nλ

What is the zero order?

Central maximum.

What is refractive index (n)?

n = c / cs

What does a higher refractive index mean?

More optically dense; light slows more.

What is refraction?

Change in direction when entering a different medium.

What is Snell’s Law?

n₁ sinθ₁ = n₂ sinθ₂

When does light bend towards the normal?

Entering a more optically dense medium.

What is the critical angle?

Angle where refracted ray is 90°.

Critical angle formula?

sinθc = n₂ / n₁

What is total internal reflection (TIR)?

Complete reflection inside a medium.

Conditions for TIR?

n₁ > n₂ and angle of incidence > critical angle.

What are optical fibres?

Thin fibres that carry light signals.

How do optical fibres work?

Using total internal reflection.

What is absorption?

Loss of signal energy.

What is dispersion?

Pulse spreading.

Types of dispersion?

Modal and material.

How to reduce dispersion?

Narrow core and monochromatic light.

What is a repeater?

Device that regenerates the signal.