Waves + Optics

Question 1

Discuss the formation of stationary waves.

Answer 1

• waves and reflection travelling in opposite directions meet
• same wavelength
• node is point of minimum
• antinode is point of maximum amplitude
• superposition of 2 waves
• energy not transferred

Question 2

How are EM waves and sound waves similar?

Answer 2

They both have same frequency as their respective sources.

Question 3

What is a stationary wave?

Answer 3

superposition of 2 progressive waves
same frequency
opposite directions

Question 4

Describe how a note of frequency 440 Hz can be produced using the correctly tuned A-string of a guitar.

Answer 4

A 1/4 length of string is used to sound the note.

Question 5

Relationship between octaves and frequencies

Answer 5

One octave = doubling of frequencies

Question 6

What’s the motion of wave particle in stationary wave?

Answer 6

oscillates about equilibrium position

Question 7

Explain, with reference to refractive index, why the pulse of red light has a shorter transit time than the pulse of blue light.

Answer 7

• Refractive index of blue > refractive index of red
• speed of red light is greater than speed of blue light

Question 8

Explain why most microwave ovens contain a rotating turnable on which the food is placed during cooking

Answer 8

position of maximum energy continually changes

Question 9

Explain why the signal strength falls to a minimum between maximas. (3 marks)

Answer 9

• path difference for two waves
• gives rise to a phase difference
• Destructive interference occurs

Question 10

Explain why the minimum intensity between maximas of double slits is not zero. (2 marks)

Answer 10

• Intensity decreases with distance.
• One wave travels further than the other and therefore the amplitudes not equal

Question 11

State and explain the effect on the signal strength as it is rotated (3 marks)

Answer 11

• The signal decreases/becomes zero
• The waves transmitted are polarised
• zero when detector at 90° to the direction of wave

Question 12

Explain how stationary waves are produced and why only waves of specific frequencies can form on the aerial wire. (4 marks)

Answer 12

• Wind produces a wave that travels along the wire
• waves travel in opposite directions
• waves superpose
• Only certain frequencies since fixed ends have to be nodes

Question 13

sag = sink, subside, or bulge downwards under weight

High winds produce large amplitudes of vibration of the aerial wire. Explain why the wire may sag when the high wind stops.

Answer 13

• Copper may be stretch beyond elastic limit
• Permanent deformation

Question 14

Discuss the properties of a step-index optical fibre. (6 marks)

Answer 14

• X is Core
• Y is Cladding
• X: Propagates light by TIR, low absorption, refractive index of core > cladding
• Y: Protects core from damage, prevents cross talk between touching fibres
• Both: Cause pulse broadening
• Material: different wavelengths => different speeds due to different refractive indices within
  the core – use monochromatic beam
• Modal: different paths have different lengths so effective time along fibre differs – use narrow core

Question 15

Suggest why the amount of reflected light changes as the fibre bends.

Answer 15

• Light may encounter impurities at different positions/angles
• Light may encounter different number of impurities