5: Waves and Particle Nature of Light

Question 1

Describe longitudinal waves in terms of pressure variation.

Answer 1

• Wave consists of alternate compressions and rarefactions
• Compressions have increased pressure
• Rarefactions have decreased pressure

Question 2

Describe longitudinal waves in terms of the displacement of molecules.

Answer 2

• Wave consists of alternate compressions and rarefactions
• Particles are displaced towards the points of compression and away from the points of rarefaction

Question 3

Describe transverse waves.

Answer 3

Vibration is at right angles to the direction of travel.

Question 4

Explain how a standing wave is set up on a string.

Answer 4

• 2 waves travelling in opposite directions superpose.
• Constructive when waves are in phase, destructive when waves are in antiphase.
• Antinodes (max amplitude) are formed from points of constructive interference, nodes (min amplitude) are formed from points of destructive interference.

Question 5

What is meant by a real image?

Answer 5

Light rays pass through the actual image.
Rays converge to a point and can be projected on a screen.

Question 6

What is a wavefront?

Answer 6

A line on a wave along which all points are in phase.

Question 7

State what is meant by diffraction.

Answer 7

Spreading out of a wave as it passes through an apature.

Question 8

State the principle of superposition of waves.

Answer 8

When two or more waves meet, the total displacement at a point is the sum of the individual displacements.

Question 9

State what is meant by refraction.

Answer 9

Change in direction of a wave, due to a change in optical density.

Question 10

State what is meant by critical angle.

Answer 10

The angle of incidence in an optically denser medium that has an angle of refraction of 90 degrees.

Question 11

State what is meant by plane polarised light.

Answer 11

Oscillations in one plane only.
Plane includes direction of propagation.
or
Oscillations in one direction, perpendicular to direction of propagation.

Question 12

Explain how the photoelectric effect supports the particle model of light. (6)

Answer 12

• Electrons emitted instantly
• No emission below the threshold frequency
• Max ke is independent of the intensity
• Rate of emission is proportional to the intensity
• One photon is absorbed by one electron
• With waves, energy can build up

Question 13

Explain the process that results in an emission spectrum. (7)

Answer 13

• Electrons absorb energy,
• Electrons move to higher energy levels,
• A photon is released when an electron drops down energy levels,
• Electrons have discrete energy levels,
• Energy of the photon = difference between energy levels,
• E = hf, so frequency is determined by difference in energy levels,
• Only certain energy changes possible, so only certain frequencies are emitted.