Specific Struggles p3b

Question 1

Describe Huygens’ theory. (3)

Answer 1

• Light is a (longitudinal) wave.
• The theory uses the idea of (secondary) wavelets.
• Every point on wavefront acts as source of secondary wavelets.

Question 2

Describe Newton’s light theory, related to a double slit experiment result. (3)

Answer 2

• Light consists of corpuscles that travel in straight lines
• (which means that) shadows are formed with sharp edges (sharp shadows)
• and only two fringes seen from a double slit.

Question 3

Explain diffraction in terms of Huygens’ theory. (5)

Answer 3

• (When wave reaches slit) each point at slit produces secondary wavelets.
• Wavelets overlap on screen.
• Path difference due to different distances between a point on the screen and the two slits.
• Path difference introduces phase differences.
• Bright fringes form where path difference is whole number of wavelengths/waves arrive in phase.
• Dark fringes where path difference is odd number of half wavelengths/waves arrive in antiphase. Do not accept ‘out of phase’.

Question 4

State what is meant by black-body radiation. [2 marks]

Answer 4

Electromagnetic radiation (1) spectrum with peak depending on temperature (of emitter alone). (1)

Question 5

Describe the ultraviolet catastrophe. [2 marks]

Answer 5

• Intensity similar at long wavelengths.
• (But rather than peak) wave theory predicts intensity increases at shorter wavelengths/infinite at very short wavelengths.

Question 6

Describe Einstein’s theory. (5)

Answer 6

• Light is made of photons
• Photoelectrons due to one photon interacting with one electron in surface of metal.
• Minimum energy (work function of metal) needed for electron to be emitted related to a threshold frequency by φ = hf(min)
• Remaining energy of photon (hf - hf(min)) becomes (max) KE of photoelectron.
• Brighter source means more photons (per second) and therefore more photoelectrons (per second).

Question 7

Which direction do electrons tunnel when a pd is applied across the narrow gap between a microscope probe tip and a surface?

Answer 7

From the negative to the positive (only).

Question 8

How is an STM probe tip above the sample used to obtain an image of a surface? (2 methods)

Answer 8

• Current is detected due to electron transfer
• Constant current: the variation of the height of the tip with time is used to map the surface
• Constant height: the variation of the current with time is used to map the surface