13) Diffraction & Interference

Question 1

Light passing through each slit spreads out. What is the name for this effect?

Answer 1

Diffraction.

Question 2

State what is meant by monochromatic.

Answer 2

Single wavelength.

Question 3

State what is meant by coherent.

Answer 3

Waves have constant phase difference and the same

frequency.

Question 4

State one safety precaution that should be taken while using a laser.

Answer 4

do not look into laser, wear goggles.

Question 5

In the double slit experiment, the laser is replaced by another laser emitting visible light with a shorter wavelength. State and explain how this will affect the spacing of the maxima on the screen.

Answer 5

• Maxima closer together.

- Spacing is proportional to wavelength and D and s are constant therefore as λ decreases so ω decreases.

Question 6

State and explain what happens to the value of angle θ for the first order beam if the wavelength of the monochromatic light decreases.

Answer 6

angle θ gets smaller because path difference gets smaller.

Question 7

State two ways in which the appearance of the fringes would change if the slit was made narrower.

Answer 7

• wider separation.

- lower intensity.

Question 8

In the double slit experiment, explain how the use of the single slit makes the light from the two slits sufficiently coherent for fringes to be observed.

Answer 8

• Single slit acts as a point light to both slits.

- The path lengths between the single slit and the double slits are the same.

Question 9

Explain how the bright fringes are formed in Young’s double slit experiment.

Answer 9

-Superposition of waves from two slits.
-Diffraction patterns from both slits overlap and interfere
constructively.
-Constructive interference at the bright fringes.
-Waves from each slit meet in phase.

Question 10

Explain how the pattern produced on the screen by an electron diffraction tube supports the idea that the electron beam is behaving as a wave rather than as a stream of particles.

Answer 10

• Wave property is shown by diffraction.
• Graphite causes electron to spread out and travel in particular directions.
• Bright rings occur where waves interfere constructively.
• Particle behaviour would only produce a spot of light.

Question 11

Explain why the signal strength falls to a minimum between two points in a double slit experiment.

Answer 11

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