2.5 Wave Properties

Question 1

Why is a laser useful in showing interference and diffraction?

Answer 1

It produces monochromatic (same wavelength/colour) light meaning diffraction and interference patterns are more defined.

Question 2

What was Young’s double-slit experiment?

Answer 2

A single light source is directed towards two slits, which each act as a coherent light source. The light interferes constructively and destructively to create an interference pattern.

Question 3

Describe the interference pattern created using white light.

Answer 3

Question 4

Why does an interference pattern form when light is passed through a single slit?

Answer 4

The light diffracts as it passes through the slit. Where the waves are in phase, constructive interference occurs making bright fringes. Where the waves are completely out of phase, destructive interference occurs making a dark fringe.

Question 5

Increasing the slit width increases the width of the central diffraction maximum. True or False?

Answer 5

False, the slit is not so close to the wavelength in size so less diffraction occurs - the central maximum becomes narrower and more intense.

Question 6

Answer 6

Single Slit.

Question 7

What is a stationary wave?

Answer 7

A wave which transfers no energy and whose positions of maximum and minimum amplitude are constant.

Question 8

What is a node?

Answer 8

A point on a stationary wave where the displacement is 0.

Question 9

What is an antinode?

Answer 9

A point on a stationary wave with maximum displacement.

Question 10

What are the conditions for a stationary wave to be produced?

Answer 10

● The waves must be of the same frequency, wavelength and amplitude.
● They must be travelling in opposite directions.
These conditions are often met when a wave is reflected back onto itself.

Question 11

How are stationary waves produced?

Answer 11

A stationary wave is formed from the superposition of 2 progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude.
● Where the waves meet in phase, constructive interference occurs so antinodes (regions of maximum amplitude) form.
● Where the waves meet completely out of phase, destructive interference occurs and nodes (regions of no displacement) form.

Question 12

Describe the first harmonic for a stationary wave with two closed ends.

Answer 12

Question 13

Describe the second harmonic for a stationary wave with one open end and one closed end.

Answer 13

It consists of two nodes and two antinodes, with one of the nodes at the closed end and one of the antinodes at the open end.

Question 14

What is phase difference and what is it measured in?

Answer 14

How much a particle/wave lags behind another particle/wave. It is measured in radians, degrees or fractions of a cycle.

Question 15

Define coherence.

Answer 15

Coherent waves have a fixed phase difference and the same frequency and wavelength.

Question 16

State 2 applications of diffraction gratings.

Answer 16

1. Splitting up light from stars to make line absorption spectra - used to identify elements
   present in stars.
2. X-ray crystallography: a crystal sheet acts as the
   diffraction grating and the X-rays pass through - used to find the spacing between atoms.

Question 17

Derive the formula dsinθ=n𝜆

Answer 17

Question 18

When light passing through a diffraction grating is changed from blue to red, do the orders get closer together?

Answer 18

The wavelength of light has increased so it will diffract more meaning the orders will become further apart.

Question 19

What is diffraction?

Answer 19

The spreading out of waves when they pass through or around a gap.

Question 20

How did Young’s double slit experiment provide evidence for the wave nature of light?

Answer 20

Diffraction and interference are wave properties hence the interference pattern of light shows that light has wave properties.

Question 21

What is path difference?

Answer 21

The difference in distance travelled by 2 waves.

Question 22

How could you investigate stationary sound waves?

Answer 22

Place a speaker at one end of a closed glass tube, lay powder across the bottom of the tube, it will be shaken from the antinodes and settle at the nodes. The distance between each node is half a wavelength.

Question 23

What is the frequency of the first harmonic of a string length 2m, mass 0.03kg with a mass of 2kg hanging off it?

Answer 23

Question 24

True or False? Only light can produce interference patterns.

Answer 24

False, interference patterns can be formed by sound waves and all EM waves too.

Question 25

What formula is associated with Young’s double slit experiment?

Answer 25

w = λD/s
w - fringe spacing
λ - wavelength of light used
D - distance from screen to slits
S - slit separation