Booklet 2B - Waves (Factual)

Question 1

What is the frequency of a wave?

Answer 1

The number of waves per second.

Question 2

What is the wavelength of a wave?

Answer 2

Wavelength is the distance from a point on one wave to the same point on the next wave.

Question 3

What is the period of a wave?

Answer 3

The time it takes one wave to pass a point.

Question 4

What is the amplitude of a wave?

Answer 4

The distace from the centre of a wave (the line of zero disturbance) to the crest or trough.

Question 5

What is meant by diffraction?

Answer 5

Diffraction is the bending of waves round an object or through a gap.

Question 6

What will increase the amount of diffraction?

Answer 6

• Longer wavelengths diffract more
• Narrower gaps cause more diffraction

Question 7

What is a coherent source?

Answer 7

Waves that have a constant phase relationship.

Question 8

What is constructive intereference?

Answer 8

This occurs when two waves meet at a point in phase.

This results in a wave of larger amplitude.

Question 9

What is destructive interference?

Answer 9

Destructive interference occurs when two waves meet at a point exactly 180º out of phase.

This results in smaller amplitude (zero if two waves are identical).

Question 10

What is the test for a wave?

Answer 10

If it can cause interference.

Question 11

Describe the interference pattern produced by a monochromatic light source and a double slit.

Answer 11

A series of bright and dark fringes symmetrical about the central, brightest fringe.

Question 12

What is a minimum?

Answer 12

A point where destructive interference occurs e.g. dark, quiet.

Question 13

What is a maximum?

Answer 13

A point where constructive interference occurs e.g. bright, loud.

Question 14

What are the conditions for constructive interference in terms of path difference?

Answer 14

The path difference is a whole number of wavelengths.

(0, 1λ, 2λ, 3λ…)

Question 15

What are the conditions for destructive interference in terms of path difference?

Answer 15

The path difference is an odd number of half wavelengths.

(½λ, 1½λ, 2½λ…)

Question 16

What are the advantages of using a grating to produce an interference pattern instead of a double slit?

Answer 16

• Fringes are brighter
• Fringes are sharper

Question 17

Using a grating how can you increase the spacing between the maxima?

Answer 17

• Use a grating with a smaller slit separation.
• Use a source with a greater wavelength of light
• Move the screen further away

Question 18

If white light is shone through a grating what does the interference pattern look like?

Answer 18

• The central order maximum will be white
• At every other maximum a spectrum will be produced
• The pattern is symmetrical
• Violet is closest to the centre and red the farthest away

Question 19

What is refraction?

Answer 19

The change in speed of light (and wavelength) as it enters a different material, which can result in a change in direction.

Question 20

What is meant by absolute refractive index?

Answer 20

It is the ratio of speed of light in a vacuum/air to the speed of light in the material.

It is also the ratio sinθ₁/sinθ₂ and λ₁/λ₂ where medium 1 is a vacuum/air.

Question 21

Describe an experiment to measure refractive index.

Answer 21

1. Setup the equipment as shown.
2. Measure the angle in air (θ_a) and in the medium (θ_p) for a range of incident angles.
3. Calculate the sin of all these angles.
4. Plot a graph of sin(θ_a) versus sin (θ_p).
5. The refractive index n is the gradient of this graph.

Question 22

What happens to the frequency when a wave enters a different medium?

Answer 22

Frequency doesn’t change - it is constant.

Question 23

A triangular prism can be used to split white light into a spectrum. Why does this occur?

Answer 23

Higher frequencies have a higher refractive index.

Question 24

What are the 4 differences in spectra produced by refraction (in a prism) and by a grating?

Answer 24

Prism Grating

One spectrum Many spectra

Produced by Refraction Produced by interference

Red light deviated least Red light deviated most

Spectrum is dim Spectrum is bright

Question 25

What is the critical angle?

Answer 25

The angle of incidence that gives an angle of refraction of ninety degrees.

Question 26

When will total internal reflection occur?

Answer 26

• Incident light must arrive at the boundary at an angle greater than the critical angle
• Occurs when light tries to pass from an optically dense material to a less dense material

Question 27

Define irradiance.

Answer 27

The power per unit area

Question 28

What is a point source?

Answer 28

This is a source where the light spreads evenly in all directions.

This means that its irradiance is inversely proportional to the square of the distance (I = k/d²)

Question 29

Describe how to investigate Irradiance and distance for a point source.

Answer 29

Set up the apparatus as shown below.

Measure the background irradiance when the lamp isn’t switched on. Subtract this reading from all subsequent measurements to correct for background.

Take corrected measurements of irradiance at regular distances away from the lamp.

Either

Plot a graph of I versus 1/d², which should be a straight line.

or

Calcualte I x d² for each point to show this value is roughly constant.

Question 30

What are the main features of the Bohr model of the atom?

Answer 30

1. Nucleus - central dense region containing most of the atoms mass. Contains positive protons and neutrons (no charge)
2. Electrons (negatively charged) are only allowed in certain energy levels
3. Electrons only emit/absorb energy when changing level.

Question 31

What is the ground state of the Bohr model of the atom?

Answer 31

The ground state is the lowest energy level in an atom.

Question 32

What is the ionisation level?

Answer 32

This is the energy needed by an electron to leave the atom altogether.

The convention is that the ionisation level is 0 as an electron reaching this level will just have escaped from the atom with no excess kinetic energy. All other energy levels are therefore negative.

Question 33

Why do the energy levels in the Bohr model have negative values?

Answer 33

Zero potential energy is defined when an electron is able to just escape from an atom (i.e. ionisation can take place). Electrons in all other energy levels are still bound to the atom so would need energy added to escape i.e. they have less than zero potential energy.

Question 34

Explain how a continuous emission spectrum is produced.

Answer 34

When atoms are packed closely together (e.g. in a solid or high pressure gas) electrons can make transitions to neighbouring atoms, meaning all energy transitions are possible and all frequencies of light are produced.

Question 35

Explain how a line emission spectrum is produced.

Answer 35

1. Each line in a spectrum is produced when an electron falls between two specific energy levels.
2. The energy levels, and therefore line spectrum, are unique to each element.

Question 36

Explain how an absorption spectrum is formed.

Answer 36

When an atom absorbs a photon with the specific energy equal to the difference in energy levels to excite electrons in a lower energy level to a higher energy level. These specific frequencies appear as dark lines in a continuous spectrum.

Question 37

What type of spectrum is this?

Answer 37

Line emission spectrum

Question 38

What type of spectrum is this?

Answer 38

Continuous emission spectrum

Question 39

What type of spectrum is this?

Answer 39

Line absorption spectrum

Question 40

If a spectral line is bright - what does this tell you?

Answer 40

More electrons are making that energy level transition producing more photons of light with the same frequency.

Question 41

Are all photons produced by energy level transitions visible?

Answer 41

No, some frequencies of photons may be in in the infrared, ultraviolet or X-ray frequency

Question 42

What do absorption lines in the Sun’s spectrum show?

Answer 42

Evidence for the composition of the outer atmosphere of the Sun.

Question 43

How are absorption lines in the Sun produced?

Answer 43

Photons of all frequencies are produced in the hot, dense core of the sun. Certain frequencies are absorbed by the relatively cooler gases in the outer atmosphere of the Sun.