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 difference and the same frequency, wavelength and velocity.

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

How can you increase the spacing between the maxima produced by a double slit?

Answer 16

• Increase the wavelength of the waves
• Increase the distance between the sources and screen/detector
• Increase the separation of the slits

Question 17

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

Answer 17

• Fringes are brighter
• Fringes are sharper

Question 18

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

Answer 18

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

Question 19

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

Answer 19

• 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 20

Describe some practical applications of interference.

Answer 20

• A hologram uses interference to display a 3D image.
• Stress analysis relies on the fact that that some materials display interference patterns directly related to the forces at that point.
• Anti-reflective coatings on camera lenses and spectacles use constructive interference to ensure brighter images.
• An interferometer uses an interference pattern to measure small changes in position.
• An interferometer was also used by Michelson and Morley to show that the speed of light is the same in all directions, which led to the development of special relativity.

Question 21

What is refraction?

Answer 21

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

Question 22

What can be said about the ratio sinθ₁/sinθ₂ when light passes from medium 1 to medium 2 at any angle other than 0.

Answer 22

It is constant.

Question 23

What is meant by absolute refractive index?

Answer 23

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 24

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

Answer 24

Frequency doesn’t change - it is constant.

Question 25

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

Answer 25

Different frequencies/colours have different refractive indices.

Question 26

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

Answer 26

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 27

What is the critical angle?

Answer 27

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

Question 28

When will total internal reflection occur?

Answer 28

• 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 29

Derive the relationship between critical angle and absolute refractive index.

Answer 29

By the definition of critical angle, when θ₂ = θ_c, then θ₁ = 90º

n = sinθ₁/sinθ₂ = sin90º/sinθ_c = 1/sinθ_c

so sinθ_c = 1/n

Question 30

Describe how to measure the critical angle.

Answer 30

1. Set up the equipment as shown, ensuring the ray is going from the medium to air at the flat edge.
2. Increase θ_p until θ_a equals 90°
3. Measure θ_p which is the critical angle.

Question 31

Describe some applications of refraction.

Answer 31

• Lenses have many uses including telescopes and correcting problems with eyesight.
• The high refractive index of diamond results in more total internal reflection and more dispersion of white light into its constituent colours. This is why diamonds sparkle so much.
• Total internal reflection is used in cycle reflectors and optical fibres.

Question 32

Define irradiance.

Answer 32

The power per unit area

Question 33

What is a point source?

Answer 33

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

This means it will obey the inverse square law of irradiance (I = k/d²)

Question 34

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

Answer 34

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 35

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

Answer 35

1. Nucleus - central dense region containing most of the atoms mass. Contains positive protons and neutrons (no charge)
2. Electrons (negatively charged) are in discrete energy levels and do not radiate energy

Question 36

Explain how a line spectrum is produced.

Answer 36

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

Question 37

Explain how an absorption spectrum is formed.

Answer 37

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 38

Explain how an emission spectrum is formed.

Answer 38

When electrons in an excited (higher) energy level drop to a lower energy level, a photon is emitted with energy equal to the difference in energy levels.

Question 39

What is the ground state?

Answer 39

This is the energy level with the least energy.

Question 40

What is the ionisation level?

Answer 40

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 41

What type of spectrum is this?

Answer 41

Line emission spectrum

Question 42

What type of spectrum is this?

Answer 42

Continuous spectrum

Question 43

What type of spectrum is this?

Answer 43

Line absorption spectrum

Question 44

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

Answer 44

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

Question 45

Are all photons produced by energy level transitions visible?

Answer 45

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

Question 46

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

Answer 46

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

Question 47

How are absorption lines in the Sun produced?

Answer 47

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.