Waves; Topic 4.4 Wave Behaviour

Question 1

What is refection

Answer 1

A wave hits a boundary between two media and does not pass through, but instead bounces back to the original medium

The law of reflection states:
The angle of incidence, i = The angle of reflection, r

Question 2

What is refraction

Answer 2

A wave changes speed and direction at the boundary between two media

Question 3

Why refraction occurs?

Answer 3

This is due to the density of the media
If the medium is more dense, the wave slows down
If the medium is less dense, the wave speeds up

Question 4

What parameter remains the same in refraction

Answer 4

When a wave refracts, its speed and wavelength change, but its frequency remains the same
This is noticeable by the fact that the colour of the wave does not change

Question 5

State general characteristics of transmission behaviour of waves

Answer 5

Transmission occurs when: A wave passes through a substance

Transmission is the more general term of a wave appearing on the opposite side of a boundary (the opposite of reflection)
When passing through a material, waves are usually partially absorbed
The transmitted wave will have a lower amplitude if some absorption has occurred

Question 6

Outline refraction is a type of transmission

Answer 6

Refraction is a type of transmission
Transmission is the more general term of a wave appearing on the opposite side of a boundary (the opposite of reflection)
Refraction is specifically the change in direction of a wave when it crosses a boundary between two materials that have a different density

Question 7

Explain different changes in the direction wave in refraction

Answer 7

The change in direction depends on which media the light rays pass between:
From air to glass (less dense to more dense): light bends towards the normal
The angle of refraction < the angle of incidence
From glass to air (more dense to less dense): light bends away from the normal
The angle of refraction > the angle of incidence
When passing along the normal (perpendicular) the light does not bend at all
This would be described as transmission

Question 8

State what happens to speed and wavelength when it passes to denser medium and rarer medium

Answer 8

When light passes from a less dense medium to a more dense medium, such as from air to glass, the refracted light has a lower speed and a shorter wavelength than the incident light

When light passes from a more dense medium to a less dense medium, such as from glass to air, the refracted light has a higher speed and a longer wavelength than the incident light

Question 9

Outline how refraction as well as reflection can also occur at the boundary

Answer 9

Together with refraction, reflection might also occur
When light travels from a less optically dense medium into an optically denser medium, both reflection and refraction always occur
Light has to be reflected from the object to the eye in order for objects to be visible
Some of the energy in the incident light is reflected back, while some is transmitted

Question 10

Explain absolute refractive index

Answer 10

Transparent media have different optical densities
Light is transmitted through these media at different speeds
The absolute refractive index, n, of a transparent medium is a measure of its optical density, and can be calculated as follows:
n= c/v

Where:
n = absolute refractive index of the medium
c = speed of light in vacuum in metres per second (m s–1)
v = speed of light in the medium in metres per second (m s–1)

Question 11

Explain according to Snell’s law the special observation which occurs when light travels from an optically denser medium into a less optically dense medium

Answer 11

According to Snell’s law, when light travels from an optically denser medium into a less optically dense medium, its speed increases and it bends away from the normal
The angle of refraction is greater than the angle of incidence
A small amount of light is also reflected back into the optically denser medium

Question 12

What is critical angle?

Answer 12

The angle of incidence which results in an angle of refraction of 90°, after which point, total internal reflection occurs

The critical angle is important because this is the point at which no light enters the new medium

Question 13

What is total internal reflection

Answer 13

If the angle of incidence is greater than the critical angle (θ1 > θc), there is no refracted ray, and all light is reflected back into material 1 (θ2 = θ1)
This is known as total internal reflection

Question 14

What is diffraction

Answer 14

Diffraction is the spreading out of waves when they pass an obstruction
This obstruction is typically a narrow slit known as an aperture

Question 15

Explain the effect on frequency because of diffraction

Answer 15

The frequency of the diffracted waves is less than that of the incident waves, since energy is distributed over a larger area

Question 16

Explain factor which affects diffraction

Answer 16

The extent of diffraction depends on the width of the gap compared to the wavelength of the wave
Diffraction is the most prominent when the width of the slit is approximately equal to or smaller than the wavelength
As the gap size increases, the effect gradually gets less pronounced until, in the case that the gap is much larger than the wavelength, the waves are no longer spread out

Question 17

State the effect of diffraction on amplitude

Answer 17

The only property of a wave that changes when its diffracted is its amplitude
This is because some energy is dissipated when a wave is diffracted through a gap

Question 18

What is interference

Answer 18

Interference occurs whenever two or more waves combine to produce a resultant wave with a new resultant displacement

Question 19

Constructive interference

Answer 19

Constructive interference happens when the resultant wave has a larger displacement than any of the individual displacements

Question 20

Destructive interference

Answer 20

Destructive interference happens when the positive displacement of one wave and the negative displacement of another wave exactly cancel out giving a resultant displacement of zero

Question 21

Explain the need for coherence for interference

Answer 21

Interference is only observable if produced by a coherent source
Waves are said to be coherent if they have:
A constant phase difference
The same frequency

A coherent beam of light contains light waves that are monochromatic and have a constant phase difference
Monochromatic light consists of light waves of a single frequency

Question 22

Explain double slit interference

Answer 22

When a coherent beam of light is incident on two narrow slits very close together, diffraction occurs at each slit (i.e. the waves spread out)
As the diffracted waves cross, they interfere with each other
If a screen is placed some distance away from the slits, a pattern of equally spaced bright and dark fringes is observed on the screen
The bright fringes form where the waves interfere constructively (i.e. a crest meets a crest or a trough meets a trough)
The dark fringes form where the waves interfere destructively (i.e. a crest meets a trough)

Question 23

What is path difference

Answer 23

The type of interference occurring at a given point (i.e. constructive or destructive) depends on the path difference of the overlapping waves
Path difference is defined as:
The difference in distance travelled by two waves from their sources to the point where they meet

Path difference is generally expressed in multiples of wavelength

Question 24

Conditions for constructive and destructive interference as per path difference

Answer 24

In general, for waves emitted by two coherent sources very close together:
The condition for constructive interference is:
path difference = nλ

The condition for destructive interference is:
path difference = (n + ½)λ

Where:
λ = wavelength of the waves in metres (m)
n = 0, 1, 2, 3… (any other integer)