Physics - waves

Question 1

What is a wave?

Answer 1

A wave transfers energy from one place to another without any overall transfer of matter.

Question 2

How is a wave created?

Answer 2

In order to make a wave something needs to vibrate. We call the thing that vibrates a medium. This can be either a collection of particles or an electromagnetic field.

Question 3

What evidence is there that it is the wave that travels and not matter?

Answer 3

When a sound wave travels through air, the air particles do not travel from one place to another. When a water wave travels across the ocean towards land, the water particles don’t all travel towards the land. In both examples, the particles simply vibrate about a fixed position and do not travel.

Question 4

What is a transverse wave?

Answer 4

In a transverse wave, vibrations are at right angles (perpendicular) to the direction of energy transfer. These vibrations cause peaks and troughs.

Question 5

What waves are transverse?

Answer 5

Waves on the surface of water (ripples) and all electromagnetic radiation such as visible light are examples of transverse waves.

Question 6

What is a longitudinal wave?

Answer 6

In a longitudinal wave, vibrations are in the same direction (parallel) as the direction of energy transfer. These vibrations cause compressions and rarefactions.

Question 7

What waves are longitudinal?

Answer 7

Sound is an example of a longitudinal wave.

Question 8

What is a compression and rarefaction in a longitudinal wave?

Answer 8

When a longitudinal wave travels through a substance, sometimes the particles are close together (compression) and sometimes they are far apart (rarefaction).

Question 9

What is the definition of frequency?

Answer 9

The frequency of a wave is defined as the number of vibrations per second. The unit is Hz (Hertz).

Question 10

What is the definition of time period?

Answer 10

Time period is defined as the time for one vibration. The unit is s (seconds).

Question 11

What is the definition of amplitude?

Answer 11

The amplitude is the maximum displacement of a vibration from its rest position.

Question 12

What is the definition of wavelength?

Answer 12

The definition of wavelength is the distance from a point on a wave to the same point on the next wave. The unit is m (metres).

Question 13

What is the relationship between time period and frequency?

Answer 13

Time period = 1 / frequency

T = f⁻¹ (Time period in seconds, s; Frequency in Hertz, Hz)

Question 14

What is wave speed?

Answer 14

Wave speed is the speed at which energy is transferred through the medium.

Question 15

How do you measure the speed of sound in air?

Answer 15

Person one should stand a large distance away from person two and make an obvious visible sound. The distance between them should be measured. Person two should start their stop clock when they see the sound being made and stop it when they hear the sound. The speed of sound can then be calculated from speed = distance / time.

Question 16

What is the relationship between wave speed, frequency and wavelength?

Answer 16

Wave speed = frequency × wavelength

v = fλ (Speed in metres per second, m/s; Frequency in Hertz, Hz; Wavelength in metres, m)

Question 17

How can you measure the speed of ripples on water surface?

Answer 17

Using a ripple tank, measure the distance across ten waves using a ruler and then divide by 10 to find one wavelength. Read the frequency from the signal generator driving the ripple tank. The speed can then be calculated using speed = frequency x wavelength.

Question 18

How would you determine the speed of waves on a string?

Answer 18

Stretch a string over a pulley using masses. Use a frequency generator to generate a wave in the string. Measure the wavelength when a clear wave is shown and note the frequency on the generator. Use the wave speed equation to determine the speed.

Question 19

What is the difference between diffuse and specular reflection?

Answer 19

Specular reflection happens when light is incident on smooth surfaces, reflecting in a predictable manner. Diffuse reflection happens on rough surfaces, scattering light in different directions.

Question 20

What is the law of reflection?

Answer 20

The angle of incidence equals the angle of reflection.

Question 21

What is a virtual image?

Answer 21

A virtual image is one from which the light rays appear to come but don’t actually come from the image, such as the image seen in a mirror.

Question 22

What is the difference between a virtual image and a real image?

Answer 22

A real image is one where the light rays do come from.

Question 23

What is refraction and why does it happen?

Answer 23

Refraction is the change in speed of a wave when it reaches the boundary between two different materials. It happens because materials have different optical densities.

Question 24

What happens to frequency and wavelength when a wave undergoes refraction?

Answer 24

When a wave undergoes refraction, its frequency doesn’t change. If a wave refracts because it speeds up, its wavelength will increase. If it slows down, its wavelength will decrease.

Question 25

What effects does the refraction of light lead to?

Answer 25

Refraction of light at water/air boundaries causes objects to appear closer closer to the water surface than they actually are. Refraction is the reason why a straw in a glass of water looks broken.

Question 26

What happens to parallel rays of light when they enter a convex lens?

Answer 26

In a convex lens, parallel rays from a distant source of light are made to converge at a point known as the focal point, a distance called the focal length from the centre of the lens.

Question 27

What do objects viewed through convex lenses look like?

Answer 27

The image formed by a convex lens depends on where the object is and the focal length of the lens.

Question 28

What happens to parallel rays of light when they enter a concave lens?

Answer 28

In a concave lens, parallel rays of light are made to diverge away from the lens. The focal point is on the same side of the lens as the object.

Question 29

What do objects viewed through concave lenses look like?

Answer 29

The image formed by a concave lens depends on where the object is and the focal length of the lens.

Question 30

How do you calculate magnification?

Answer 30

Magnification can be calculated using the equation: Magnification = object height / image height. Both image and object heights need to be measured using the same unit (usually cm). Magnification is a ratio and so has no unit. ## Footnote This equation will be given to you on the equation sheet in an exam.

Question 31

What happens to white light when it is shone through a prism?

Answer 31

When white light is directed through a prism, the separate colours of the spectrum are seen. This is called dispersion and happens because different colour (wavelength) light changes speed by different amounts.

Question 32

Why do opaque objects appear the colour they do?

Answer 32

Coloured objects reflect only the wavelengths (colours) of light that they appear and absorb all others. White objects reflect all wavelengths, black objects absorb all wavelengths.

Question 33

What is a transparent object?

Answer 33

A transparent object transmits light that is incident on it.

Question 34

What is a translucent object?

Answer 34

A translucent object transmits light that is incident on it only partially.

Question 35

What is a filter?

Answer 35

A filter only allows light of the wavelengths corresponding to its colour to pass through it (be transmitted); all other wavelengths are absorbed.

Question 36

What happens to the colour of an object if it is viewed through a colour filter?

Answer 36

If the filter allows the wavelength of light corresponding to the object to pass, it will look the colour it is. If the filter does not allow the wavelength of light corresponding to the object to pass, it will look black.