P1c

Question 1

What is a trough?

Answer 1

Point on maximum negative displacement.

Question 2

What is a crest?

Answer 2

Point on maximum positive displacement.

Question 3

What is amplitude?

Answer 3

The amplitude is the displacement from the rest position to the crest. (Not from the trough to the crest)

Question 4

What is the wavelength?

Answer 4

The wavelength is the length of a full cycle of the wave, e.g. from crest to crest.

Question 5

What is the frequency?

Answer 5

Frequency is the number of complete cycles or oscillations passing a certain point per second. Frequency is measured in hertz (Hz). 1 Hz is 1 wave per second.

Question 6

What do all electromagnetic waves do?`

Answer 6

Travel at the same high speed in space or a vacuum.

Question 7

What is the equation for wave speed?

Answer 7

Wave speed = frequency x wavelength

Question 8

High frequencies can often be given in kHz or MHz, what are they in Hz?

Answer 8

1kHz (kilohertz) = 1000Hz

1MHz (megahertz) = 1000000Hz

Question 9

What should you do if the speed is in standard form?

Answer 9

Put it in standard for in the calculator?

Question 10

What do electromagnetic waves travel in?

Answer 10

Straight lines through a particular medium.

Question 11

What happens when light reflects from an even surface?

Answer 11

When it reflects from an even surface (smooth and shiny like a plane mirror) then it’s all reflected at the same angle and you get a clear reflection.

Question 12

What is the law of reflection?

Answer 12

Angle of incidence = angle of reflection
These two angles are always defined between the ray itself and the normal. The normal is the imaginary line that’s perpendicular (at right angles) to the surface at the point of incidence (where the light hits the surface).

Question 13

What happens when a beam of light reflects from an uneven surface such as a piece of paper?

Answer 13

The light reflects off at different angles.

Question 14

What does refraction involve?

Answer 14

A change in direction of a wave due to the wave passing from one medium into another.
Changing the speed of a wave can change its direction. Waves travel at different speeds in substances which have different densities. So when a wave crosses a boundary between two substance from glass to air, say, it changes speed.

Question 15

What happens if a light wave hits the boundary ‘face on’?

Answer 15

It slows down but carries on in the same direction. It now has a shorter wavelength but the same frequency.

Question 16

What happens if a wave meets a different medium at an angle?

Answer 16

Part of the wave hits the denser layer first and slows down, while another part carries on at the first, faster speed for a while. So the waves change direction - it has been refracted.

Question 17

What happens when light passes from air into the glass of a window pane (a denser medium)?

Answer 17

It slows down - causing the light to refract towards the normal. When the light reaches the ‘glass to air’ boundary on the other side of the window, it speeds up and refracts away from the normal.

Question 18

What happens if waves travel along the normal (the angle of incidence is zero)?

Answer 18

They will change speed, but are not refracted - they don’t change direction. Waves are only refracted if they meet a new medium at an angle.

Question 19

What happens when waves pass through a gap or pass an object?

Answer 19

All waves spread out (diffract) at the edges.

Question 20

What is the significance of the size of the opening or barrier relative to the wavelength?

Answer 20

The amount of diffraction depends on the size of the gap relative to the wavelength of the wave. The narrower the gap, or the longer the wavelength, the more the waves spread out. A narrow gap is one about the same size as the wavelength of the wave. So whether a gap counts as narrow or not depends on the wave.
If the gap is much wider than wavelength, there is little diffraction. If the gap is a bit wider than the wavelength, diffraction only happens at edges. If the gap is the same as the wavelength, there is maximum diffraction.

Question 21

Can you hear someone through an open door if you can’t see them?

Answer 21

Light has a very small wavelength so it can be diffracted but it needs a really small gap. This means you can hear someone through an open door even if you can’t see them, because the size of the gap and the wavelength of sound are roughly equal, causing the sound wave to diffract and fill the room. But you can’t see them unless you’re directly facing the door because the gap is about a million times bigger than the wavelength of light, so it won’t diffract enough.
if a gap is about the same size as the wavelength of a light, you can get a diffraction pattern of light and dark fringes.

Question 22

Can you get diffraction around the edges of obstacles too?

Answer 22

Yes. The shadow is where the wave is blocked. The wider the obstacle compared to the wavelength, the less diffraction it causes, so the longer the shadow.

Question 23

What are the seven types of electromagnetic waves that comprise the spectrum in ascending order of frequency?

Answer 23

Increasing frequency and decreasing wavelength.

Radio waves, microwaves, infra red, visible light, ultra violet, x rays, gamma rays.

Question 24

What is an example of a communicative use of a radio wave, microwave, infrared and visible light?

Answer 24

Radio wave - radio
Microwave - mobile phones
Infrared - transmitting information between mobile phones or computers - only over short distances.
Visible Light - optical fibres

Question 25

What are the limiting effects of diffraction on waved based sensors?

Answer 25

Optical microscopes have to be small, because you usually use them to look at small samples of tiny things in the lab - you want to collect light coming from a very small area only. Their small size makes it hard to get a good resolution - the gap needs to be very small, so you still get some diffraction even though light has a small wavelength. Bigger telescopes give us better resolution because they cause less diffraction, so the information is clearer. Telescopes with small gaps compared to the wavelengths they're looking for have limited resolving power - they are said to be diffraction-limited.

Question 26

Relate the size of a communications receiver to the wavelength for radio, microwave, infrared and visible light.

Answer 26

We use different receivers (sensors) to pick up the different types of EM waves used for communication - e.g. telescopes, satellite dishes, microscopes etc. The minimum size of receiver needed is linked to the size of the wavelength of the wave - the longer the wavelength, the larger the receiver should be. So radio waves need the biggest receivers, then microwaves, then infrared, then light waves. This is because of diffraction - when a wave enters a receiver it passes through a gap. If a wave is diffracted, it spreads out and you lose detail.