Module 4 EM Spectrum, diffraction, polarisation

Question 1

EM - What is the electromagnetic spectrum?

Answer 1

in physics, electromagnetic radiation consists of waves of the electromagnetic field, which propagate through space and carry electromagnetic radiant energy.

It includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays . All of these waves form part of the electromagnetic spectrum.

Question 2

EM - What defines electromagnetic waves

Answer 2

can travel in a vacuum, travel at the speed of light in a vacuum

Question 3

EM - What type of waves are in the EM spectrum?

Answer 3

Transverse waves, progressive therefore transferring energy

Question 4

EM - Name the EM waves in order of decreasing wavelength

Answer 4

radio
microwaves
infrared
visible
ultraviolet
x-ray
gamma

Question 5

EM - EM spectrum in order of decreasing frequency

Answer 5

gamma
x-ray
ultraviolet
visible
infrared
micro
radio

Question 6

EM - Typical wavelength for visible light

Answer 6

700-400nm red to blue (7-4x10^-7m)

Question 7

EM - Typical wavelength for gamma rays

Answer 7

10^-10 to 10^-16 (typical 10^-14m)

Question 8

EM - Typical wavelength for x-rays

Answer 8

10^-8 to 10^-13 (typical 10^-9m)

Question 9

EM - Typical wavelength for ultraviolet light

Answer 9

4x10^-7 to 10^-8 (typical 10^-8m)

Question 10

EM - Typical wavelength for infrared light

Answer 10

10^-3 to 7x10^-7 (typical 10^-5m)

Question 11

EM - Typical wavelength for microwaves

Answer 11

10^-1 to 10^-3 (typical 10cm or 10^-2m)

Question 12

EM - Typical wavelength for radiowaves

Answer 12

> 10^-1 (typical 10^3m)

Question 13

Diffraction - Definition of diffraction

Answer 13

when a wave passes through an aperture and spreads out into the space beyond

Question 14

Diffraction - Conditions for maximum diffraction

Answer 14

when the wavelength of the source is = to the aperture through which the wave is diffracting
(same magnitude ie power of ten)

Question 15

Diffraction - Why is the diffraction of sound more noticeable then the diffraction of light

Answer 15

• sound has longer wavelengths
• aperture needs to be the same size as the wavelength of the wave
• Wavelength of sound is around 1m vs light which has one of 5x10^-7m
• Common gaps of obstacles are of the order of 1m so diffraction of sound is more commonly observed
• light requires very narrow slits to diffract

Question 16

Diffraction - Describe how diffraction can be observed using a ripple tank (describe how the tank is put together)

Answer 16

-Plane bar of wood driven by electric motor (or use an oscillating paddle)
-wood oscillates at the frequency of the motor
-wavelength and frequency of the water waves depend on frequency of the motor
-wooden planks can be used to make obstacles and gaps to observe the diffraction of water waves

Question 17

Diffraction - Explain why long wave radio waves can be detected behind a mountain but short wave radio waves cannot be

Answer 17

-Mountain is closer in size to the wavelength of long wave radio waves
-More diffraction takes place and radio wave is reformed closer to the mountain
-Little diffraction happens with short wave radio waves

Question 18

Diffraction - Describe how you could observe diffraction in a laboratory using a loudspeaker

Answer 18

-Connect a signal generator to a loudspeaker in front of a slit board
-Aperture comparable to wavelength of sound generated
-Sound should spread beyond aperture which can be observed using a microphone attached to an oscilloscope

Question 19

Polarisation - What is polarisation?

Answer 19

where the vibrations of a light wave occur in a single plane at 90º to the the direction of energy transfer

Note: light is made up of electric and magnetic fields both at 90º to the direction of energy transfer

Question 20

Polarisation - Function of a polaroid or polarising filter?

Answer 20

polarises visible light

Question 21

Polarisation -Describe what happens when when vertical polaroid is placed in front of vertically polarised light and is rotated through from 0-180º

Answer 21

• at 0⇒90º intensity of light gradually goes from max to 0
• 90⇒180º it goes from 0 to max
• When direction polarised light and polaroid are parallel light is let through, perpendicular non is let through

Question 22

Polarisation - Describe and explain how you can find out the direction of plane polarisation of a set of microwave transmitter and receivers

Answer 22

• transmitter emits polarised microwaves
  receiver can only detect if in same plane
  orientate so they are in the same plane by turning the transmitter until max current is shown on the ammeter
• place a metal grille between the transmitter and receiver
• if the grill is parallel to direction of plane polarisation of the microwaves then no current is detected as waves are absorbed by metal wires which cause electrons in the metal to vibrate
• therefore, rotate the grille until no current is detected, the direction of the wires is the direction of polarisation

Question 23

Polarisation - What direction does a metal grill absorb polarised microwaves?

Answer 23

same plane as polarisation

Question 24

Diffraction - What is a word which can be used to describe gaps for diffraction?

Answer 24

Aperture