waves - Refraction, diffraction and interference

Question 1

What is path difference?

Answer 1

The difference in the distance travelled by two waves

Question 2

What is a coherent light source?

Answer 2

The same frequency and wavelength and a fixed phase difference

Question 3

What does young’s double slit experiment demonstrate?

Answer 3

Interference of light from two sources

Question 4

How can you form an interference in young’s double slits experiment ?

Answer 4

Use two coherent light sources or use single source with a double split.

Question 5

What should you do if you don’t have a coherent source of light in young’s double slit experiment?

Answer 5

Place a single split before the double slit to make the light have a fixed phase difference and a filter to make the light monochromatic

Question 6

When are light fringes formed in young’s double slit experiment?

Answer 6

When the path difference of the waves are n lambda - difference between the waves is a whole number of wave lengths

Question 7

When are dark fringes formed in young’s double slit experiment?

Answer 7

When the phase difference of the waves are completely out of phase (antiphase) and the path difference will be whole number and a half wavelengths n1/2 lambda

Question 8

What happens if white light is used instead of a monochromatic laser?

Answer 8

-Wider maxima
-less intense diffraction pattern
-Central white fringe
-Alternating bright fringes which are spectra , violet closest to central maxima red the furthest

Question 9

What hazards do lasers provide and what can be done to minimise the risk of these hazards.

Answer 9

Hazard - lases can permanently damage eyesight
steps taken to minimise risk:
-wear laser safety googles
-Don’t shine laser at reflective surfaces
-Display a warning sign
-Never shine the laser at another person

Question 10

How can the type of interference seen in young’s slits be displayed using sound waves?

Answer 10

• Use two speakers connected to the same single generator
• measure the intensity of the wave using a microphone to find maxima and minima

Question 11

What did young’s double slit experiment provide evidence for light/EM waves and why?

Answer 11

That light has a wave nature as diffraction and interference are wave properties so EM waves must act as waves most of the time

Question 12

What is diffraction?

Answer 12

The spreading out of waves when they pass through or around a gap

Question 13

When does the greatest diffraction occur?

Answer 13

When the gap is the same size as the wavelength

Question 14

What happens when monochromatic light is diffracted through a single split onto a screen?

Answer 14

-Pattern has a bright central fringe, which is double the width of all other fringes
-Alternating dark and bright fringes on either side
-bright fringes caused by constructive interference
-dark fringes caused by destructive interference
-intensity of bright fringe decreases from central fringe

Question 15

What happens when white light is diffracted through a single split onto a screen?

Answer 15

-Different wavelengths in white light are diffracted by different amounts
-Get a spectrum of colour
-Has a central white maximum
-Alternating fringes which are spectra, red furthest away and violet closest to central maxima

Question 16

What happens if you increase the widths of the slits in a diffraction grating

Answer 16

-decreases amount of diffraction
-central maximum becomes narrower
-light intensity increases

Question 17

What happens if you increase the wavelength of light going through a diffraction grating

Answer 17

-increases amount of diffraction
-central maximum becomes wider
-light intensity decreases

Question 18

What is a diffraction grating?

Answer 18

A slide containing many equally spaced slits very close together

Question 19

Why is monochromatic light’s interference pattern much sharper and brighter through a diffraction grating than a double slit?

Answer 19

Many more rays of light reinforcing the pattern

Question 20

What are some application for diffraction gratings?

Answer 20

1.split up light from stars using a diffraction grating to get line absorption spectra which can be used to find elements in the star
2.X-ray crystallography which is where x-rays are directed at a thin crystal sheet which acts as a diffraction grating to form a diffraction pattern, this is because the wavelength of x-rays
is similar in size to the gaps between the atoms. This diffraction pattern can be used to measure the atomic spacing in certain materials

Question 21

What is refractive index?

Answer 21

A property of a material which measures how much it slows down light passing through it.

Question 22

What is refraction?

Answer 22

when a wave changes direction either towards or away from the normal depending on the refractive index

Question 23

What happens if n1 > n2 in snells law?

Answer 23

n2 is less optically dense than than n1 so ray of light will bend away from the normal

Question 24

What happens if n2 > n1 in snells law?

Answer 24

n1 is less optically dense than than n2 so ray of light will bend toward the normal

Question 25

What is the critical angle?

Answer 25

The angle of incidence when the angle of refraction is 90 degrees

Question 26

When does total internal reflection occur?

Answer 26

When angle of incidence is greater than the critical angle and incident refractive index n1 is greater than the refractive index of the material at the boundary n2.

Question 27

How does TIR occur in fibre optic cables?

Answer 27

They have an optically dense core surrounded by cladding with a lower optical density

Question 28

What does the cladding do in a fibre optic cable apart from allow TIR?

Answer 28

-protects the core from damage
-prevents signal degradation through light escaping the core

Question 29

What cause signal degradation?

Answer 29

absorption and dispersion

Question 30

What is absorption?

Answer 30

Where signals energy is absorbed by the fibre reducing the amplitude of the signal - can lead to loss of infomation

Question 31

What does dispersion cause?

Answer 31

Pulse broadening

Question 32

What is pulse broadening?

Answer 32

Where the received signal is broader than the original transmitted signal. Broadening can cause signals to overlap causing information loss

Question 33

What is modal dispersion?

Answer 33

-where light enters the optic fibre at different angles
-so light takes different paths along the wire
-rays take different times to travel down the wire causing pulse broadening

Question 34

How can modal dispersion be reduced?

Answer 34

Make the core narrow making possible difference in path lengths smaller

Question 35

What is material dispersion?

Answer 35

-where light has different wavelengths
-light rays will travel at different speeds along the fibre
-leads to pulse broadening

Question 36

How can material dispersion be reduced?

Answer 36

Use a monochromatic light.

Question 37

How can both modal and material dispersion be reduced?

Answer 37

Use a optical fibre repeater - regenerates the signal on the way to its destination.