Light Waves

Question 1

What is the normal?

Answer 1

An imaginary line perpendicular to a boundary between two materials or a surface.

Question 2

Which way does light bend if the light travels into a more dense material?

Answer 2

Towards the normal.

Question 3

Which way does light bend if the light travels into a less dense material?

Answer 3

Away from the normal.

Question 4

What are the three equations you need to know for Snell’s law?

Answer 4

n = sin(i) ÷ sin(r)
n = c₁ ÷ c₂
n₁ x sin(θ₁) = n₂ x sin(θ₂)

Question 5

Why does white light split into different colours when it goes through a prism.

Answer 5

White light is composed of light of different wavelengths so when the light is refracted, each colour goes off in different directions depending on it’s wavelength.

Question 6

What is total internal reflection?

Answer 6

Where the light will travel along the boundary or even be reflected off the boundary.

Question 7

What is the criteria for total internal reflection?

Answer 7

The incident material must have a large refractive index than the other material.
The angle of incidence must exceed the critical angle.

Question 8

How do you calculate the critical angle?

Answer 8

sin(θ𝒸) = n₂ ÷ n₁

Question 9

Why do diamonds sparkle?

Answer 9

Because diamonds have a very high refractive index causing them to have a low critical angle meaning the light can totally internally reflect many times. This gives it a shiny look.

Question 10

What can optical fibres be used for?

Answer 10

Endoscopes to see inside the body and in communications to send information.

Question 11

How do optical fibres work?

Answer 11

Light enters in one end and totally internally reflects all the way to the other end.

Question 12

What is the purpose of cladding in optical fibres?

Answer 12

The cladding has a lower refractive index so that the critical angle is smaller meaning the light is more likely to totally internally reflect. This prevents light loss and makes the signal stronger.

Question 13

Why is the core very narrow in optical fibres?

Answer 13

The core being narrow forces the light to take the same path which reduces modal dispersion.

Question 14

Why must the light in optical fibres be monochromatic?

Answer 14

Because white light would split up into multiple colours causing the different signals to become mixed up.

Question 15

Why must the light in optical fibres be monochromatic?

Answer 15

Because white light would split up into multiple colours resulting in different speeds of light causing the different signals to become mixed up.

Question 16

What happens when two coherent light sources enter double slits and are projected upon a surface?

Answer 16

The light sources create a fringe pattern of bright and dark fringes.

Question 17

Why does shining a single light source through a double slit cause a fringe pattern?

Answer 17

The two sources of light are coherent and interfere with each other causing areas where the light is brighter and areas where the light is cancelled out.

Question 18

What is the double slit equation?

Answer 18

w = λd ÷ s 
w = fringe separation
λ = wavelength
D = distance
s = slit separation

Question 19

What is coherent light?

Answer 19

Light with the same frequency and constant phase difference.

Question 20

Will two different light sources form an interference pattern?

Answer 20

No, because the light sources change at random meaning that the points reinforcement and destruction keep changing preventing an interference pattern.

Question 21

Red and Blue light are shone through a double slit creating an interference pattern. Aside from colour, what are the differences and why?

Answer 21

The fringe separation for red light will be larger because red has a longer wavelength than blue.

Question 22

What is monochromatic light?

Answer 22

Light of only one colour.

Question 23

What happens if light is diffracted through one slit?

Answer 23

The light creates a diffraction pattern of bright fringes and dark fringes. The intensity of light decreases the further the light gets away from the center.

Question 24

How do fringes change with wavelength?

Answer 24

The longer the wavelength, the wider the fringes.

Question 25

What is diffraction grating?

Answer 25

A plate with lots of closely spaced parallel slits on it.

Question 26

What is the diffraction grating equation?

Answer 26

d x sin(θ) = nλ
d = distance between slits
λ = wavelength
n = order of maxima
θ = angle

Question 27

What happens if you shine white light through diffraction grating?

Answer 27

The white light splits up into different colours with the purple/blue colours being closer to the middle as it doesn’t separate as much.