Waves and Optics

Question 1

What is the equation for threshold frequency?

Answer 1

f_0 = ϕ/h

Question 2

What happens when a light ray moves into a more optically dense medium?

Answer 2

The light ray bends towards the normal line.

Question 3

What happens when a light ray moves into a less optically dense medium?

Answer 3

The light ray bends away form the normal line.

Question 4

What are the conditions for total internal reflection (TIR) to occur?

Answer 4

The ray would be moving into a less optically dense medium if TIR didn’t occur and the angle of refraction must be greater than 90 degrees.

Question 5

What is path difference?

Answer 5

The amount by which the path travelled by one wave is longer than the path travelled by the other wave.

Question 6

What is phase difference?

Answer 6

The difference in the phase angle between two waves.

Question 7

What is the equation for the speed of a wave?

Answer 7

V = fλ

Question 8

What are the equations for the wavelength of a standing wave in the 1st, 2nd and 3rd harmonic?

Answer 8

1st: λ = 2l
2nd: λ = l
3rd: λ = 2l/3

Question 9

What is the equation for tension in a string creating a standing wave?

Answer 9

T = mg

Question 10

what is the equation for μ?

Answer 10

μ = m/l (mass per unit length)

Question 11

What is the single slit used for in young’s double slit experiment?

Answer 11

To keep the waves coherent.

Question 12

What are light and dark fringes?

Answer 12

Light fringes are caused by light waves in phase superposing and interacting constructively. Dark fringes are caused by light waves in antiphase superposing and interacting destructively. There is still light in dark fringes, it is just cancelled out by destructive interference.

Question 13

Define the terms in the equation for fringe spacing:

w = λD/s

Answer 13

w is fringe spacing, λ is wavelength, D is distance from the slits to the screen and s is slit spacing measured from the centre of each slit.

Question 14

What is a node?

Answer 14

A stationary point on a standing wave caused by destructive interference of two waves superposing in antiphase.

Question 15

What is an antinode?

Answer 15

A point of a standing wave where displacement is maximum. This is caused by constructive interference between two waves superposing in phase.

Question 16

What is the equation for refractive index?

Answer 16

n = c/v where n is the refractive index, c is the speed of light in vacuo and v is the speed of light in the material.

Question 17

What is Snell’s law?

Answer 17

n = sin θ_I/sin θ_R

Question 18

What is the equation for the law of refraction?

Answer 18

n_1 sin θ_I = n_2 sin θ_R

Question 19

What is the critical angle?

Answer 19

Where the angle of refraction is 90 degrees.

Question 20

What is the equation for a diffraction grating?

Answer 20

d sin θ = n λ where d is the slit spacing, n is the order number and λ is the wavelength.

Question 21

What is the equation for lines per meter in a diffraction grating?

Answer 21

1/d where d is slit spacing.

Question 22

How do you convert lines per millimetre on a diffraction grating into lines per metre?

Answer 22

Multiply lines per millimetre by 10^3 then take the reciprocal.

Question 23

Why are there black lines on an absorbtion spectrum?

Answer 23

Because excited gas atoms absorb the photons of the wavelengths of the blackspaces.

Question 24

Define the terms in the equation for the first harmonic:

f = 1/2l √(T/μ)

Answer 24

f = frequency of oscillation
l = length of the string
T = Tension in the string
μ = mass per unit length of the string

Question 25

How does increasing the slit width affect the amount of diffraction and the intensity of the central maximum?

Answer 25

Decreases diffraction, increases intensity of the central maximum.

Question 26

How does increasing the wavelength affect the amount of diffraction and intensity of the central maximum?

Answer 26

Increases diffraction, lowers intensity of the central maximum.

Question 27

How does using a diffraction grating instead of just two slits affect the diffraction pattern?

Answer 27

Same shape but bright bands are brighter and narrower and the dark areas between are darker. The pattern is really sharp which allows for more accurate measurements.

Question 28

What do the maxima look like from monochromatic light?

Answer 28

Monochromatic, sharp lines.

Question 29

What do the zero, first and second orders look like from the diffraction of white light?

Answer 29

The zero order (middle) is white then the first order is a clear rainbow then the second order is a wider, more faded rainbow etc.

Question 30

How does the refractive index of a material affect the speed of light within it?

Answer 30

The higher the refractive index is, the slower the light is within it.

Question 31

What is modal dispersion?

Answer 31

When light rays enter the fibre at different angles so take different paths so reach the end in different amounts of time.

Question 32

What is material dispersion?

Answer 32

When light rays reach the end of the fibre at different times due to their differing wavelengths.