# Quanta and Waves part II Flashcards

What is the condition for **constructive** interference in terms of optical path difference?

Constructive interference occurs when

**Optical path difference = mλ**

Stationary waves can only be formed by interference at very specific frequencies.

What name is given to these frequencies?

Resonant frequencies.

A **polariser** and **analyser** combination can be used to increase or decrease the transmission of a light wave.

What would be observed if a laser beam was shone through a polariser and then an analyser which were at 90 degrees to each other?

There would be no light transmission through the polariser and analyser combination.

The relationship shown below can be used to determine the wavelength of a laser going through a double slit.

Define each of the quantities in the relationship.

Δx = distance between adjacent bright fringes (m)

λ = Wavelength of the laser light (m)

D = distance from double slit to screen (m)

d = distance between slits (m)

Under which condition will a wave undergo a phase change of λ/2 upon reflection?

The wave will be travelling from a medium with a lower refractve index and reflected by a medium with a higher refractive index.

What is the condition for **destructive** interference in terms of phase?

Two waves **meet** completely out of phase by half a wavelength- crest meets trough.

The phase difference Φ = π radians.

An experiment designed to show ‘thin wedge’ interference is set up as shown below:

What is the relationship used to describe thin wedge interference, and what does each quantity represent?

∆x = distance between bright fringes (m)

λ = wavelength of light (m)

l = length of ‘wedge’ (m)

d = height of ‘wedge’ at end (m)

What is the condition for **destructive** interferece in terms of optical path difference?

Constructive interference occurs when

**Optical path difference = (m+1/2) λ**

What is the relationship between the brewster angle and the refractive index of a material?

n = tan i_{p}

Plane polarised light can be produced by partial reflection from a glass surface as shown below.

The reflected ray will be plane polarised if the angle of incidence is equal to the brewster angle (i_{p}).

What condition must be met to find the brewster angle, and produce plane polarised light?

The angle between the reflected and refracted rays must be **90 degrees** as shown below. This will produce a reflected ray which is plane polarised parallel to the surface of the glass.

A stationary wave is formed in a piece of elastic string under tension, created by a vibration generator set to 250Hz as shown below.

The distance between adjacent nodes is 0.15m

- Determine the wavelength of this wave.
- Determine the speed of this wave.

- The wavelength of the wave is
**0.3m**(The distance between nodes is equal to half a wavelength). - The speed of the wave = frequency x wavelength

= 250 x 0.3

**= 75 ms ^{-1}**

What does it mean if a wave is said to be **plane** **polarised**?

If the oscillations of the wave medium are restricted to one dimension only the wave is said to be **plane polarised.**

What is the relationship between geometrical path length and optical path length?

Optical path length = n x geometrical path length.

What is the unit of phase angle, Φ?

Radians (rad).

Shown below is an example of interference by division of amplitude- a lens is ‘bloomed’ with a non-reflective coating.

Derive the expression **d = λ/4n** where ‘d’ is the thickness of coating required to give destructive interference.

**Optical PD = λ/2** for destructive interference

also from the diagram, **Optical PD = 2nd**

therefore

2nd = λ/2

**d = λ/4n**