3.3.2 - Refraction, Diffraction & Interference

A measure of the relative speed of light in a material compared to in a vacuum (3 x 10^8 ms^-1)

Light changing velocity when it travels across the boundary between two materials.

A more optically dense material (higher refractive index) causes it to slow down and bend towards the normal and vice versa.

Wave speed and wavelength change.

Frequency stays the same

n = c (vacuum) / c (material) = λ (vacuum) / λ (material

Air is considered a vacuum as it doesn’t slow light down significantly so has a refractive index of one

n1 sinθ1 = n2 sinθ2

• n1 is the refractive index of material 1

• n2 is the refractive index of material 2

• θ1 is the angle of incidence of the ray in material 1

• θ2 is the angle of refraction of the ray in material 2

All light gets reflected off of a surface instead of passing through and being refracted.

• Light has to be more from a more optically dense medium (higher n value) into a less optically dense medium (lower n value).

• Angle of Incidence > Critical angle

The angle of incidence which causes light to travel alongside the boundary due to an angle of refraction = 90*

sinθ c = n2 / n1

where n1 > n2

TIR is used in optical fibres which carry information in the form of light signals.

A light pulse is sent down an optical fibre and is detected at the other end generating a signal.

Flexible thin tube of plastic or glass.

Inner core is more optically dense and surrounded by cladding which is less optically dense

• protects the core from damage

• prevents signal degradation through light escaping the core, which can cause information to be lost

• Absorption

• Dispersion

Parts of the signal’s energy is absorbed by the fibre reducing the overall amplitude of the signal - could cause a loss of information

Causes Pulse Broadening which is when the received signal is broader then the original transmitted signal - broadened signals can overlap which causes loss of information.

• Modal

• Material

Light rays enter fibre at different angles so take different paths hence they could take different amounts of time to travel along the fibre

Make the core very narrow to reduce the possible path differences the light could have.Mater

Light consisting of different wavelengths will travel at different speeds in a material

Use monochromatic light

The difference in the distance travelled by two waves

A light source where all light waves emitted have the same frequency and wavelength with a constant phase difference.

Light of a single wavelength

To demonstrate the wave properties of light.

• Place single slit before a double slit to make the light have a constant phase difference

• Use a filter to make the light monochromatic

A series of bright (maxima) and dark (minima) fringes - the central fringe is always a bright fringe.

The path difference for light from one slit is different from the other slight so light meets on the screen in different phases.

Light meets in phase and interferes constructively which occures when the path difference is a whole number of wavelengths (nλ).

Light meets out of phase and interferes destructively which occurs when the path difference is a whole number and a half wavelengths ((n+½)λ).

sw = λD

where -

• s = slit separation (m)

• w = fringe separation (m)

• λ = wavelength (m)

• D = distance from screen (m)

slit separation is measured from the centre of one slit to the centre of the next

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

The greatest diffraction occurs when the gap is the same size as the wavelength - when the gap is smaller most waves get reflected.

A bright(est) central fringe double the width of all the other fringes with alternating dark and bright fringes on either side.

Intensity of fringes decreases from central fringe,

Increasing slit width = less diffraction so central maxima is narrower and more intense

Increasing the wavelet will increase how much light diffracts causing a thicker less intense central maxima

More distinct dark and bright fringes.

The central fringe is called the zero order line and then the first order lines and then the second order lines etc.

A slide containing many equally spaced slits.

d = 1/N

where -

• d = distance between the slits

• N = number of slits per metre

d sinθ = nλ

where -

• d = distance between the slits

• θ = the angle between the zero order line and next order line

• n = the order

• λ = the wavelength