Diffraction

Diffraction

The spreading out of waves as they pass an obstacle or through an opening

When is the effect of diffraction the greatest?

When the size of the gap is similar to or smaller than the wavelength of the waves

Path difference

When λ ∈ Z+, constructive interference occurs, light points

When λ is not an integer, destructive interference occurs, dark points

Coherence

• Same frequency

• Constant phase difference

Fringe spacing equation

w = λD/s

• w: fringe spacing

• λ: wavelength

• D: distance from slits to screen

• s: slit separator

Properties of single slit diffraction

• Intense, double width central fringe

• Less intense, narrower fringes each side

Properties of double slit diffraction

• Every fringe same width

• Intensity greatest for central fringe, then varies

If a grating is stated as having N lines per mm, what is the slit separation?

d = 1/N

How are spectroscopes used?

In telescopes to help scientists analyse the materials that make up stars and nebulae

How does a spectroscope work?

• Light coming into the telescope is filtered through a tiny hole in a metal plate, to isolate light from a single area / object

• This light is bounced off a special grating which splits the light into its different wavelengths (just like a prism makes rainbows)

• The split light is focused onto a detector, forming a spectrum