diffraction

Definitions Single slit diffraction Multiple slits Diffraction grating Applications

Definition

Diffraction = waves spread out as pass aperture

e.g. sound

water

radio waves Amount diffraction depends on ratio of

to aperture width, a

Diffraction of light - single slit

light 400 – 700 nm V. small slit needed! Huygen’s principle – predicts future

position of wavefront each point on wavefront is a point

sources of wavelets Wavelets superpose + interfere to form

future wavefronts

Diffraction pattern = series of bright and dark fringes

Central maximum Secondary maxima

Red filter used …

More diffraction if 1. increased2. Aperture width decreased

What would pattern look like if blue filter used?

Diffraction - multiple slits

Effects similar to Young's interference pattern

As number slits increases, maxima = brighter + sharper

Each slit acts as point source of 2° wavelets individual diffraction patterns interference Superposition of single slit pattern on

multiple patterns

Diffraction grating

Transmission grating Reflection grating Use: produce spectra to measure

accurately 2nd order

1st order

Zero order

1st order

2nd order

Laser

Coarse/fine grating Increasing coarseness = increasing number

of orders (and orders closer together)

Theory

Angle is such that wave B in phase with wave A

path difference is for 1st order principle maxima 2 for 2nd order 3 for 3rd order n for nth order

ABC:

Sin = AC = AC = n AB d d

Or n = d Sin

Spectrometer

Collimator parallel light diffraction grating Pattern observed measured find

Applications

Accurate method of measuring Can separate s close together Identify v. small quantities of material

since different elements = different spectra

Analysis of light from stars, nebulae and interstellar gas determine of structure