Waves- Diffraction and interference

Diffraction

Diffraction refers to the bending of waves when they encounter an obstacle or pass through a narrow gap.

What factors influence diffraction

• Wavelength, the degree of diffraction increases as wavelength increases.

• Gap or obstacle size, if the gap is much larger than the wave, it passes through largely undisturbed, if the gap is much smaller, very little wave energy makes it through.

When is diffraction most notable?

When the wavelength is similar in size to the obstacle

Geometrical shadow

The region behind an obstacle where we would expect no waves if waves travelled in straight lines.

What is interference?

Interference is the phenomenon that occurs when two or more waves overlap and combine

What is an interference pattern?

A pattern of alternating high and low intensities on a screen

Set up for Young’s double slit

• A single coherent light source is shone through two closely spaced slits.

• The light emerging from the slits acts as two coherent sources.

• On a screen some distance away, an interference pattern of bright and dark fringes appears.

Fringe pattern explanation

Constructive interference (bright fringe) occurs where path difference = an integer number of wave lengths. Destructive interference (dark fringe) occurs where path difference = an integer number of wavelengths over 2

What is a diffraction grating

A tool that uses multiple evenly spaced slits to create highly defined interference patterns

Diffraction of white

The central maximum is white since all wavelengths overlap at 0. At higher order, each wavelength diffracts by a different angle, producing spectra. Red diffracts more than violet, because it is of a longer wavelength.

Application of diffraction gratings

Spectroscopy (identifying elements in an atmosphere), Optical instruments, Astronomical spectrographs (measuring redshift).