two source interference and single slit diffraction

The principle of superposition

When two or more waves of the same type meet in one place the total displacement is the vector sum of the individual displacements

Conditions for interference

• same frequency

• Same wavelength

• Constant phase difference

What is path difference

The difference in the distance travelled by 2 waves

Equation for two source interference

W = lambda D / s

How to make maxima further apart

• increase wavelength (eg. Red light)

• Increase distance to screen

• Smaller slit separation

Interference pattern for double slit

• highest intensity in middle fringed

• Equally spaced

• Destructive interference - intensity is zero

• Dark and light fringes

Conditions for single slit diffraction

• wavelength similar to gap/obstacle

Interference pattern for single slit diffraction

• central fringe 2x width of outer fringes

• Peak intensity decreases with distance from centre

• Outer fringes are same width

• Central maximum much brighter/ more intense

What happens if the slit is narrower

Maxima are more spread out

Observation for single slit diffraction

A white central maximum surrounded by coloured fringes