In this chapter of AS/A-level Physics, we will learn about diffraction.
In AS/A-level Physics, Amount of diffraction depends on the wavelength compared to the size of the gap
o Gap greater than the wavelength = diffraction unnoticeable o Noticeable diffraction when gap several wavelength wide
o Most diffraction = same size wavelength
o Gap smaller = waves reflected back
Light shone through a narrow slit can form diffraction patterns
o Need monochromatic [same wavelength and frequency], coherent light source same colour
o Central bright fringe (central maximum), with dark and bright fringes alternating on either side. The dark and bright fringes are caused by destructive and constructive interference of light waves
In AS/A-level Physics, Diffraction of white light creates a spectrum
o Central maxima = white
o The black fringes are closer together and constructive fringes create different colours (spectra)
Intensity of light means number of photons
- Central maxima are the brightest due to having the highest intensity
- Intensity = power/ cross sectional area
- For monochromatic light, photons have the same energy, so an increase in the intensity means an increase in the number of photons per second
- More photons per unit area hitting the central maximum per second than the other bright fringes--> p = n(hf), power equation
--> Increasing slit width decreases the amount of diffraction meaning central maximum is narrow and the intensity is higher
--> Increasing the wavelength increases the amount of diffraction meaning the central maximum is wider, and the intensity of the central maximum is lower
Two source interference in water and sounds
- In AS/A-level Physics, Need coherent sources--> same wavelength and frequency
- Need to use the same oscillator
Two source interference in light-->Thomas Young’s Double Slit
- Shine a laser though the two slits on a screen-->slit should be roughly same size as the wavelength of the laser light
Can do similar with microwave
2. Danger of working with lasers/ prevent:
o Damage retina, warning signs outside, wear laser safety goggles, turn off laser when not needed
Young’s Double Slit formula
Fringe spacing
Fringe spacing – Distance from the centre of one minimum to the centre of the next minimum or from one central maximum to the other
In AS/A-level Physics, Young’s experiment: Evidence of wave nature of EM radiation
Light could refract/ diffract and interfere
Light made up of tiny corpuscles
This is the end of the topic!
