In this chapter of AS/A-level Physics, we will learn about diffraction gradient.
Diffraction gradient
- In AS/A-level Physics, Can repeat Young’s double slit experiment with more than two equally spaced slits
--> same pattern but the bright bands are brighter and narrower and the dark becomes darker
- Monochromatic light is passed through a grating with hundreds of slits per mm so the interference patters is sharp --> more accurate measurement
In AS/A-level Physics, Monochromatic light on a diffraction grating gives sharp lines
1. Monochromatic light --> all the maxima’s are sharp lines
2. There’s a line of maximum brightness at the centre called the zero order line 3. Lines just either side of the central one are called 1st orders
4. For a grating with slit distance d apart, the angle between the incident beam and the nth order maximum 𝒅𝐬𝐢𝐧𝜽= 𝒏𝝀
Deriving the equation
- At each slit, incoming waves are diffracted. These diffracted waves interfere with each other to produce an interference pattern
- Consider the first maximum, happens at the angle when the wave from one slit line-up with the waves from the next slit that are exactly one wavelength behind
- Angle between first order and incoming light 𝜃
- Angle 𝜃 (using geometry), d is the slit spacing and the path difference is 𝜆
- Therefore d sin 𝜃 = 𝜆 --> other maxima’s can occur when there is a different path difference so to make the equation general you put an n in front where n is a integer
--> If 𝜆 bigger, sin 𝜃 is bigger and so 𝜃 is bigger therefore larger wavelength = more spread out
--> If d is bigger, sin 𝜃 is smaller therefore the coarser the grating the less the pattern will spread out
--> Sin 𝜃 can’t have 𝜃 greater than 1 therefore you’ll get to a certain 𝜃 there is no more orders that exist
In AS/A-level Physics, Diffraction Gratings help to identify elements and calculate atomic spacing
- White light is a mixture of colours, if you diffract white light you get a spectra due to all the different wavelengths --> spread out by different amounts
- Each order = spectrum, red on the outside and violet on the inside. The zero order maximum stays white because all 𝜆 pass through
- Astronomers and chemists need to study spectra to identify elements --> X ray have
similar wavelength to the spacing between atoms in crystalline solids --> x ray forms
diffraction patterns and therefore x ray crystallography --> structure of DNA
This is the end of the topic!