IB DP Physics: HL復(fù)習(xí)筆記9.4.3 Resolvance of Diffraction Gratings

Category: IB課程, 教材筆記, 福利干貨 Date: 2022年8月1日下午6:22

Resolvance of Diffraction Gratings

In order to know if a diffraction grating is able to resolve two wavelengths, the?resolving power?of the diffraction grating must be found
- This is based on the?Rayleigh criterion?as applied to diffraction gratings and their output
The resolving power,?R, of a diffraction grating is given by:

Worked Example

A student is using a diffraction grating to resolve two emission wavelengths from calcium in the 3^rd?order of the spectrum. These wavelengths are 164.99 nm and 165.20 nm.

Determine the minimum number of lines per mm needed if a beam of width 0.25 mm is incident upon the diffraction grating.

Step 1: List the known values

- Order of diffraction,?m?= 3
- Wavelength,?λ₁?= 164.99 nm
- Wavelength,?λ₂?= 165.20 nm
- Beam width = 0.25 mm

Step 2: Determine values for?λ?and?Δλ

- The value of the incident wavelength, λ, can be determined from the mean of the two wavelengths:

Exam Tip

In the worked example, the answer may look as though it has been rounded incorrectly but we are looking for the actual number of lines here, not fractions of lines

Rounding down to 1048 would leave the grating nearly half a line short, which can't happen, so always?round up to the nearest whole number

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