5.7 The Diffraction Grating Flashcards
What does a diffraction grating consist of
A plate with many closely spaced parallel slits ruled on it
What happens when a parallel beam of monochromatic light is directed normally at a diffraction grating
Light is transmitted by the grating in certain directions only as:
- The light passing through each slit is diffracted
- The diffracted light waves from adjacent slits reinforce each other in certain directions only including the incident light direction and cancel out in all other directions
What is the central beam referred to as
The zero order beam
What is the zero order beam in the same direction as
The incident beam
The angle of diffraction between each transmitted beam and the central beam increases if:
Light of a longer wavelength is used (eg. by replacing a blue filter with a red filter)
A grating with closer slits is used
Diffraction grating: As each diffracted wavefront emerges from a slit, it _______ a wavefront from a slit _______ to it
Reinforces
Adjacent
Diffraction grating equation
d Sin ϴ = nλ
The number of slits per metre on the grating N =
1/d
Where d is the grating spacing
For a given order and wavelength, the ______ the value of d, the ______ the angle of diffraction. In other words, the ______ the number of slits per metre, the ______ the angle of diffraction
Smaller
Greater
Larger
Bigger
Fractions of a degree are usually expressed either as a decimal or in minutes (abbreviated ‘) where 1° =
60’
To find the max number of orders produced, substitute ϴ=90° in the…
…grating equation and calculate n using n = d/λ
The max number of orders is given by the value of…
… d/λ rounded down to the nearest whole number
How can diffraction gratings be made (glass)
By cutting parallel grooves very close together on a smooth glass plate
Each transmits some incident light and reflects or scatters some so the grooves act as coherent emitters of waves just as if they were slits
However effective slit width needs to be much smaller than grating spacing so that diffracted waves spread out widely
Otherwise higher order beams much less intense than lower order
Diffraction grating in a spectrometer
To study the spectrum of light from any light source and to measure light wavelengths very accurately
Designed to measure angles to within 1 arc minute which is a sixtieth of a degree
Angle can be measured accurately using light of known wavelength
What do most industrial and research laboratories ow use
A spectrum analyser
What is a spectrum analyser
An electronic spectrometer linked to a computer that gives a visual display of the variation of intensity with wavelength
A minimum in the single slit diffraction will…
…supress a diffracted order from the grating and there will be a missing order in the pattern as a result
The number of maxima observed is…
2n + 1
Where n is the greatest order
What are the types of spectra
Continuous, line emission, line absorption
Example of continuous spectra
The spectrum of light from a filament lamp
CONTINUOUS SPECTRA: What does the most intense part of the spectrum depend on
The temperature of the light source
The hotter the light source, the shorter the wavelength of the brightest part of the spectrum
CONTINUOUS SPECTRA: How can you measure the temperature of the light source
By measuring the wavelength of the brightest part of a continuous spectrum
Example of line emission spectra
A glowing gas in a vapour lamp or discharge tube
Emits light at specific wavelengths so its spectrum consists of narrow vertical lines of different colours
LINE EMISSION SPECTRA: What are the wavelengths of the lines characteristic of?
The chemical element that produced the light
LINE EMISSION SPECTRA: How can you identify elements?
If the glowing gas contains more than one element, the elements can be identified by observing its line spectrum
What is a line absorption spectrum
A continuous spectrum with narrow dark lines at certain wavelengths
LINE ABSORPTION SPECTRUM: Example of the spectrum of light from a filament lamp
Observed after passing it through a glowing gas
Thin dark vertical lines are observed superimposed on the continuous spectrum
LINE ABSORPTION SPECTRUM: What is the pattern of the dark lines caused by?
The elements in the glowing gas
These elements absorb light of the same wavelengths they can emit at so the transmitted light is missing these wavelengths.
Atoms of glowing gas that absorb light then emit light subsequently but not necessarily in same direction as transmitted light
