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Physics Lesson 12.4.4 - Diffraction Gratings
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Welcome to our Physics lesson on Diffraction Gratings, this is the fourth lesson of our suite of physics lessons covering the topic of Interference and Diffraction of Light, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
Diffraction Gratings
A diffraction grating tool is very similar in concept to the double-slit system discussed earlier but with more openings. It has a very large number of parallel lines on a glass or plastics obtained through grating process as shown in the figure.
The figure below shows an educational diffraction grating tool whose parameters (number of gratings per mm) are written under each glass.
Obviously, we cannot see the gratings on naked eye, as they are very close and thin.
A diffraction grating is often (but not only) used to split the white light into colours. It is also used to determine the wavelength of a light wave similarly to the methods mentioned earlier in single-slit diffraction and Young Double-Slit Experiment.
If we enlarge a part of a diffraction grating and see it in profile, the view obtained will be as shown below.
We can use the equation for the condition of constructive interference
to determine the wavelength of a certain light beam. Let's consider an example.
Example 3
Monochromatic (one-coloured) light is incident at right angle to a diffraction grating having 2000 lines per centimetre. The angular separation between the zeroth and the fourth-order maxima is 300. Calculate the wavelength of the incident light.
Solution 3
First, let's work out the separation a between two adjacent gratings. We have
= 0.0005 cm
= 5 × 10-4 cm
= 5 × 10-6 m
Given that N = 4 and sin 300 = 0.5, we obtain
5 × 10-6 × 0.5 = 4 × λ
2.5 × 10-6 = 4 × λ
λ = 2.5 × 10-6/4
= 0.625 × 10-6 m
= 6.25 × 10-7 m
You have reached the end of Physics lesson 12.4.4 Diffraction Gratings. There are 4 lessons in this physics tutorial covering Interference and Diffraction of Light, you can access all the lessons from this tutorial below.
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- Continuing learning optics - read our next physics tutorial: Dispersion of Light
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