How Do Grating Lines and Wavelength Affect Diffraction Patterns?

• physicoo
In summary, the conversation is about diffraction grating and its effects on the angle and spread of spectra. The first question discusses how a decrease in the grating constant results in an increase in angles and spread, while the second question talks about how a change in wavelength also affects the angles and spread. The third question is about the limit of the order of spectrum in diffraction grating, which cannot exceed 90 degrees due to the grazing of the diffracted rays on the plane of the grating. The conversation also touches on the multiple images and colors seen when looking through a diffraction grating, which is caused by the grating acting like a prism and spreading out the light into its component colors.
physicoo
Hi guys, I currently have a few doubts with diffraction grating (mostly theoretical-wise), so minimal calculations are involved.

I basically did an experiment on diffraction grating, involving looking through a diffraction grating and observing the spectra, aiming a laser beam at the grating etc.

What I would like to clarify is:

1) Suppose a grating with more lines per unit length (meaning the grating constant d, decreases) is used, I assume the angles and spread of spectra both increase, mainly due to the grating equation of d.sin(theta) = m(lambda)?

2) Now suppose the wavelength of light used is increased (E.g. From green to red), similarly to the previous question, I am again assuming the angles and spread of spectra also increases, again due to the grating equation.

3) I understand that there is a limit to the order of spectrum that can be observed in diffraction grating, and I've read that it's primarily because the angle theta cannot exceed 90 degrees, so the limit of the order is mainly (grating constant/wavelength). Could anyone explain to me why it can't exceed 90 degrees?

Appreciate the help! :)

you have to find what would be constant. Like are you viewing a single value of m with a certain diffraction grating. So then theta and lambda are proportional.

I do remember this lab from first year physics and that is how i thought of it. We were viewing m= 1, 0, -1 and you know that 0 always is constant while the position of 1 and -1 change based on the wavelength as long as the grating is constant.For the 90 degree thing, how does light go behind a grating? The only way would be by reflections, so light cannot exceed an angle greater then parallel to its source

Hope that helps

Yes, I've thought of that too. Which is how I derived my stand from the equation of d.sin(theta) = m(lambda).

Say, in regards to the first question, if d decreases, and the wavelength and m is held constant, I find the angles increase (in fact, quite significantly). And same goes for the spread of the spectra, after having calculated the result, which also increases when d decreases.

Cheers! :)

physicoo said:
Yes, I've thought of that too. Which is how I derived my stand from the equation of d.sin(theta) = m(lambda).

Say, in regards to the first question, if d decreases, and the wavelength and m is held constant, I find the angles increase (in fact, quite significantly). And same goes for the spread of the spectra, after having calculated the result, which also increases when d decreases.

Cheers! :)

you are quiet right sir :)

Thanks :)

As for the third question, is anyone able to explain to me why the angle is unable to exceed 90 degrees? I do not understand what would happen, if it exceeds.

θ is the angle between the diffracted ray and the normal. When θ = 90 degrees, the diffracted rays graze the plane of the grating. Hence θ cannot be more than 90 degrees.

Yup, I've read up on other sites and realized the reason why it can never exceed 90 degrees :) Thanks though!

I've got a last question though it's somewhat a little more of I-just-wanna-know-why. When holding the diffraction grating to my eye and looking at light sources, I saw multiple images of the light source, in different colors (not sure if I'm supposed to see this though!). I am assuming this is due to the grating (kind of obvious) which 'diffracts' the light into a spectrum of colors which was observed.

But how exactly does the grating piece give this kind of effect when we look through it? I'm thinking it's somewhat related to the multiple colors we see when we hold the disc under a light source.

Diffraction depends on the wavelength of the light source. when a white light passes through the grating, just like a prism, different colors come out of the grating in different direction.

So I suppose this diffraction grating piece basically acts like a prism, 'spreading out' the light into its component colors, one of which end has the highest wavelength while the other end has the lowest wavelength?

physicoo said:
So I suppose this diffraction grating piece basically acts like a prism, 'spreading out' the light into its component colors, one of which end has the highest wavelength while the other end has the lowest wavelength?

Yes. But the order of the colors are reversed in the spectrum.

Think I understand now :) Thank you!

1. What is a diffraction grating experiment?

A diffraction grating experiment is an optical experiment that involves passing light through a grating with evenly spaced slits. This causes the light to diffract, or spread out, into its component wavelengths, creating a spectrum. This allows for the analysis of the different wavelengths present in the original light source.

2. How does a diffraction grating work?

A diffraction grating works by utilizing the principle of diffraction, which is the bending of light as it passes through a narrow opening or slit. In a diffraction grating, the light passes through multiple slits in the grating, causing the light to diffract and create a spectrum.

3. What is the difference between a diffraction grating and a prism?

A diffraction grating and a prism both work to separate light into its component wavelengths, but they do so in different ways. A prism uses refraction, or the bending of light as it passes through a medium, to create a spectrum. A diffraction grating uses diffraction, or the interference of light waves, to produce a spectrum.

4. What factors affect the quality of a diffraction grating experiment?

The quality of a diffraction grating experiment can be affected by several factors, including the number of slits in the grating, the spacing between the slits, and the wavelength of the light being used. Additionally, factors such as the angle of incidence and the distance between the grating and the detector can also impact the results.

5. What are some real-world applications of diffraction grating experiments?

Diffraction grating experiments have many practical applications, including spectroscopy, which is the study of the interaction between matter and electromagnetic radiation. This can be useful in fields such as chemistry, astronomy, and forensics. Additionally, diffraction gratings are used in devices such as CD and DVD players, where they are used to read the information encoded on the discs.

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