# Diffraction Grating: Possible variables for Experiment

• KDPhysics
In summary, changing the distance between two CDs will change the distance ##y## that the laser light travels and can be used to measure the number of bits that can be stored on the CD.
KDPhysics
Homework Statement
Devise an experiment investigating the properties of a CD diffraction grating (Can't change wavelength).
Relevant Equations
Diffraction Grating equation for maxima: ##d\sin\theta = n\lambda##
For my High School Physics course, I have been tasked to design an experiment investigating the properties of a CD diffraction grating, and we MUST make a graph. Unfortunately, we only have two lasers of different wavelength, so changing the wavelength and measuring ##theta## would be a bad idea.

I have come up with two ideas, one which is theoretically possible, and one which I am not so sure about.

Idea 1: changing the distance from the CD to the viewing screen (which we will call ##L##)
We use the diffraction grating equation for maximum interference: ##d\sin\theta = n\lambda## where d is the distance between the bumps on the CD, and define the distance from the laser to the screen L, the angular distance from the principal maxima to the nth order maxima by ##theta##, and their distance by ##y##.
\begin{align} d\sin(\arctan(\frac{y}{D})) &= n\lambda\\ \arctan(\frac{y}{D}) = \arcsin(n\frac{\lambda}{d})\\ \frac{y}{D} = \tan(\arcsin(n\frac{\lambda}{d}))\\ y = \tan(\arcsin(n\frac{\lambda}{d})) D \end{align}
I can then vary the distance D and measure y for the first order maxima (##y = \tan(\arcsin(\frac{\lambda}{d})) D##). plot them and use the slope of the regression fit to find the distance between the bumps. From there I can calculate the number of bits the CD can hold, and maybe compare with a DVD.

Is the mathematics correct? I am afraid that I may have forgotten some special conditions that make this formula incorrect.

Idea 2: changing the distance between two CDs acting as double diffraction gratings
I place two CDs which act as double diffraction gratings. I change the distance between them, and measure the distance ##y##.
Unfortunately, I have no idea if there will be any noticeable difference if i add this second CD, let alone how to develop the mathematics and physics behind this experiment.

What is D? Same as L? What distance is y?
I would have thought that with a laser source the distance from there to the CD was irrelevant.
What about the angle of the beam to the CD?

By the way, note that:
$$\tan(\arcsin(x))) = \frac{x}{\sqrt{1 - x^2}} ~~~~~ \text{where } x < 1$$

[EDIT] Fixed the equation to reflect the fact that the argument of arcsin function is not an angle. I don't know what I was thinking (or even thinking at all) before..

Last edited:
gneill said:
By the way, note that:
$$\tan(\arcsin(\theta))) = \frac{\theta}{\sqrt{1 - \theta^2}} ~~~~~ \text{where } \theta < 1 \text { is in radians}$$
Surely the argument to arcsin is not usually an angle. Why would it be in radians?

haruspex said:
Surely the argument to arcsin is not usually an angle. Why would it be in radians?
Whoops! That's a brain fart on my part. Should have used x instead of θ, and specified that x < 1. I'll edit the post. Sorry about that!

## 1. What is a diffraction grating?

A diffraction grating is an optical component made of a large number of evenly spaced parallel slits or grooves. It is used to separate light into its component wavelengths, creating a spectrum.

## 2. How does a diffraction grating work?

When light passes through a diffraction grating, it is diffracted, or bent, by each slit or groove. The diffracted light waves interfere with each other, creating a pattern of bright and dark spots called an interference pattern. This pattern allows for the separation of different wavelengths of light.

## 3. What are the possible variables to consider when conducting an experiment with a diffraction grating?

The possible variables to consider include the distance between the grating and the light source, the distance between the grating and the screen, the number of slits or grooves on the grating, and the wavelength of the light being used.

## 4. How can the distance between the grating and the screen affect the results of the experiment?

The distance between the grating and the screen affects the angle at which the light is diffracted. A larger distance results in a smaller angle and a wider separation of the different wavelengths of light, while a smaller distance results in a larger angle and a narrower separation.

## 5. What is the relationship between the number of slits or grooves on the grating and the quality of the spectrum produced?

The more slits or grooves on the grating, the higher the resolution of the spectrum. This means that the different wavelengths of light will be more clearly separated, resulting in a higher quality spectrum.

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