# Question about Defraction Grating?monochromatic light?

1. Jul 31, 2009

### graphicer89

1. The problem statement, all variables and given/known data

A) What kind of pattern of light would you get if you shone monochromatic light on a diffraction grating??

What pattern of light would you get if you shone white light on a diffraction grating???

B) Is it correct to say that the difference in the patterns produced by monochromatic and white light is caused by the fact that monochromatic light waves interere with each other when they pass through a diffraction grating, while the rays of white light do not? Yes , no and why?

2. Relevant equations

None.....

3. The attempt at a solution

I took a shot..so this is my guess

A) There would be an interference pattern (light,dark,light,dark).If white light was shone it would aslo be a interference pattern

B) No, because both monochromatic light and white light are of single frequency....thats what i got....the Beginning of B tells me that im completely wrong...please help me out....

2. Jul 31, 2009

### kuruman

Re: Question about Defraction Grating??monochromatic light?

A) Take a CD disk which acts pretty much like a diffraction grating and look at the side that has the recording on it under white light. What do you see? I don't think it is (light, dark, light, dark). Much like a prism, a diffraction grating splits light that has many frequencies mixed together into individual frequencies.

B) Monochromatic light is a single frequency but white light is all visible frequencies mixed together. Does that help?

3. Jul 31, 2009

### graphicer89

Re: Question about Defraction Grating??monochromatic light?

A) Well i see a rainbow but the colors change on the way i tilt it( when the angle changes) so i see colors that blend in which each other...

B) oh well kinda...so because white light has alot more frequencies that does interact with each other then???

4. Jul 31, 2009

### kuruman

Re: Question about Defraction Grating??monochromatic light?

And where is the rainbow that you see in the CD? White light from the room hits the CD and rainbow comes out. What is the role of the CD in all this? Very correctly you observed that when you change the angle, the color changes. This means that different colors bounce from the CD at different angles. So to summarize white light goes in many colors come out at a different angle each.

5. Jul 31, 2009

### graphicer89

Re: Question about Defraction Grating??monochromatic light?

Well the certain colors come out at certain angles....the role of the CD is just transmitting the light and reflecting it no?? So white light goes in many colors and comes out at a diffrent angle ...so for A...

A) What kind of pattern of light would you get if you shone monochromatic light on a diffraction grating??

The monochromatic light would form an array of colors......no?
If white light was shone on a diffraction grating then a certain color would appear depending on the angle....no?

B) Is it correct to say that the difference in the patterns produced by monochromatic and white light is caused by the fact that monochromatic light waves interere with each other when they pass through a diffraction grating, while the rays of white light do not? Yes , no and why?

B)Yes because white light goes in many colors come out at a different angle each or different frequency while monochromatic light is just one frequency

Sorry if im not getting this...

6. Jul 31, 2009

### kuruman

Re: Question about Defraction Grating??monochromatic light?

You are OK with (A). Now for (B). Suppose you shine a single color of light, say red, on the CD. Do you think you will see red light bounce off no matter at what angle you look at it or do you think that you will see red light bounce off only at one maybe two different angles?

7. Jul 31, 2009

### graphicer89

Re: Question about Defraction Grating??monochromatic light?

Well would that be the only color that would show..because the other colors are absorbed and not reemitted...??So Yes i think so...it would bounce off no matter at what angle you look at it .....no???

8. Jul 31, 2009

### kuruman

Re: Question about Defraction Grating??monochromatic light?

No. If red light appeared at all angles, then so should green and yellow and blue which means that if you shone all these colors together (white light) on the CD they will bounce off at all angles and remain mixed together which means that you will not be seeing a rainbow which is what you said you saw. Red light bounces off at selected angles and so does blue and all the other colors. But the angles for red are different from the angles for blue and all the other colors. Because of this difference, you see a rainbow. All the colors incident on the CD (or diffraction grating) are re-emitted and none are absorbed. The reason that red appears at certain angles and not others is because there is interference, mostly destructive, at almost all angles and red light appears at those angles that correspond to constructive interference. So, because of interference effects different colors appear at different angles.

I have provided you with what you need to answer (B). Now you need to understand it and put it in your own words. Remember white (or polychromatic) light is many frequencies all mixed together.