# CD's have little bumps on them that act like a grating right?

1. Jan 31, 2006

### UrbanXrisis

CD's have little bumps on them that act like a grating right? but how is it that the little bumps can split white light into the rainbow of colors when I look at it?

2. Jan 31, 2006

### Staff: Mentor

Think of the many grooves as forming a diffraction grating. Each color creates a diffraction pattern at a slightly different angle, thus producing a rainbow effect.

3. Jan 31, 2006

### UrbanXrisis

how does each color create a different diffraction pattern? do you mean light seen from different angles become different colors due to diffraction?

4. Jan 31, 2006

### Staff: Mentor

Different colors will diffract at different angles. (Ordinary light is a mix of colors.)

5. Jan 31, 2006

### UrbanXrisis

so say I was in the nanometer world and I am sitting on one of the CD 'bumps.' I view white light coming towards the CD and then depending on where it hits, it bounces off in a different direction due to the 'bumps'. The light that bounces off constructively and destructively interfers to create different colors. how is looking at a CD from a different angle going to change the color of the CD when the constructive and destructive interference of the light have already created a certain color? why would a color change just because it bounces off the same bumps but from a different angle?

6. Jan 31, 2006

### Staff: Mentor

Think of it this way. If there was no diffraction, then the different colors in the white light would reflect at the same angle and the colors would not be visible to the eye. But on the CD, each color diffracts at a slightly different angle. When you look at the CD, each spot on the CD makes a different angle with your eye, thus reflects a different color. (Note that each spot reflects all colors, but you only see one of them depending on the angle that it makes with your eye.) Make sense?

7. Jan 31, 2006

### Smilecdl

different coloured light has different wavelengths and therefore scatters by a different amount

8. Jan 31, 2006

### UrbanXrisis

how does the little bumps on the CD split light into different colors?

9. Jan 31, 2006

### Staff: Mentor

10. Jan 31, 2006

### UrbanXrisis

but why does refraction gratings work the same way as diffraction gratings?

11. Jan 31, 2006

### Staff: Mentor

What do you mean by "refraction" grating?

12. Jan 31, 2006

### UrbanXrisis

instead of light going though slits, it is reflected

13. Jan 31, 2006

### Staff: Mentor

Ah, you mean reflection grating (as opposed to transmission grating). To see a diffraction pattern, all you need is a regular pattern of light sources with the appropriate phase relationship. It doesn't matter if these light "sources" are due to transmission or reflection.