Scattering of Blue Light and Red Light

Blue light is scattered more efficiently than red light. As we shall see, this explains why skim milk and the sky overhead are blue, but sunsets are red.

Discussion

After the diffraction grating is moved in front of the screen, the first-order images display the spectrum of the white light reflected by the screen, with the blue band closest to the center and the red farthest. (The blue and green bands you may notice on the right and left of the first order images belong to the second order diffraction pattern.) When the container of water is moved over the aperture, there is no change in the spectrum of the transmitted light, but adding milk to the water in the container results in both striking changes to the spectrum of transmitted light and to scattering of light to the side. Before milk is added, the beam of light passing through the container is almost invisible from the side and the spectrum is that of white light; after milk is added, the beam can be seen quite clearly from the side and the intensity of the short wavelength part of the spectrum (blue and green) is much reduced in comparison to the long wavelength part (red and orange).

When the grating is removed, it is clear that red and orange are dominant colors in the unscattered light that passes through the milk suspension.

Finally, when the polarizing sheet is placed in front of the container and screen and rotated, the intensity of scattered light changes with the orientation of the polarizing sheet (somewhat more light passes through the polarizing sheet after it is rotated), but the intensity of transmitted light reflected by the screen is independent of the orientation of the polarizing sheet. (Polarization effects for the scattered light are much more obvious in the next movie, where a more dilute suspension of milk is used.) This result means that scattered light is polarized, but transmitted light is not.