If you are scanning a previously-printed image, such as a magazine photograph, you will need to 'de-screen' the image (in Photoshop, the Unsharp Filter is a good tool to use) , blurring it slightly to avoid the effect below:
This interference is called a moiré pattern (pronounced more-ay). In a scanned image, Moiré patterns are caused by interference between two sets of fine pattern grids, the scanner samples and the halftone screen in the original image. Any scanner will do this, it's a simple fact of life.
Any image printed on a printing press (like a book, magazine, newspaper, postcard, calendar, etc.) is printed with halftone screen patterns. The printed image is composed of a pattern of dots. A strong magnifying glass will show them. The halftone dots are printed entirely in black if a B&W image, or there are four screens in each of the three primary colors plus black (CMYK) if a Color image. These fine dots cause optical problems in a scanned image because the scanned image is also composed of fine dots.
The two patterns of dots, the printed magazine's 133 or 150 lpi screened pattern, and the 300 or 600 dpi scanner CCD cells, combine into maximums or minimums every several pixels in the image, depending on the spacing of the dots. It affects the overall light intensity in periodic patterns that become very visible. The pattern is named Moiré.