# Decrease in intensity of light on undergoing successive reflections

1. Jun 2, 2006

### Amith2006

Sir,
1) A thick plane mirror shows a number of images of the filament of an electronic bulb. It is said that of these images the first image is the brightest. Is it true? If so, is it because some energy is lost in traversing through the dense medium?
2) It is said that the intensity of light at a large distance r from a long, thin cylindrical source of light is proportional to r^0. Is it true? I know that intensity of a point source is proportional to r^(-2). In what way is a long, thin cylindrical source of light different? Can't the cylindrical source be treated as a point source for large distances? Here the symbol ^ stands for power.

Last edited: Jun 2, 2006
2. Jun 2, 2006

### Astronuc

Staff Emeritus
1) Some light is scattered within the solid medium and at the interface of the solid and atmosphere. So some intensity (number of photons) is lost.

2) A line is essentially a continuous series of point sources, and each contributes to the light being received at some distance. If one does the integral, then one sees that the light intensity is inversely proportional to the distance r (or r-1) from the line.

Keep in mind this applies to a very long line, which is observed along a line perpendicular to the line of light sources.

If the line is short, then the intensity would approach that of a point source a long distance.

Also the orientation (angle) of the 'short' line with respect to the line of observation would have to be considered.