# Just Wondering

1. Nov 4, 2005

### EIRE2003

When I was in class today we were doing an experiment to measure the wavelength of monochromatic light. my teacher said ''focus the light on a distant object'', but the light source that we use is very close so why do we have to focus it on a distant object?

2. Nov 4, 2005

### Integral

Staff Emeritus
A more complete description of your experiment would help, but...

A near by light source does not produce plane waves, while a source at infinty does. Many experiments of this type require plane waves to get meaningful results. My guess is that you are trying to approximate plane waves.

3. Nov 13, 2005

### starfield

if you were trying to find the focal length ....this is the explanation.
because to find the focal length, the object has to be at infinity. only when the object is at infinity will the waves coming from it be parallel to the axis of the lens and only then will the image be formed exactly at at a distance equal to the focal lenth of the lens.