Re: Why can't film be exposed to light after taken out of the camera?

K'Rina,

Thanks for the question.  The reason film can not be exposed to light even 
after you have taken pictures with it is that it is still sensitive to 
light.  

Here is an analogy.  Imagine a piece of paper and a water color marker.  
You can draw a picture on the paper with the marker.  However, once you are 
done drawing your picture someone else can come along and use the marker to 
draw over your picture.  You have to do something extra like cover your 
drawing with a sheet of clear plastic to prevent any further marks being 
added to the paper.

The situation is similar with photographic film.  When you take a picture 
light is shaped by the lense of the camera and briefly strikes the film to 
form a "latent" image on the film. The image is latent because even if you 
could look at the film without messing it up you would not see anything 
different about it.  The light that strikes the film uses up a small amount 
of the light sensitive chemicals on the film, but there is typically plenty 
more waiting to absorb more light. If you open the back of the camera after 
taking a picture then light floods the film like the villan scribbler 
overwrites the picture in the analogy.  

At this point you may be wondering how it is ever possible to look at film 
in the light without destroying the negative.  What happens is that the 
film is typically rolled up into a light-tight cannister (for 35 mm film) 
and taken out of the camera.  At the photo lab the film is first developed, 
to intensify the latent image into a visible image on the film.  Next the 
film is "fixed".  This means it is made insensitive to light so that light 
will no longer wash out the picture that is already on the film.  Fixing 
typically uses a chemical to dissolve the light sensitive part of the film 
while leaving the exposed chemical behind.  Once film is fixed, washed, and 
dried then it becomes the negatives that you get back with the pictures.  
The negativea are pieces of the film that you once had in your camera.

I hope that clears up what is going on with the film.

Regards,

Everett Rubel