Slices of glass or gelatin placed in front of the lens of a camera to prevent certain frequencies of light from reaching the film are called:

I believe the filters are the slices of glass or gelatin put in front of the lens of the camera or printer to reduce certain frequencies of light reaching the film. The lens on the other hand gathers light from the scene and transmits it onto the flat surface of the film or video chip to form an image that represents size, depth, and other dimensions of the scene; each type of lens will render perspective in different ways.

The answer is Filters.  Slices of glass or gelatin placed in front of the lens of a camera to prevent certain frequencies of light from reaching the film are called Filters.  Filters are used in cinematography to change the image either for technical reasons to correct the colour of the light to something that is required by the film we are using.

The rocks expand when there hot/warm  and contract when they are cold.