Question

In a dark room of photography, generally red light is used. The reason is

Answer 1

Hint: In dark room photography we use a special type of film (light image capture film) which is very sensitive to light that has high energy within it. This may burn down the film completely so in order to protect the film we use a low energy light ray.
The energy of a light ray is determined by:
$E = \dfrac{{hc}}{\lambda }$
Where $\lambda $is the wavelength, $c$ is the speed of light and $h$is the Plank's constant.
Thus, $E \propto \dfrac{1}{\lambda }$
As rest $c$ the speed of light and $h$ Plank's constant remain unchanged.

Complete answer:
Now we know that ultraviolet rays of light or blue light which lie in the visible spectrum has the maximum energy (for light in visible spectrum) and they have the minimum wavelength compared to any other colour in a visible spectrum as:
$E = \dfrac{{hc}}{\lambda }$
Where $\lambda $is the wavelength, $c$ is the speed of light and $h$ is the Plank's constant.
So, if we use red light in dark room photography, it has low wavelength means it will have low energy in the visible spectrum as:
$E \propto \dfrac{1}{\lambda }$
This low energy of the light will protect our photographic film from being burnt down thus we can protect the film if we use red light in dark room photography

Note:
Apart from red light we even use the infrared light (in the form of radiation) in dark room photography as the usual objects that are in normal temperature condition emit infrared radiation (from the vibration of the particle present in the body). Infrared light has even less energy compared to that of normal light thus we can take better chances in protecting the film from being damaged.