In black and white photography, a photon energy of about 4.00 x 10 to the negative 19th power I is needed to bring about the changes in the silver compounds used in the film. Explain why a red light used in a darkroom does not affect the film during developing.
for red light λ=0.62 – 0.76 μm
max f=c/ λ=3•10⁸/0.62•10⁻⁶=4.84•10¹⁴ Hz
ε=hf=6.63•10⁻³⁴•4.84•10¹⁴ =3.2•10⁻¹⁹ J
Since 3.2•10⁻¹⁹ J < 4•10⁻¹⁹ J red light does not affect the film .
A red light used in a darkroom does not affect the film during developing due to the difference in photon energy. The energy of a photon is directly proportional to its frequency, and inversely proportional to its wavelength. Red light has a longer wavelength compared to other colors in the visible spectrum.
In black and white photography, silver compounds are used to capture light and create the image on the film. These silver compounds are sensitive to higher energy photons, like ultraviolet and blue light. However, red light has lower energy photons, which are not sufficient to bring about changes in the silver compounds.
The photon energy needed for the silver compounds to react and create an image on the film is roughly 4.00 x 10^(-19) Joules. Red light, with its longer wavelength, has a lower energy per photon than this threshold. As a result, red light does not have enough energy to affect the silver compounds, and thus does not cause any changes in the film during developing.
Using a red light in a darkroom allows the photographer to work without exposing the film to light that could potentially ruin the image. The red light provides enough illumination for visibility and minimal disruption of the developing process, as it falls below the energy requirements for silver compound reactions.
In black and white photography, the film contains silver compounds that undergo a chemical reaction when exposed to light. Specifically, the photons of light interact with the silver compounds in the film and cause them to change their structure.
The energy of a photon is directly related to the color of light. Photons with higher energy, such as those in the blue and ultraviolet spectrum, have more energy than photons in the red spectrum. The energy of a photon can be calculated using the equation E = hf, where E is the energy, h is Planck's constant, and f is the frequency of the light.
Now, the given photon energy is 4.00 x 10^(-19) J. To convert this energy into frequency, we can use the equation E = hf, and rearrange it to solve for f: f = E / h.
Using Planck's constant (h ≈ 6.63 x 10^(-34) J·s), we can calculate the frequency of the light needed to produce this energy.
f = (4.00 x 10^(-19) J) / (6.63 x 10^(-34) J·s)
Calculating this yields a frequency of approximately 6.03 x 10^14 Hz.
Here comes the interesting part. The frequency of red light generally ranges between 4.28 x 10^14 Hz and 4.91 x 10^14 Hz, which is below the calculated frequency. Therefore, the red light used in a darkroom does not possess sufficient energy (photon energy) to bring about the necessary changes in the silver compounds used in the film during developing.
This is why a red light can be safely used in a darkroom, as it does not affect the film during the developing process. The red light has a frequency lower than what is required to trigger the chemical changes in the film's silver compounds, making it a suitable light source for maintaining the darkness of the darkroom while working with the film.