Before the halftone process (introduced in 1881), photographs could not be directly reproduced in print media. Images in newspapers were limited to hand-drawn illustrations, woodcuts, and engravings. The halftone process converted continuous-tone photographs into a pattern of tiny dots of varying sizes, which could be printed alongside text using standard printing presses. This meant that for the first time, photographs could appear in newspapers and books, creating the conditions for photojournalism — the use of photographs to document news events and drive public opinion. Combined with improvements like dry plates, smaller portable cameras, and George Eastman's Kodak box camera (1888) with roll film, the halftone process transformed print journalism from a primarily word-based medium to a powerful visual medium, enabling photographic documentation of wars, social conditions, and major public events.

Homolytic bond cleavage: each atom takes one electron, forming free radicals (e.g., methyl radical •CH₃). Heterolytic fission gives carbocations or carbanions.

For two Carnot engines in series where the work output is equal ($W_A = W_B$):

$Q_1 - Q_2 = Q_2 - Q_3$

Since $Q \propto T$ for a Carnot cycle, we can write:
$T_1 - T_2 = T_2 - T_3$

Rearranging for $T_2$:
$2T_2 = T_1 + T_3 \implies T_2 = \frac{600 + 400}{2} = 500 \text{ K}$.

Antibiotics target bacterial structures/processes (cell wall, protein synthesis). Salmonella typhi (bacterium) causes typhoid; antibiotics work. Herpes (virus) and polio (virus) require antiviral drugs.