Concept:
In geometry, two figures are said to be similar if:
• their corresponding angles are equal, and
• their corresponding sides are proportional.
Similarity means the shape remains exactly the same, although the size may increase or decrease.
On the other hand:
• Congruent figures have the same shape and the same size.
• Similar figures have the same shape but may have different sizes.
Step 1: Understand what happens during enlargement or reduction.
When a photograph is enlarged:
• every length increases in the same proportion,
• all angles remain unchanged,
• the shape remains identical.
Similarly, when a photograph is reduced:
• every length decreases proportionally,
• the overall appearance remains the same,
• only the size changes.
Step 2: Compare with definition of similarity.
In enlarged or reduced photographs:
• corresponding angles remain equal,
• corresponding sides remain proportional.
Therefore, the photographs satisfy all conditions of similar figures.
Step 3: Explain why they are not congruent.
Congruent figures must have:
\[
\text{same shape + same size}
\]
But enlarged or reduced photographs have different sizes.
Hence they cannot be congruent.
Step 4: Final conclusion.
Since the shape remains the same but the size changes proportionally:
\[
\boxed{\text{Reduced and enlarged photographs are similar}}
\]
Final Answer:
\[
\boxed{\text{similar}}
\]