GATE 2021 AE – Question 39

Correct answer: (C) shadowgraph

Explanation

Optical methods for visualising density gradients in compressible flow:

• **Interferometry:** measures the density itself through fringe shifts.
• **Schlieren:** a knife edge placed at the focus cuts off part of the light, so the intensity shows the **first derivative** of the density (the density gradient).
• **Shadowgraph:** needs **no knife edge or focusing optics**. The light rays bent by the density gradient make a pattern of bright and dark regions on a screen, which shows the **second derivative** of the density (so it is best for shocks).
• **Holography:** records the full wave front using a reference beam.

The arrangement without a knife edge is a **shadowgraph** (option C).