The Elastic Ruler

Light undergoes refraction when passing from water into air, causing submerged objects like a ruler to appear compressed or shorter than their actual length. This optical illusion occurs because light rays leaving the ruler at a steep angle bend toward the horizontal as they exit the water's surface. When these rays reach the eye at a shallower angle, the brain perceives the source of the light as being closer to the surface than it truly is. This effect intensifies as the observer's eye level approaches the water's surface. Conversely, lifting the ruler out of the water creates a stretching sensation as the light rays transition back to a direct path through the air. This demonstration illustrates the fundamental physical principle that light changes direction when crossing at an angle between two transparent media of different densities.