When light passes by the edge of an obstacle or through a narrow slit and bends into the region of geometrical shadow, the phenomenon responsible for this bending of light is called what?

Introduction / Context:

Wave optics explains several phenomena that cannot be understood by treating light purely as rays. One such phenomenon is the bending and spreading of light when it encounters obstacles or apertures comparable in size to its wavelength. This question asks for the correct term used to describe the bending of light around a corner or through a narrow slit into the shadow region.

Given Data / Assumptions:

• Light behaves as a wave with a definite wavelength.
• Light encounters an obstacle edge or passes through a narrow opening.
• The bending happens into regions that would be dark according to simple ray optics.

Concept / Approach:

Diffraction is the phenomenon in which waves bend around obstacles or spread after passing through narrow apertures. It is significant when the size of the obstacle or aperture is of the same order as the wavelength of the wave. Reflection involves bouncing of light from a surface, refraction involves change in direction at an interface due to change in refractive index, and total internal reflection is a special case of refraction at large incident angles in denser media. None of these describe bending into geometrical shadow caused by obstacles or slits; that is specifically diffraction.

Step-by-Step Solution:

Verification / Alternative check:

Classic experiments with single and double slits demonstrate diffraction patterns, where light spreads out and interferes, producing bright and dark fringes on a screen. These effects cannot be explained by straight line ray diagrams. Huygens principle and wavefront analysis show that each point on a wavefront acts as a secondary source, leading to bending around edges, which is the essence of diffraction.

Why Other Options Are Wrong:

Option A: Reflection describes light returning from a surface with the angle of incidence equal to the angle of reflection; it does not explain bending behind obstacles.

Option B: Refraction is bending at a boundary between different media due to a change in speed of light, not bending into shadow regions behind an obstacle.

Option D: Total internal reflection is a special refraction effect where light is completely reflected back into a denser medium at angles above the critical angle.

Common Pitfalls:

Students sometimes mix diffraction and refraction, especially because both involve changes in direction. A useful distinction is that refraction involves interfaces between media, whereas diffraction involves obstacles and apertures causing spreading of wavefronts. Remember that diffraction explains why you can hear sound around corners and why light forms fringes in slit experiments.

Final Answer:

The bending of light around a corner or slit is due to diffraction.