When a ray of light passes obliquely from one transparent medium into another and its direction changes at the interface, this phenomenon of bending of light is called what?

Introduction / Context:
This question is about a basic phenomenon in optics. Light rays often pass from one medium to another, for example from air into glass or water. When this happens obliquely, the direction of the light ray changes. Understanding the correct name of this phenomenon is essential for studying lenses, prisms and many optical instruments that rely on the bending of light at boundaries.

Given Data / Assumptions:

• A ray of light travels from one transparent medium into another (for example, air to water).
• The ray strikes the boundary at an angle, not along the normal.
• A change in direction of the ray is observed at the interface.
• We are asked to name this phenomenon of change in direction.

Concept / Approach:
The change in direction of light when it passes from one medium to another of different optical density is called refraction. Refraction occurs because the speed of light is different in different media. When light enters a denser medium, it usually slows down and bends towards the normal; when it enters a rarer medium, it speeds up and bends away from the normal. Diffraction, in contrast, is the bending of waves around obstacles or through small openings. Propagation is a general term for the travel of waves and does not necessarily imply bending. Therefore, the correct term for this boundary phenomenon is refraction.

Step-by-Step Solution:
Step 1: Recall that refraction is defined as the change in direction of a light ray when it passes obliquely from one medium to another. Step 2: Understand that this change in direction is caused by the change in speed of light between the two media. Step 3: Recognise that diffraction describes the spreading of waves around obstacles or through slits, which is a different situation. Step 4: Note that propagation simply means movement or transmission and does not specifically refer to bending. Step 5: Since the question clearly states that the direction of light changes at the boundary between media, identify this as refraction.

Verification / Alternative check:
A familiar example is the apparent bending of a stick when it is partially immersed in water. The part of the stick under water appears shifted because light from that part is refracted at the water air boundary. Similarly, lenses in spectacles or cameras bend light rays to converge or diverge them by refraction. These everyday observations match the definition and confirm that the described phenomenon is refraction, not diffraction or simple propagation.

Why Other Options Are Wrong:
Diffraction involves bending and spreading of waves around edges or through narrow openings, not the change in direction at a flat interface between two media. Propagation is a general term for wave travel and does not specifically mean a change in direction due to entering a new medium. No option is correct is wrong because refraction is a standard textbook term that exactly matches the description in the question.

Common Pitfalls:
Students sometimes confuse refraction and diffraction because both involve some form of bending of waves. A useful way to remember the difference is that refraction happens at a boundary between different media, while diffraction happens around obstacles or through slits in a single medium. Keeping this distinction clear will help you select the correct term in exam questions.

Final Answer:
The change in direction of light when it passes from one medium to another is called Refraction.