The phenomenon in which the direction of light changes when it passes from one transparent medium to another is called what?

Introduction / Context:
When light travels from one medium into another, such as from air into water or glass, its speed changes. This change in speed leads to a change in direction, a phenomenon observed in many optical effects and devices. The correct term for this bending of light at the boundary between media is refraction. This question asks you to identify that term, distinguishing it from related but different phenomena like reflection and dispersion.

Given Data / Assumptions:

• Light travels from one transparent medium into another with a different refractive index.
• The direction of the light ray changes at the boundary between the two media.
• The media are homogeneous and transparent, so light can pass through them.
• We are not considering cases where light is entirely reflected.
• We assume classical geometric optics applies.

Concept / Approach:
Refraction is defined as the change in direction of a light ray when it passes from one medium to another in which its speed is different. Snell law describes this relationship quantitatively as n1 * sin theta1 = n2 * sin theta2, where n1 and n2 are refractive indices and theta1 and theta2 are angles of incidence and refraction. Reflection, by contrast, refers to light bouncing off a surface, while dispersion describes the separation of white light into different colours due to wavelength dependent refraction. Propagation and scattering are more general terms and do not specifically mean the bending at an interface.

Step-by-Step Solution:
Step 1: Identify that the question describes a change in direction of light when crossing a boundary between two media.Step 2: Recall that such bending occurs because light speed changes when it enters a medium with a different refractive index.Step 3: Recognise from basic optics that the phenomenon of bending at the interface is called refraction.Step 4: Confirm that reflection, dispersion, and scattering refer to different, though related, optical effects.

Verification / Alternative check:
Common examples support this conclusion. A stick partly immersed in water appears bent at the surface due to refraction. Lenses use controlled refraction to focus light and form images. Rainbows, while involving dispersion, fundamentally rely on refraction occurring at the surfaces of water droplets. Descriptions of these phenomena always mention refraction of light at interfaces, confirming that this is the correct term for a change of direction when passing from one medium to another.

Why Other Options Are Wrong:
Option a is wrong because propagation is a general term meaning travel of light and does not specifically refer to bending at a boundary. Option b is incorrect because reflection involves light bouncing back from a surface, not passing into a new medium. Option d is not correct because dispersion is the separation of white light into its component colours, which depends on refraction but is not simply the bending itself. Option e is unrelated here since scattering involves random deviations of light due to particles, not the orderly bending at a smooth interface.

Common Pitfalls:
Students sometimes confuse refraction with reflection because both occur at boundaries. Another frequent mistake is to identify any colourful effect, such as a rainbow, purely with dispersion and forget that refraction is the underlying bending mechanism. Keeping the definitions clear helps: refraction is bending at a boundary due to speed change, reflection is bouncing back, and dispersion is wavelength dependent refraction. Learning examples for each helps you remember the distinctions.

Final Answer:
Refraction, when light bends on entering a different medium.