In basic optics, the bending of light rays when they pass from a rarer medium to a denser medium is known as which phenomenon?

Introduction / Context:
When light travels from one transparent medium to another, such as from air to water or from air to glass, it often changes direction at the boundary. This change in direction is responsible for many familiar optical effects like apparent bending of a stick in water and the working of lenses in spectacles, cameras, and microscopes. The question asks for the correct name of this phenomenon when a light ray bends while entering a denser optical medium from a rarer medium. Recognising the correct term is fundamental to understanding optical instruments and visual illusions.

Given Data / Assumptions:
• A light ray travels from a rarer medium to a denser medium. • At the interface between the two media, the direction of the light ray changes. • The question focuses only on naming the phenomenon, not on quantitative laws or angles.

Concept / Approach:
The basic phenomena involving light at a boundary between two media include reflection, refraction, dispersion, diffraction, and scattering. Reflection is the process where light bounces back into the same medium. Refraction is the change in direction of a light ray as it passes from one medium to another with a different optical density, due to a change in speed. Dispersion is the splitting of white light into its component colours, typically by a prism. Diffraction is the bending and spreading of light around obstacles or through narrow openings. Deflection is a more general term that does not refer to a specific standard wave phenomenon. For a light ray changing direction at the interface between media of different refractive indices, the correct term is refraction.

Step-by-Step Solution:
Step 1: Identify that the light ray is passing from one medium into another (rarer to denser). Step 2: Recognise that the ray bends at the interface when its speed changes in the new medium. Step 3: Recall that this change in direction due to a change in speed in different media is defined as refraction. Step 4: Conclude that the bending of light rays in a denser medium is called refraction.

Verification / Alternative check:
Every school level physics textbook describes Snell law, which relates the angles of incidence and refraction when light passes between media with different refractive indices. The very word refractive index is derived from refraction. Lenses work by refraction of light at curved surfaces. Apparent depth of objects in water is explained by refraction. All these examples involve a light ray entering a medium where its speed changes and its path bends. Therefore, the phenomenon described in the question clearly matches refraction and not the other effects.

Why Other Options Are Wrong:
Option a (Reflection): Reflection occurs when light bounces back into the original medium instead of entering the second medium. Option b (Dispersion): Dispersion refers to the splitting of white light into its component colours due to different refractive indices for different wavelengths. Option d (Deflection): Deflection is a generic word for change in direction but is not the standard physical term for this specific optical process. Option e (Diffraction): Diffraction is the bending and spreading of waves around obstacles or through narrow slits, especially visible when slit width is comparable to the wavelength.

Common Pitfalls:
Students sometimes confuse refraction with dispersion because prisms cause both a change in direction and splitting of colours. It is important to remember that refraction refers to bending of light due to a change in medium, while dispersion is about the separation of colours. Another mistake is to use the word reflection whenever a light ray changes direction, forgetting that reflection keeps the ray in the same medium. To avoid confusion, always consider whether the ray enters a new medium or stays in the original one.

Final Answer:
The bending of light rays in a denser medium is called Refraction.