GCSE Physics Tutorial: Refraction of Waves and Velocity Change

Refraction is a phenomenon that occurs when waves change direction as they pass from one medium to another with a different optical density. This change in direction is a result of the change in the wave's velocity as it transitions between mediums. In this tutorial, we'll explain why waves refract when their velocity is changed.

Change in Wave Velocity:

When a wave transitions from one medium to another, its velocity can change due to differences in the medium's properties, such as density or stiffness. The change in velocity causes the wave to refract or bend.

Principle of Least Time:

The principle of least time states that light (and other waves) will follow a path that minimises the time taken to travel between two points. When a wave encounters a boundary between two mediums at an angle, it travels slower in the denser medium and faster in the less dense medium. To minimise the time taken to travel from the source to the destination, the wave changes direction at the boundary.

Example: Light Refraction:

Imagine light passing from air into water. Light travels slower in water than in air due to the higher optical density of water. As the light encounters the water surface at an angle, it refracts towards the normal (an imaginary line perpendicular to the surface). This bending of light allows it to change direction while obeying the principle of least time.

Real-World Example:

• Straw in a Glass of Water: When you place a straw in a glass of water, it appears to be bent at the water-air boundary due to refraction. This is an example of how waves, including light, refract when their velocity changes between media.

Implications and Applications:

• Lenses: Refraction is essential for the functioning of lenses in cameras, eyeglasses, and microscopes.

• Prisms: Refraction in prisms separates white light into its component colors, creating a rainbow effect.

• Mirages: Atmospheric refraction can create mirages, where distant objects appear to be displaced or elevated due to changing air densities.

Summary:

Refraction occurs when waves change direction as they pass from one medium to another with a different optical density. This change in direction is a result of the change in the wave's velocity. The principle of least time explains why waves refract, allowing them to optimise their path while transitioning between mediums. Refraction has practical applications in optics, engineering, and everyday observations.

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