What refraction is

Refraction is the bending of waves as they pass from one medium to another with different densities. This happens because the wave changes speed when it enters a new medium, causing its path to curve. For example, light waves bend when moving from air into water, which is why a straw looks broken when placed in a glass of water.

Key features of refraction

• Medium - A substance or material that waves travel through, such as air, water, or glass
• Density difference - Refraction occurs only when waves move between media with different densities, like from less dense air to denser glass
• Wave bending - The direction change helps explain everyday observations, such as objects appearing distorted underwater

This bending is a fundamental property of all waves, including light waves.

How waves bend when moving between different media

When a wave enters a new medium, its path bends relative to an imaginary line called the normal. The normal is a straight line drawn perpendicular to the surface where the two media meet. The amount and direction of bending depend on whether the wave is entering a denser or less dense medium.

Bending toward the normal

Waves refract toward the normal when they enter a denser medium. This occurs because the wave slows down in the denser material, causing the part of the wave that enters first to move more slowly while the rest catches up, pulling the wave path closer to the normal.

Bending away from the normal

Waves refract away from the normal when they enter a less dense medium. Here, the wave speeds up in the less dense material, so the part of the wave that enters first moves faster, spreading the wave path farther from the normal.

These rules apply to all waves, but the effect is more pronounced with greater density differences between media.

Ray diagrams for refraction

Ray diagrams are simple drawings that show how light waves bend during refraction. They use straight lines called rays to represent the wave paths and include key elements to illustrate the process clearly.

Key parts of a refraction ray diagram

• Incident ray - The incoming ray that approaches the boundary between two media
• Refracted ray - The ray that continues inside the second medium after bending
• Emergent ray - The ray that leaves the second medium and continues into a third medium, often bending again
• Normal line - A dashed line drawn perpendicular to the surface at the point where the ray hits, used as a reference for measuring bending

Steps to draw a refraction ray diagram

1. Draw the boundary line between the two media, such as air above and glass below.
2. Add the normal line as a perpendicular dashed line at the point of incidence.
3. Draw the incident ray approaching the boundary at an angle to the normal.
4. Show the refracted ray bending toward or away from the normal inside the second medium, depending on density.
5. If the ray exits the medium, draw the emergent ray bending again as it enters the new medium.

These diagrams help visualize refraction in objects like lenses or prisms, where multiple bendings occur.

Changes in wave properties during refraction

During refraction, some wave properties stay the same while others change. This is because the wave must adjust to the new medium's density, affecting how it travels.

Properties that change

• Wave speed - The speed decreases in denser media and increases in less dense media, causing the bending effect
• Wavelength - The distance between wave crests shortens in denser media (where speed is slower) and lengthens in less dense media (where speed is faster)

Properties that remain constant

Frequency is the number of waves passing a point per second and stays the same, as it is determined by the wave's source and does not change with the medium.

These changes explain why refraction occurs without altering the wave's basic identity.

Dispersion of white light through prisms

Dispersion is the splitting of white light into its different colors when it passes through a prism, demonstrating refraction in action. A prism is a triangular piece of glass that refracts light twice—once when entering and once when exiting.

How dispersion happens in a prism

1. White light (made of all colors) enters the prism as an incident ray and refracts toward the normal because glass is denser than air.
2. Inside the prism, different colors refract at slightly different angles because each color has a unique wavelength and travels at a slightly different speed in glass (shorter wavelengths like blue bend more than longer ones like red).
3. At the exit side, the rays refract away from the normal as they enter less dense air, spreading the colors further.
4. The emergent rays form a spectrum of colors, from red (least bent) to violet (most bent).

This process shows how refraction causes wavelength changes while frequency remains constant, separating white light into a rainbow-like display.