Refraction

When waves pass from one medium to another and change direction (bend), this is called refraction. This occurs due to the waves changing speed at the boundary between two transparent media of different densities.

For example, due to the refraction of light, this spoon looks broken.

The spoon clearly hasn’t snapped in half, it is sitting in the cup as normal. However, it looks distorted because of refraction.

Light reflects off the top of the spoon that isn’t submerged in the water. At this part of the spoon, light is travelling through the same medium (air). However, as light travels into the water to reach the bottom of the spoon, it has to cross to a different medium, from air to liquid. Because of this, light doesn’t travel in a straight line to the bottom of the spoon and back to our eye.

When light enters the new medium, it changes direction. The light has to change direction when entering and leaving a new medium. From air to liquid, then again from liquid to air. This makes the spoon appear distorted.

To understand refraction, we must examine how light changes direction when transitioning between two mediums.

For example, when a torch is pointed at glass.

1. As light moves from air to glass, it changes direction.

  • The reason light changes direction is because it is changing speed, slowing down as it enters the glass

2. When the light leaves the glass, moving back into the air, it changes direction again.

  • This time, the light speeds up as it moves back into the air.

During refraction, only the wavelength and wave speed change. The frequency of the waves stays the same.

Refraction Ray Diagrams

Let’s look at a refraction ray diagram.

On a refraction ray diagram, you can calculate the angle of incidence, angle of refraction and angle of emergence.

The angle of incidence = The angle of emergence

The angle of incidence is equal to the angle of emergence because, in both instances, light is travelling in the air. Before light enters the glass and when light leaves the glass.

  • When light enters a denser medium, it slows down, causing the ray to bend towards the normal.
  • When light enters a less dense medium, it speeds up, so the ray bends away from the normal

Keep in mind that the ‘normal’ is represented by the dotted line in the diagram.

So, if light enters a more dense medium, the angle of refraction is smaller and if light enters a less dense medium, the angle of refraction is greater.

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