When is refraction abrupt

This is the same change in direction as for light when it goes from a fast medium to a slow one. When going from the grass to the footpath, the front wheels can move faster and the mower changes direction as shown. This, too, is the same change in direction as for light going from slow to fast. Figure 3. The change in direction of a light ray depends on how the speed of light changes when it crosses from one medium to another.

The speed of light is greater in medium 1 than in medium 2 in the situations shown here. This is analogous to what happens when a lawn mower goes from a footpath to grass. This is analogous to what happens when a lawn mower goes from grass to footpath. The paths are exactly reversible.

The amount that a light ray changes its direction depends both on the incident angle and the amount that the speed changes. For a ray at a given incident angle, a large change in speed causes a large change in direction, and thus a large change in angle. The incoming ray is called the incident ray and the outgoing ray the refracted ray, and the associated angles the incident angle and the refracted angle. Snell was not aware that the speed of light varied in different media, but through experiments he was able to determine indices of refraction from the way light rays changed direction.

A classic observation of refraction occurs when a pencil is placed in a glass half filled with water. Do this and observe the shape of the pencil when you look at the pencil sideways, that is, through air, glass, water. Explain your observations. Draw ray diagrams for the situation.

Find the index of refraction for medium 2 in Figure 3a, assuming medium 1 is air and given the incident angle is The index of refraction for air is taken to be 1 in most cases and up to four significant figures, it is 1. This is the index of refraction for water, and Snell could have determined it by measuring the angles and performing this calculation.

He would then have found 1. Today we can verify that the index of refraction is related to the speed of light in a medium by measuring that speed directly. Suppose that in a situation like that in Example 2, light goes from air to diamond and that the incident angle is This means there is a larger change in direction in diamond.

The cause of a large change in direction is a large change in the index of refraction or speed. In general, the larger the change in speed, the greater the effect on the direction of the ray. Figure 6. A ray of light passes from one medium to a third by traveling through a second. Skip to main content. Geometric Optics. Search for:.

Snell's law relates the directions of the wave before and after it crosses the boundary between the two media. Notice that as the wavefronts cross the boundary the wavelength changes, but the frequency remains constant.

In the above animation a spherical wave pulse propagates in a medium where the wave speed is constant in all directions. The wave expands outwards as an ever expanding circle, with the wave traveling at the same speed in all directions.

Since the wave speed is the same everywhere, there is no refraction, and the wave does not change direction as it propagates. In acoustics, however, sound waves usually don't encounter an abrupt change in medium properties. Instead the wave speed changes gradually over a given distance. The first student reached the tape, slowed down, and observed the rest of the students marching ahead at the original speed.

The change in direction of the line of students only occurs at the boundary when the students change speed and approach at an angle. The Marching Soldiers analogy provides an excellent analogy to understanding the cause of light refraction.

The line of students approaching the masking tape are analogous to a wavefront of light. The masking tape is analogous to a boundary between two media. The change in speed that occurred for the line of students would also occur for a wave of light. And like the marching students, a light wave will not undergo refraction if it approaches the boundary in a direction that is perpendicular to it.

The same two conditions that are necessary for bending the path of the line of students are also necessary for bending the direction of a light ray. Light refracts at a boundary because of a change in speed. There is a distinct cause-effect relationship. The change in speed is the cause and the change in direction refraction is the effect. Physics Tutorial. My Cart Subscription Selection.

Student Extras. Then the plane tangent to the boundary is the boundary itself. Reflection at rough, irregular boundaries is diffuse reflection.

The smooth surface of a mirror reflects light specularly, while the rough surface of a wall reflects light diffusely. The reflectivity or reflectance of a surface material is the fraction of energy of the oncoming wave that is reflected by it. The reflectivity of a mirror is close to 1.

Refraction Refraction is the change in direction of propagation of a wave when the wave passes from one medium into another, and changes its speed. Light waves are refracted when crossing the boundary from one transparent medium into another because the speed of light is different in different media.

Assume that light waves encounter the plane surface of a piece of glass after traveling initially through air as shown in the figure to the right. What happens to the waves as they pass into the glass and continue to travel through the glass? The speed of light in glass or water is less than the speed of light in a vacuum or air.