BBC Bitesize - Higher Physics - Refraction of light - Revision 3
For example, this happens when light passes from water to air or from glass to water. Angle of incidence and angle of refraction through water and air. When light travels through something else, such as glass, diamond, n, which is defined as the ratio of the speed of light in vacuum to the speed of light in the medium: The index of refraction can also be stated in terms of wavelength: second medium is related to the angle of incidence by Snell's law. angle of incidence i in degrees angle of refraction r in degrees. Table 2 Refraction by a glass block; angles of incidence and.
Typical values for the index of refraction of glass are between 1.
The distance between wave fronts will therefore be shorter in the glass than in air, since the waves travel a smaller distance per period T. Now consider wave fronts and their corresponding light rays approaching the surface at an angle. We can see that the rays will bend as the wave passes from air to glass. The bending occurs because the wave fronts do not travel as far in one cycle in the glass as they do in air.
As the diagram shows, the wave front halfway into the glass travels a smaller distance in glass than it does in air, causing it to bend in the middle.
Refraction of light
Thus, the ray, which is perpendicular to the wave front, also bends. The situation is like a marching band marching onto a muddy field at an angle to the edge of the field. The rows bend as the speed of the marchers is reduced by the mud.
The amount of bending depends on the angle of incidence and on the indices of refraction of glass and air, which determine the change in speed. This is Snell's law, or the law of refraction. When light passes from one transparent medium to another, the rays are bent toward the surface normal if the speed of light is smaller in the second medium than in the first.
The rays are bent away from this normal if the speed of light in the second medium is greater than in the first. The picture on the right shows a light wave incident on a slab of glass.
Waves Tutorial 6
One part of the wave is reflected, and another part is refracted as it passes into the glass. The rays are bent towards the normal.
At the second interface from glass into air the light passing into the air is refracted again. The rays are now bent away from the normal. Reflectance and transmittance At a boundary between two transparent media, light is partially reflected and partially refracted.
If the light hits the interface at any angle larger than this critical angle, it will not pass through to the second medium at all.
Instead, all of it will be reflected back into the first medium, a process known as total internal reflection. For any angle of incidence larger than the critical angle, Snell's law will not be able to be solved for the angle of refraction, because it will show that the refracted angle has a sine larger than 1, which is not possible.
Reflection and Refraction
In that case all the light is totally reflected off the interface, obeying the law of reflection. Optical fibers are based entirely on this principle of total internal reflection.
An optical fiber is a flexible strand of glass. A fiber optic cable is usually made up of many of these strands, each carrying a signal made up of pulses of laser light. The light travels along the optical fiber, reflecting off the walls of the fiber. With a straight or smoothly bending fiber, the light will hit the wall at an angle higher than the critical angle and will all be reflected back into the fiber.
Even though the light undergoes a large number of reflections when traveling along a fiber, no light is lost. Depth perception Because light is refracted at interfaces, objects you see across an interface appear to be shifted relative to where they really are. If you look straight down at an object at the bottom of a glass of water, for example, it looks closer to you than it really is.
Looking perpendicular to an interface, the apparent depth is related to the actual depth by: An example A beam of light travels from water into a piece of diamond in the shape of a triangle, as shown in the diagram.
Step-by-step, follow the beam until it emerges from the piece of diamond. The speed can be calculated from the index of refraction: A diagram helps for this. In fact, let's look at the complete diagram of the whole path, and use this for the rest of the questions.
The angle we need can be found from Snell's law: What isthe angle between the normal and the beam of light inside the diamond at the air-diamond interface?