Refraction

If a ray of light goes from a rare substance such as air into a denser medium, such as glass it will be bent towards the normal to the surface of seperation;if from a denser to a rarer material, it will bend away from the normal. The amount of the deflection will not be constant but will change with the inclination of the incoming ray. Fig 4

FIGURE 4 Credits: Freeman Ira

shows a number of cases of refraction of a ray by glass.

Many common observations are connected with refraction. A spoon standing in a cup of water seems to be bent where it passes through the surface (Fig 6a). This is because any underwater object is seen by light reflected by it and coming up through the surface. Rays coming from any point are refracted as in Fig 6b and so appear to come from another point which lies closer to the surface. The result is that the immersed part of the object seems to be bent upward.

Light that passes completely through a parallel- sided piece of glass is refracted, the rays being bent equal but opposite amounts at each

FIGURE 6 Credits: Freeman Ira

surface-towards the normal in one case and away in another. As a result the beam that comes through is parallel to the original one but is displaced to 1 side(Fig 6c)

The observed direction of a refracted ray can be accounted for by the wave theory of light. Think of a parallel beam of light coming through air and striking a flat glass surface, as in Fig 3. AB is a plane wave front that is just about to enter the glass at the point A, while CD is a wave front that has just passed completely in. During the time that the light waves travel a distance d_a in the air, they evidently travel a distance d_g in the glass, so we can conclude

V_a /V_g =d_a /d_g

Where V_g is the speed of light in glass; V_a is the speed in air, which we can take to be practically the same as c, the vacuum speed, the difference being unimportant for most purposes. The ratio of the speed of light in a vacuum to the speed in a given material is called the index of refraction of the material. This quantity, represented by the symbol n, determines the extent of the bending of the rays. Thus the law of refraction may be considered to give the change in

FIGURE 7 Credits: Freeman Ira

direction of a ray in terms of the value of n and the construction of Fig 7. It states also that the incident ray, the normal to the surface and the refracted ray all lie in one plane.