REFRACTION

Refraction is the bending of a ray of light when it goes from one medium to another.

TERMS

When Light travels from ...      (MIX)
When light travels from a rarer to a denser medium it is refracted towards the normal.
When light travels from a denser to a rarer medium it is refracted away from the normal.

The Laws of Refraction of Light     (MIX)
1st Law: The incident ray, the normal ray at the point of incidence and the refracted ray all lie in the same plane.
2nd Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.
i.e. , where n is a constant

NOTE

The second law of refraction is also called Snell's law.

Refractive Index of a Medium      (MIX)
The refractive index of a medium is the ratio of the sine of the angle of incidence to the sine of the angle of refraction
when light travels from a vacuum into that medium.

The Refractive Index Between Two Media      (MIX)
The refractive index between two media is the ratio of the sine of the angle of incidence to the sine of the angle of refraction
when light travels from one of those media into the other.

From the diagrams above we can see that .

MANDATORY EXPERIMENT: To verify Snell's law.

Apparatus ray box, semi-circular glass block, protractor. Method Record i, the angle of incidence. Record r, the angle of refraction. Repeat for different values of i. Graph sin i against sin r. A straight line through the origin verifies Snell's law. The slope of the graph gives a value for the refractive index.

REAL DEPTH AND APPARENT DEPTH

The part of the ruler viewed through the glass appears to be closer than the rest of the ruler.

When viewed from above:

MANDATORY EXPERIMENT:
To measure the refractive index of a liquid.

Record the real depth, R.D. Record the apparent depth, A.D. Use the formula to calculate a value for n, the refractive index of the liquid. Repeat for different sizes of container. Take an average of your results.

REFRACTIVE INDEX IN TERMS OF RELATIVE SPEEDS.

When light travels from medium 1 to medium 2, the relative refractive index is equal to the ratio of the speeds of light in the two media.

Since light travels just about as fast in air as it does in a vacuum:

TOTAL INTERNAL REFLECTION

Critical Angle     (MIX)
When light travels from a denser to a rarer medium
the angle of incidence whose corresponding angle of refraction is 90°
is called the critical angle (C) for those two media.

Total Internal Reflection     (MIX)
When light going from a denser to a rarer medium
strikes the second medium with an angle of incidence greater than the critical angle,
it does not enter the second medium.
It is reflected back into the denser medium. This is called total internal reflection.

Glass is denser than air. As the ray leaves the glass block it refracts away from the normal. If the angle of incidence is greater than the critical angle, total internal reflection occurs.

If n is the refractive index of a medium, and C is the critical angle then

OPTICAL FIBRES

Optical Fibre     (MIX)
A very thin transparent rod (usually of glass) through which light can travel
by total internal reflection is called an optical fibre.

Advantages of Using Optical Fibres in Telecommunications:

• They have a much higher bandwidth
• Energy losses are much less
• They are much harder to tap information from.

Disadvantages

• They cannot be joined together (spliced) as easily as copper wires.
• They are more expensive than copper wires (but the extra bandwidth compensates for this)

A MIRAGE

On a hot summer's day the road ahead may appear to be wet. The air just above the road is hot. Layers of air higher up are cooler. This causes continual refraction. Total internal reflection may occur. The observer sees an image of the sky, which looks like a wet patch.