Refraction

Refraction

Refraction of LightLight travels at 3.0 x 108 m/s

in a vacuumWhen light hits a medium,

its speed and direction change

The incident ray is split: it reflects and refracts

Refraction

• Refraction is the bending or changing direction of light as it travels from one medium into another.

REFRACTION OF LIGHTThe Refracted Ray: The ray that is bent upon entering a second medium

Angle of Refraction: The angle between the normal and a refracted ray

Note: Reflection is still occurring!

• Light bends because it changes speed when it moves between materials with different densities.

• Light travels more slowly in thicker/denser material.

• The bending of light makes an object’s image appear to be in a different position from where the object really is.

The Direction of Bending

• Refraction is the bending of the path of a light wave as it passes from one material into another material.

• The refraction occurs at the boundary and is caused by a change in the speed of the light wave upon crossing the boundary.

• Light travels more slowly in thicker/denser material.

• The bending of light makes an object’s image appear to be in a different position from where the object really is.

• Refraction only takes place at the boundary.

“Fast to Slow” “Slow to Fast”When light travels from a

medium in which its speed is fast into a medium in which its speed is slow:The refracted ray bends

towards the normal

When light travels from a medium in which its speed is slow into a medium in which its speed is fast:The refracted ray bends

away from the normal

As you look through the side of the glass at the portion of the pencil located above the water's surface, light travels directly from the pencil to your eye. Since this light does not change medium, it will not refract.

As you look at the portion of the pencil which was submerged in the water, light travels from water to air. This light ray changes medium and subsequently undergoes refraction. As a result, the image of the pencil appears to be broken. The portion of the pencil which is submerged in water also appears to be wider than the portion of the pencil which is not submerged.

The Speed of Light

• The speed of light has been measured in different media since 1862.

• Speed of light (c) in a vacuum is 3.0 x 108 m/s. Use this value as the speed of light in air as the measurement is very close.

• c in water is 2.26 x 108 m/s.

• c in acrylic is 1.76 x 108 m/s.

The Index of Refraction

• The index of refraction for a medium is the ratio of the speed of light in a vacuum to the speed of light in the substance.

Index of refraction

Speed of light in medium

Speed of light in vacuum

Because c and v are measured in m/s, the units cancel out, leaving n as a dimensionless quantity.

Sample Calculation

• The speed of light in olive oil is 2.027 x 108 m/s. Calculate the index of refraction for olive oil.

• Recall c = 3.00 x 108 m/s• n = c/v

»n = 3.00 x 108 m/s ÷ 2.027 x 108 m/s» n = 1.48

Example 2

• The speed of light in sodium chloride is 1.96 x 108 m/s. Calculate the index of refraction for sodium chloride.

• Recall c = 3.00 x 108 m/s• n = c/v

»n = 3.00 x 108 m/s ÷ 1.96 x 108 m/s» n = 1.53

Rearrange the Formula Example

• Calculate the speed of light in vegetable oil. The n for vegetable oil is 1.47.

• Recall c = 3.00 x 108 m/s• n = c/v• v = c/n

»v = 3.00 x 108 m/s ÷ 1.47» v =2.04 x 108 m/s

Another example

• The index of refraction for diamond is 2.147. What speed does light travel through it?

• Recall c = 3.00 x 108 m/s• n = c/v• v = c/n

»v = 3.00 x 108 m/s ÷ 2.147» v = 1.40 x 108 m/s

• The same index of refraction can be calculated using the sines of the angles:

• n = sin <i / sin < R

Angle of incidence

Angle of refraction

Sample Calculation

• Calculate the index of refraction for glass and for water, based on the diagram.

Glass• n = sin < i/sin < R• n = sin 60/sin 34.5• n = 0.8666÷0.5664• 1.529

Water• n = sin < i/sin < R• n = sin 60/sin 40.6• n = 0.8666÷0.6508• 1.331

Angles of Incidence and Refraction

• When light travels from one medium into another, some light is refracted and some light is reflected.

• As light enters a more dense medium, it slows down and bends towards the normal.

• As light enters a medium that is less dense, it speeds up and bends away from the normal.

• The angle of refraction is larger than the angle of incidence.

• The angle of refraction will continue to increase as the angle of incidence increases.

The Critical Angle• Eventually the angle of refraction will become

90o. The angle of incidence at this point is called the critical angle.

• The critical angle is the angle of incidence that will produce a refracted angle of 90o.

Total Internal Reflection

• If you increase the angle of incidence past the critical angle, the refracted ray will no longer exist. Instead it will be reflected back into the medium.

Beyond the critical angle, light will be reflected back into the denser medium (water, in our example)

This is called total internal reflection

• Total internal reflection occurs when:• Light is travelling more slowly in the first medium that

in the second medium.• The angle of incidence is large enough that no

refraction occurs in the second medium. Instead, the ray is reflected back into the first medium.

Fibre Optics

• Fibre Optics is a technology that uses light to transmit information along a glass cable.

• Light must not escape as it travels along the cable. To achieve this, the cable must have a small critical angle (light entering it will have a larger incident angle).

• Triangular prisms exhibit total internal reflection. By changing the orientation of the prism, you can change the direction of the emergent ray by 90 or 180 degrees.