Refraction. The Optical Density of a Medium The better a medium transmits light, the lower its optical density. The slower light is transmitted by a medium,

Refraction

The Optical Density of a Medium

• The better a medium transmits light, the lower its optical density.

• The slower light is transmitted by a medium, the higher its optical density is. • H

Law of Refraction

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1. When a ray of light travels obliquely from a medium of lesser optical density into a medium of greater optical density, it will bend towards the normal.

2. When a ray of light travels obliquely from a medium of greater optical density into a medium of lesser optical density, it will bend away from the normal.

Law of Refraction

3. The incident and refracted rays will be in the same plane.

Index of Refraction

• The refractive index (or index of refraction) of a medium is a measure of how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical soda-lime glass has a refractive index close to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed of light in a vacuum. The formula for the index of refraction is:

•N = vvacuum / vmedium

• The index of refraction has no units

The Speed of Light

• The speed of light in a vacuum is designated by the small letter, c.• Light has a speed of 2.9979 x 108 m/s in a vacuum. • Light has a speed of 2.9970 x 108 m/s in air.

• The index of refraction for light in air is 1.0003

• Unless special accuracty is required, the speed of light in a vacuum or in air is normally given as 3.0 x 108 m/s

Snell’s Law

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Snell’s Law Derivation

Objects appear shallower in water due to refraction

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The Critical Angle• The critical angle is the minimum angle at which light

begins to internally reflect as it passes from a medium of higher refractive index into a medium of lower refractive index.

The Critical Angle• Going from a medium of higher to lower refractive index,

when the incident angle passes a certain minimum critical angle it will reflect back at the medium boundary.

Calculating the Critical Angle• Using Snell’s Law, the Critical Angle can be calculated if

the angle of refraction is taken to be 90 degrees (In experiments, a refraction of 90 degrees and internal reflection happen at the same time). In Snell’s Law Formula, the sine of 90 degrees is 1 which allows for the Critical Angle (the angle of incidence) to be calculated, given the indices of refraction of the two media.

Applications of the Critical Angle

• Since the critical angle for glass is about 42 degrees (depending on the kind of glass), an incident angle of 45 degrees (going from greater to lesser refractive index media) causes total internal reflection. Prisms are used to reflect light in many optical devices.

Applications of the Critical Angle• Prisms can reflect light at various angles as long as the

incident angle is over the critical angle.

Applications of the Critical Angle• Periscopes can be made with mirrors or prisms. Since

mirrors absorb some of the light they reflect, passing on less than 100 percent, they are not as desirable as prisms which reflect 100 percent.

Applications of the Critical Angle• Binoculars use various arrangements of prisms to shorten

the length of the optical system.

Total Internal Reflection as Experienced in Water• Aquatic organisms see many internal reflections when they

look upward at the water’s surface.

Why Do Diamonds Sparkle So Much?

• Diamond has a high refractive index which means it has a lower critical angle also. Diamonds are specially cut to produce as much internal reflections as possible. Many internal reflections redirect light rays out of the top as flashes of light to the eye.

Applications of the Critical Angle• Total Internal Reflection means that light can be sent

inside fibers, always relecting since the incident angle is over the critical angle.

Total Internal Reflection

• In the picture below, a laser is off-camera to the right, shining light into the plastic tube. This light can be “piped” in circles and around a corner until it comes out towards your eye. All along the way, light reflects internally since it is always at angles greater than the critical angle.

Total Internal Reflection

• Light can be piped into hard to illuminate areas like places in the human body and lenses to take pictures can likewise be inserted in hard to get at places.

Fiber Optics

• Light-conducting fibers are referred to as fiber optics.• Fiber optic systems can carry signals faster than electricity

and do not lose energy like electrical system do. (Electrical systems have internal resistance in wires which generates energy loss as heat)

• Fiber optic cables can carry many more messages at a much faster rate than conventional electric systems.

Refraction Effects in Nature

• As light enters earth’s atmosphere it is refracted. This causes the eye to form images of objects which are not really where they appear to be.

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The Formation of Mirages

• The hot air above the sand causes a refraction of light rays from the sky. Thus what looks like blue water is actually refracted rays from the blue sky. (See next slide)

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Shimmer Effects

• Hot air rising and mixing through cooler air (convection currents) causes vibrating refraction of light. This produces a shimmer effect or at night, a twinkling effect when viewing stars.

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