Four Chapter 11

Applications of Curved Mirrors

• Parabolic mirrors are used in large telescopes:

Newtonian focus telescope

Cassegrain focus telescope

• Concave reflectors are used in solar heaters and furnaces.
  • Substances to be heated are placed at the focal point of the reflector

Refraction

• Light refracts as do all waves when they pass from one medium to another with different wave speed.
• The law of refraction applies and the direction of refraction is as previously learned.
• Partial reflection and partial refraction – The phenomena whereby when a wave passes from one medium to another, some portion of the wave is refracted, yet some portion is reflected by the boundary between media.
• Light shows partial reflection and refraction
• Remember that optically dense media have
  • Lower wave speed
  • Higher absolute index of refraction.
• The law of refraction is sometimes called “Snell’s law” since Snell was the first person to prove the relationship.

• n₁₂ = the relative index of refraction for a light passing from medium 1 into medium 2 and it equals a constant.

Total Internal Reflection

• When light travels from an optically dense medium to a less dense medium, speed increases, and refraction is away from the normal.
• As with all other waves, when the angle of incidence is increased, the angle of refraction increases as well. This means that for this circumstance eventually a critical angle is reached when the angle of refraction = 90^o.
• Therefore, total internal reflection occurs with light as with other waves.
• You should review what you have already learned about total internal reflection.

Applications of Total Internal Reflection

• Total internal reflection reflects 100% of the light. Mirrors do not reflect 100% of the light.
• In binoculars and cameras, by using prisms instead of mirrors, brighter pictures or images occur.
  • In binoculars, prisms are used to shorten the length of the binoculars’ by folding the path of the light.
• Fiber optics – A thin fibre of glass or plastic that ensures that the angle of incidence on the inside surface of the fibre is always greater than the critical angle.
• Light pipe – The fibre in fibre optics.
• Fiber optics is being used in telecommunications.
  • Laser light intensity is modulated by telephone signals.
  • The pulse of laser light can travel the length of the fiber.
  • Advantages include
    • More signals are possible
    • Communication in both ways on the fiber
    • Less loss of energy
• Total internal reflection is also used in medicine.
  • Fiber optic bundles transmit pictures of internal organs without the need for major surgery.

Applications of Refraction

• Ponds appear shallower than they really are due to refraction. As the light leaves the pond, it refracts away from the normal. Our eyes do not perceive this bending of the light, and so the pond looks more shallow than it is.

• The sun is visible for about 5 minutes before true sunrise and for about 5 minutes after true sunset because of refraction. Light from the sun enters the atmosphere which is optically more dense than the vacuum of space. As a result, light bend down towards the Earth's surface. As a result, even though the sun is below the horizon, we can still see it.

• When the sun is close to the horizon, light from the lower portion is refracted more than light from the upper portion. This gives the sun a flattened appearance.
• The large size of the moon and sun when near the horizon is not because of refraction, but because we compare the size of them to objects visible to us on Earth. This fools our brain into thinking they are larger than they really are.
• Stars twinkle because of the fact that the atmosphere is in motion. This causes different density of air in different spots. Sometimes the light refracts towards us because of the non uniform density, and we see the star. At other times it refracts away from us and the star blinks out.
  • Telescopes are placed on mountain tops and in orbit to reduce or escape this effect.
• Schlieren effect is caused due to refraction of light through the hot non-uniformly dense air above hot surfaces. This produces the wavy pattern that one sees for example, over hot pavement in parking lots.
• Mirages form because light approaching the Earth enters progressively warmer and warmer and therefore, less optically dense air. Therefore refraction is away from the normal and away from the normal etc. until we see an image of the sky or the object forming the mirage.

• Notice how the angle of incidence gets larger and larger. Eventually the angle of incidence is greater than the critical angle and reflection occurs to give the image of the sky or object as well.
• Refraction is used in eye glasses to correct vision.
• Total internal reflection (a refraction effect) is used in
  • Periscopes (instead of mirrors to give a brighter image)
  • Binoculars (as previously stated)
  • Projectors (to make the image erect)
  • Diamonds (to make them sparkle).

• Diamonds have an index of refraction of 2.42 and a critical angle of 24.5^o.
  • Light entering the diamond from the top either enters the diamond or reflects.
  • The bottom of the diamond is cut so that light that enters the diamond, strikes the bottom with an angle of incidence greater than 24.5^o. Therefore, it reflects back out the top.
  • Some light will leave the bottom, but the better the cut, the smaller the amount leaving the bottom.

Dispersion

• Dispersion –The splitting of light into its colors.
• To split light into its different colors, requires a dispersive medium.
• In dispersive media, different colors (frequencies) of light have different indexes of refraction due to different speeds.
• Dispersion created a problem of unusual coloration in Newton’s and Lippershey’s first telescope.
• In trying to solve the problem, Newton investigated dispersion.

• All colors of light have the same angle of incidence in Newton's experiment, but they have different indexes of refraction in the prism (n₂). The law of refraction tells us:

• Rearranging this expression gives:

• From this equation, we see that those colors which have a larger index of refraction in the prism, will refract through a smaller angle since n₁ and i are the same for all colors.
• From his observations, Newton concluded that white light was made up of many colors.
• Spectrum – The band of colors produced when light is dispersed.
• Note that red refracts the least and violet the most.
• Recomposition – The bringof the colors back together to produce white light.
  • can be achieved by:
    • lenses
    • two prisms
    • rotating wheels with the colors painted on them.
• Sunlight passing through a prism produces a spectrum
  • Beyond the red and blue ends, light that is not visible is present.
  • As evidence of this, thermometers placed in these invisible regions rise in temperature.
• Infrared – light beyond the red end of the spectrum.
  • More than 50% of sunlight is infrared.
  • Penetrates clouds, smoke, and haze.
  • Can be used for photographs
  • Heat photographs of the human body can help detect cancer.
  • Used to keep food warm in restaurants.
  • Used to dry paint.
• Ultraviolet light – light beyond the blue end of the spectrum
  • discovered because some objects are fluorescent when placed in this region.
  • Causes tanning and sun burns.
  • Accelerates the manufacture of vitamin D in the skin.
  • Most of this light from the sun is absorbed by the atmosphere.

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November 19, 2013