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Light - Other Applications of Light

This GCSE Physics quiz looks at other applications of light. The obvious use of light waves is to enable us to observe things with our eyes, however, light is also used for communications and lasers. The word laser is made up from the initial letters of the words 'Light Amplification by Stimulated Emission of Radiation'. Lasers produce beams of light that are extremely intense and narrow. Laser light is described as being monochromatic and coherent which means that the crests and the troughs of the individual rays of light are perfectly aligned and so the light beam doesn't spread out much and is very intense. Scientists working for the NASA space agency have been able to measure the distance of the Moon extremely accurately by reflecting laser light off special reflectors left behind after the Apollo Moon landings.

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You can probably think of several uses of lasers - laser pointers, light shows and even cat toys; astronomers fire lasers into the sky to create an artificial star that they can use to improve the quality of the images they record. The uses that you are supposed to know about for the GCSE are cutting, cauterising and burning. Industrial lasers are extremely powerful, so powerful in fact that they can be used to cut through many different materials, including metals. The advantages of laser cutting are that the material is easier to hold in place as there is no cutting edge that is biting into it; the laser beam doesn't wear out; the cut is very smooth; very complex shapes can be cut as the laser is computer controlled and one or two others. The main disadvantage is that they don't have the sort of power to be able to cut through very thick materials like steel plate.

Cauterising is a medical term that refers to the use of heat to either remove part of the body or to seal part of it in order to reduce bleeding and to avoid further damage. Doctors and surgeons are beginning to use lasers to remove growths known as skin tags in place of the conventional method involving a heated wire. The heat from the laser beam cuts through the skin tag and seals the blood vessels, reducing the bleeding from this minor operation. You are probably more familiar with laser eye surgery which is used to correct a number of different problems with the eye by re-shaping the cornea. In most people, laser eye surgery means that they no longer have to wear contact lenses or glasses although for a few people, it creates more problems that it solves. Shining any laser, however low powered, has dangers, even in the hands of doctors carrying out eye surgery with them.

Visible light can be sent along glass fibres. This is because of total internal reflection (TIR). TIR occurs when light inside an object hits the surface at an angle that is greater than the critical angle. At angles less than this, light only partially reflects and the rest refracts at the surface, leaving the medium. Glass fibres are long and thin so light rays already inside the fibre will always hit the outside surfaces at angles greater than the critical angle and will remain inside the fibre. It is only at the end of the fibres is it possible for light to leave.

The critical angle depends on the refractive index of the transparent material so it will be different for glass and perspex. The refractive index is equal to the reciprocal of the sine of the critical angle.

As the name suggests, total internal reflection means that all the light is reflected inside the glass fibre, so it is a very efficient method of moving light from one place to another. It is used to transfer digital signals in high speed broadband and in endoscopes (devices used to look inside of body cavities).

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  1. Which formula correctly states the relationship between refractive index and critical angle?
    Different transparent media have different refractive indices
  2. What is the definition for critical angle in optics?
    Total internal reflection is the reason that glass fibres can be used to transmit light efficiently from one place to another
  3. Which of the following are examples of optical fibres?
    Optical fibres have found their way into most parts of our modern day lifestyles now. They are preferred to more conventional methods as they are more efficient and can be made smaller and more cheaply
  4. What is a laser?
    Coherent means that all the lightwaves are 'in phase' (all their crests and troughs are lined up). Monochromatic means that the light waves in a laser beam all have the same wavelength
  5. Which of the following is NOT an application of total internal reflection?
    Telescopes rely on light passing through a series of lenses or reflection off a curved mirror to bring it to a focus
  6. Optical fibres can be used to transmit which of the following?
    An optical fibre is a bundle of glass fibres of high quality. Optical fibres are very flexible
  7. Calculate the refractive index of a medium which has a critical angle of 52o.
    sin c = 0.788, 1/0.788 = 1.27. Refractive index has no units and is always quoted as just a number
  8. Lasers can be used for a variety of applications. Which of the following are lasers useful for?
    The strength of the laser should be considered when dealing with each situation, as some may require stronger lasers than others
  9. What is the critical angle of a medium if its refractive index is 1.5?
    Rearrangement of the refractive index equation
  10. What is the normal range of sight for a human eye?
    As you get older, the near point (the closest point to your eye that is in sharp focus) usually increases which is why you may see older people holding things further away to read what is written

Author: Martin Moore

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