The properties of light include:
CRITICAL ANGLE & TOTAL INTERNAL REFLECTION
When light is transmitted through a semi-circular Perspex block and the angle of incidence is refracted, the Angle of refraction is less than 90o.
When light is transmitted through a semi-circular Perspex block and the refracted ray travels along the straight edge of the Perspex block, the angle of incidence is called the critical angle The Angle of refraction is 90 o .
When light is transmitted through a semi-circular Perspex block at a greater angle than the critical angle, then the ray is total internally reflected.
TOTAL INTERNAL REFLECTION
When light is travelling from a more dense medium to a less dense medium, there is an angle of incidence, called the critical angle, for which the refracted angle is 90 o. If the angle of incidence is greater than the critical angle, the light ray is totally internally reflected, and then the incident angle equals the reflected angle. Total internal reflection occurs only at a boundary where light goes from a more dense medium to a less dense medium; e.g from glass to air or from water to air.
Word Bank: air(x2) less reflected greater more 90 internally critical
OPTICAL FIBRES & ENDOSCOPES
The following diagram shows a light ray moving through a section of optical fibre.
Name the process that allows the ray of light to be transmitted through the fibre. Total internally reflection
Use the information on page 35 in Science World 9 to explain what an endoscope is and how it works.
The endoscope has a long flexible tube containing optical fibres that is inserted through the patient’s mouth and oesophagus. The endoscope contains bundles of optical fibres, each one about 10 micrometres in diameter. The optical fibres act like a flexible torch. Light shines through one end of the optical fibres in the endoscope and it comes out the other end, no matter how much the tube is twisted or bent.
Use the information on page 272 in Science World 9 to explain how optical fibres are used in communications.
The optical fibres used in the telephone network are very thin, pure glass fibres. Each fibre consists of a glass core, a glass cladding and a protective outer jacket, and the whole fibre is thinner than a human hair.
At telephone exchanges the electrical signals from local telephones are converted to pulses of laser light. These laser pulses are narrow high-intensity light beams of a single colour
(wavelength). They are digital (on or off) and they travel through the optical fibre by total internal reflection. That is, the light can travel around bends and even loops by reflecting off the inside surface of the inner glass core. One optical fibre can carry up to 2 billion pulses of light per second. DISPERSION
Read and then use the information on page 39 & 51 of Science World 9 textbook to complete the following questions.
1. Identify the spectrum of colours (ROYGBIV).
2. Define dispersion:
Below is a labelled diagram to show how white light is dispersed through a prism.
3. Explain why violet light appears at the bottom of the spectrum and red light at the top.