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Transcript
Refraction: from air to glass These lines are the crests of waves or WAVEFRONTS: the distance between successive wave fronts is called the WAVELENGTH Glass block Refraction A RAY is a line drawn at right angles to the wave fronts: the arrow indicates direction Here, a ray is in air travelling at the speed of light: it strikes a glass block AIR – a less dense medium GLASS – a more dense medium Refraction A ray striking a new substance or This is called a ‘normal’ medium is called the INCIDENT RAY The angle it makes with a ‘normal’ at the surface is called the angle of incidence GLASS – a more dense medium Refraction A ray entering a new substance or medium is called the REFRACTED RAY The angle it makes with a ‘normal’ at the surface is called the angle of REFRACTION As the ray enters the glass, the wavelength SHORTENS but the FREQUENCY stays the same so the ray slows down GLASS – a more dense medium Refraction The refracted ray has bent TOWARDS the normal inside the dense medium GLASS – a more dense medium Refraction The ray reaches the boundary with air again: as it enters air it speeds up and the wavelength increases: it bends AWAY FROM the normal as it enters the less dense medium AIR – a less dense medium The final direction is parallel to the original direction and the ray returns to the speed of light Refraction and Internal Reflection This ray reaches the glass and refracts towards the normal. The ray within the block hits the side at a large angle of incidence: if this angle is large enough, the entire ray reflects and stays inside the block This is TOTAL INTERNAL REFLECTION This happens inside an optical fibre The ray moves through the glass at a slower speed as the wavelength shortens If the ray reaches the far end of the block, it refracts AWAY from the normal as it leaves the block Refraction and Internal Reflection The ray moves through the glass at a slower speed as the wavelength shortens If the ray reaches the far end of the block, it refracts AWAY from the normal as it leaves the block Refraction and Internal Reflection Semi-circular glass block normal Air The ray leaves the glass and refracts away from the normal Glass Air The ray enters the glass block at right angles to the surface so it goes straight in and slows down Some of the light is reflected inside the block. This is called INTERNAL REFLECTION Refraction and Internal Reflection This ray leaves the glass and refracts further away from the normal at 900 Air Glass Air The ray enters the glass block at right angles to the surface so it goes straight in and slows down This angle of incidence inside the block is called the CRITICAL ANGLE Refraction and Internal Reflection Air Glass NO PART of the ray leaves the glass Air The ray enters the glass block at right angles to the surface so it goes straight in and slows down It reflects TOTALLY INSIDE the block This angle of incidence inside the block is greater than the CRITICAL ANGLE Refraction and Internal Reflection This happens in an optical fibre Air Glass Internal Reflection This happens in an optical fibre Air Glass fibre (solid glass) Optical fibres are used to carry information as pulses of infra-red laser light: the pulses are ON or OFF (digital) They are also used for internal medical examinations using an ENDOSCOPE Refractive Index • 1. Set up a semi circular block as in Fig.2 p.212 Nelson Thornes. Draw around the block. Draw the normal. • 2. Vary the angle of i and measure the angle r. Do for three angles of i. Draw on your rays, angle i and r. Calculate sin i and sin r, then sin i/sin r. Calculate the average value to find the refractive index n of this glass. • n = sin i / sin r Critical Angle & Total Internal Reflection 3. Set up your glass block as in Fig. 3 page 214. Draw around it and draw on the normal. Find the angle at which total internal reflection occurs c. n = 1/sin c OR sin c = 1/n Does this agree with your previous value of n? The Endoscope Describe how the endoscope works. Describe how laser light can be used in endoscopy. What safety precautions should be used when dealing with laser light?
