1 Light Reflection of light Reflection is when light bounces off an object. If the surface is smooth and shiny, like glass, water or polished metal, the light will reflect at the same angle as it hit the surface. The angle of incidence equals the angle of reflection. Types of reflection Smooth reflection is when light hits an object and reflects in lots of different directions. This happens when the surface is rough. Most of the things we see are because light from a source has reflected off it. The angle at which light hits a reflecting surface is called the angle of incidence, and the angle at which light bounces off a reflecting surface is called the angle of reflection.
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Light Reflection of light
Reflection is when light bounces off an object. If the surface is smooth and shiny, like glass, water or polished metal, the light will reflect at the same angle as it hit the surface. The angle of incidence equals the angle of reflection.
Types of reflection
Smooth reflection is when light hits an object and reflects in lots of different directions. This happens when the surface is rough. Most of the things we see are because light from a source has reflected off it.
The angle at which light hits a reflecting surface is called the angle of incidence, and the angle at which light bounces off a reflecting surface is called the angle of reflection.
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Mirrors
Concave mirrors
Light rays travel towards the mirror in a straight line and are reflected inwards to meet at a point called the focal point.
Concave mirrors are useful for make-up mirrors because they can make things seem larger. This concave shape is also useful for car headlights and satellite dishes.
Convex mirrors
Convex mirrors curve outwards, like the outside of a balloon.
Convex mirrors are useful for shop security and rear-view mirrors on vehicles because they give a wider field of vision but will make things appear smaller.
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Questions
1. Which one of the mirrors is concave?
2. Which one of the mirrors is convex?
3. Draw a Convex shaped mirror.
4. Draw a Concave shaped mirror.
5. Which mirror brings the light to a single point or focus?
6. Which shape is used in satellite dishes to concentrate the signal strength?
7. Which mirror is used for security because it has a wider field of vision?
8. Indicate the incident ray, the reflected ray, the angle of incidence, the angle of reflection, the normal in the diagram below.
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Refraction
Refraction is caused by the difference in speed of the wave as it goes between 2 different mediums. Remember the speed of light is constant 3x 108 m/s. But it will slow down in a material which is more optically dense.
Optically dense materials are diamond, prisms, glass. That’s why a diamond sparkles the light is captured and reflected inside the diamonds surfaces.
The Law of refraction.
When a ray of light passes from one medium to a more optically dense material the ray bends towards the normal.
When a ray of light passes from one medium to a less optically dense material it bends away from the normal.
In the space below
draw the normal
draw the light going into the more optically dense glass
draw the light coming out of the glass.
air
glass
It should look like the diagram below.
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Questions
1
2
3
6
4
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Continue line B On the diagram above and place where the stone appears to be.
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Refractive index. The refractive index is a measure of how much the light (or wave) is bent. It is also a measure of how much the light slows down as it passes into a more optically dense medium.
Refractive index of water = speed of light in vacuum / speed of light in water
material Speed of light m/s Refractive index Vacuum 2.998 x 108 1.0000 exactly Air 2.997 x 108 1.0003 Water 2.3 x 108 Perspex 2.0 x 108 Glass 1.8 to 2.0 x 108 diamond 1.25 x 108
Use a calculator to calculate the refractive index of water, perspex, glass, diamond. (to 1 decimal place 1.3, 1.5, 1.7, 2.4)
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Worked example 1
Refractive index = sin i / sin r
Calculate refractive index.
Measure angle of incidence. =
Sin (i ) =
Measure angle of refraction =
Sin (r) =
Refractive index = sin i / sin r
=
Glass
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Worked example 2
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Refractive index = sin i / sin r
Calculate refractive index.
Measure angle of incidence. =
Sin (i ) =
Measure angle of refraction =
Sin (r) =
Refractive index = sin i / sin r speed of light in perspex =
=
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Total Internal Reflection
In the diagram below you can see 2 rays of light coming out of the block of plastic. One is reflected and one is refracted.
Refracted ray
Reflected ray Ray in (incident ray)
We see this every day when we try to look into a river or the see. We see instead the light being reflected by the surface of the water. That’s why we have to wear glasses to cut out bright reflected sunlight and only let the light in which is refracted.
Critical Angle The critical angle is the angle of incidence which results in the angle of refraction = 90 o .
So none of the light is allowed to escape. At angles greater than the critical angle all of the light is totally reflected back into the block. This is known as total internal reflection.
A B C D
The critical angle depends on the material, water 49o glass 42o, diamond 25o so light entering diamonds gets bounced around before they exit making them sparkle.
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Use of Total Internal Reflection
Fibre optics carry light using TIR.
Light bounces along inside the fibre optic. Each time it strikes the inside of the fibre the angle of incidence is greater than the critical angle so no light gets lost. The fibre can be curved and the glass is very pure so none of the energy in the light wave is absorbed and lost.
Fibre optics use Total Internal Reflection. Light travels along the fibre optic carrying data. Without this technology the internet would not be possible. The reflection is total and the glass is pure. The light can travel many kilometeres without data loss.
Fibre optics are used in medicine. An endoscope is used to see inside a patient’s body without surgery. The endoscope has a fibre to carry light down and may have have a cutting tool built in to do small operations without surgery.
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Questions.
1. What is TIR?
2. Why do diamonds sparkle?
3. Diamonds have a small or large critical angle?
4. Data travels along fibre optics at the speed of light 3 x 108 m/s. true or false?
5. Which industries use TIR?
6. What do you call an instrument which uses fibre optics to see inside a patient and perform minor surgery?
7. Sketch a diagram how light travels down a fibre optic.
8. The critical angle of water is 49o. If a ray of light hits the upper surface of the pond at an angle of incidence of 45o. What will happen?
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