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Chapter 3

Refraction of LightChapter 3

Refraction• Refraction is the bending of light

as it passes from one medium to another medium.

• It bends because the light slows down as it enters the

new medium.

Table of Observations# Activity

1 Invisible coin

2 Broken pencil

3 Glass block

4 From air to water

5 From water to air

6 Dispersion of light

7 Light ray thru’ glass

Observations

1.1. Coin disappears when Coin disappears when viewed from sideviewed from side

2.2. Pencil appears to bendPencil appears to bend

3.3. Coin disappears when Coin disappears when viewed from sideviewed from side

4.4. Light bends as it enters Light bends as it enters containercontainer

5.5. Light bounces off waterLight bounces off water

6.6. Light spreads out into Light spreads out into colours ROYGBVcolours ROYGBV

7.7. Light bends twice as it Light bends twice as it enters and exits the glassenters and exits the glass

Why is the Sky Red at Sunset?• Dust particles in the air scatter light from

the sun and the sky.• Blue light is scattered more than red.• When the sun is high in the sky, it looks

yellow-white because the light travels vertically through a fairly clear and thin atmosphere, so there is little refraction.

• At dusk, the sun’s rays travel horizontal and through more dust filled air.

• More of the blue light is scattered so that only the reddish rays hit our eyes.

Dispersion• Dispersion is the spreading out of

light into its component colours ROYGBV.

• It does this because the different colour light rays slow down at different speeds as they enter a new medium.

Recomposition• Colours of the spectrum may be

recombined by means of a lens to form white light in a process called recomposition.

• A series of mirrors can shine the different coloured lights together.

• Newton’s colour disk works because of the persistence of vision, in which the image of colours remain on the retina and combine to form white.

Rainbows…

• A rainbow is the sun’s spectrum produced by water droplets in the atmosphere.

• Light enters the spherical rain drop where it is refracted, reflected internally and finally dispersed.

…Rainbows

• The rainbow arc appears at specific points in the sky because only droplets of water that are located along that arc will reflect the spectrum at the correct angle in to the eye of the observer.

• The sunlight is behind the observer, while person has to angle his/her vision up by 42 o.

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Refraction

• White light can be separated into its constituent colours by dispersion.

• A prism can be used to do this.• It can be recombined with a

Newton Disc or a lens.

Refraction

• Refraction is the bending of a light ray as it enters a new medium and changes speed.

• The angle is measured from the normal – an imaginary line at 90° to the point of intersection.

• Light bends towards the normal if it enters an optically denser substance and v.v.

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Refractive Index - n

• The refractive index, n, is a measure of how much light bends as it enters the substance.

• n = c/v, where c = 3x108 m/s• v = velocity of light in new medium• Air has a refractive index of 1.• Diamond bends light the most (n= 2.42).• Table of n values – page 79.

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Snell’s Law

• In air, n = sin Θi / sin Θ r

• Snell’s Law: n1sinΘ1 = n2sinΘ2

• The left side refers to the medium in which the light is incident.

• The right side refers to the medium to where the light exits.

Activity

• P. 81, Q. 1-3• P. 83, Q. 1-2• P. 86, Q. 3-5, 7

Total Internal Reflection

• This occurs when a ray of light passes from an optically dense material (big n) to an optically LESS dense material (low n).

• If the angle of incidence is greater than a certain angle – the critical angle - the light will NOT refract out, but will reflect internally.

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TOTAL INTERNAL REFLECTION (TIR)

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Critical Angle

• In TIR situations, there comes a point at which the angle of refraction increases until it leaves the medium.

• In this case the angle of refraction can be considered to be equal to 90o.

• The angle of incidence at which an angle of refraction of 90° first occurs is the Critical Angle.

• Thus for Critical Angle questions, the angle of refraction is assumed to be 90°.

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Total Internal Reflection

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Snell’s Law & TIR

• n1sinΘ1 = n2sinΘ2

• Thus the Θ2 is 90o, always.

• The Θ1 is called Θc , the critical angle.

• As n increases, the Θc decreases causing more TIR, which is why diamonds appear so brilliant.

• Page 88, Q. 1, 2, 6

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Fibre Optics

• This is especially useful in fibre optics.

• Light enters a optic fibre and reflects on the inside of the cable instead of escaping.

• So signals can be transmitted at the speed of light, much faster than the speed of electricity.

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Lab Activity

• Change: Draw a 10 cm by 10 cm cross (with accurate 90o angles) on a blank sheet of paper.

• Make a table as in the book up to 60o.• In pairs, do the lab.• Report due beginning of next class • Determine the Critical Angle of Lucite.

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Chapter Summary• Dispersion is the separation of white light

into its component colours by a prism.• The colours of the spectrum, when

recombined, form white light.• White objects tend to reflect light. Black

objects tend to absorb light.• Subtractive Theory of Colour: the colours

of the spectrum contained in white light are subtracted by filters or dyes until the desired colour remains.

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• Additive Theory of Colour: any other

colour of the spectrum can be produced by mixing additive primary colours (RBG).

• IR and UV light are radiations beyond the red and violet areas of the spectrum, respectively. Neither is visible to the human eye.

• Rainbows are formed by the dispersion and internal reflection of the white light from the sun by water droplets in the atmosphere.

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• Light refraction terms: incident ray, angle of incidence, normal, reflected ray, angle of refraction.

• When light enters a more optically dense medium, its speed decreases.

• n = c/v• Law of Refraction

• Snell’s Law n = sin i / sin r• The incident ray and refracted ray are

on opposite sides of the normal

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• When a ray of light passes into an optically denser medium, it bends towards the normal.

• When a ray of light passes into an optically less dense material, it bends away from the normal.

• When a ray of light passes into a medium at an angle of 90 ۫ , it does not refract.

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• Total Internal Reflection TIR• The ray of light passes from one

medium into another of lower optical density.

• The angle of incidence is greater than the critical angle.

• When the angle of incidence is greater than the critical angle, the refracted angle is 90 ۫.

• n1sin Θ1 = n2 sin Θ2

Activity

• P. 93, Q. 1-7

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