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Chapter 3 & 4Chapter 3 & 4Refraction & Refraction & LensessLensess

Refraction of LightRefraction of Light

Chapter 3Chapter 3

RefractionRefraction Refraction is the bending of light as it Refraction is the bending of light as it

passes from one medium to another passes from one medium to another medium.medium.

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

new medium.new medium.

Table of ObservationsTable 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 Why is the Sky Red at Sunset?Sunset? Dust particles in the air scatter light from the sun Dust particles in the air scatter light from the sun

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

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

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

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

DispersionDispersion Dispersion is the spreading out of light Dispersion is the spreading out of light

into its component colours ROYGBV.into its component colours ROYGBV. It does this because the different colour It does this because the different colour

light rays slow down at different speeds light rays slow down at different speeds as they enter a new medium. as they enter a new medium.

RecompositionRecomposition Colours of the spectrum may be Colours of the spectrum may be

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

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

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

RainbowsRainbows A rainbow is the sun’s spectrum A rainbow is the sun’s spectrum

produced by water droplets in the produced by water droplets in the atmosphere.atmosphere.

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

The sunlight is behind the observer, The sunlight is behind the observer, and the viewer must angle his/her and the viewer must angle his/her vision up by 42 vision up by 42 oo..

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

RefractionRefraction

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

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

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

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Refractive Index - nRefractive Index - n

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

n = c/v, where c = 3x10n = c/v, where c = 3x108 8 m/sm/s v = velocity of light in new mediumv = velocity of light in new medium Air has a refractive index of 1.Air has a refractive index of 1. Diamond bends light the most (n= 2.42).Diamond bends light the most (n= 2.42). Table of n values – page 79.Table of n values – page 79.

99

Snell’s Law In air, n = sin ΘIn air, n = sin Θi i / sin Θ/ sin Θ r r

Snell’s Law: nSnell’s Law: n11sinΘsinΘ11 = n = n22sinΘsinΘ22

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

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

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

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Total Internal ReflectionTotal Internal Reflection

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

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

1111

TOTAL INTERNAL REFLECTION TOTAL INTERNAL REFLECTION (TIR)(TIR)

Critical AngleCritical Angle

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

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

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

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

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Total Internal ReflectionTotal Internal Reflection

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Snell’s Law & TIRSnell’s Law & TIR

nn11sinΘsinΘ11 = n = n22sinΘsinΘ2 2

Thus the ΘThus the Θ2 2 is 90is 90oo, always., always.

The ΘThe Θ1 1 is called Θis called Θc c , the critical angle., the critical angle.

As n increases, the ΘAs n increases, the Θcc decreases causing decreases causing

more TIR, which is why diamonds appear so more TIR, which is why diamonds appear so brilliant.brilliant.

Page 88, Q. 1, 2, 6Page 88, Q. 1, 2, 6

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Fibre OpticsFibre Optics

This is especially useful in fibre optics.This is especially useful in fibre optics. Light enters a optic fibre and reflects on Light enters a optic fibre and reflects on

the inside of the cable instead of the inside of the cable instead of escaping.escaping.

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

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Lab ActivityLab Activity

Change: Draw a 10 cm by 10 cm cross (with accurate Change: Draw a 10 cm by 10 cm cross (with accurate 9090oo angles) on a blank sheet of paper. angles) on a blank sheet of paper.

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

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Chapter SummaryChapter Summary Dispersion is the separation of white light into its Dispersion is the separation of white light into its

component colours by a prism.component colours by a prism. The colours of the spectrum, when recombined, The colours of the spectrum, when recombined,

form white light.form white light. White objects tend to reflect light. Black objects White objects tend to reflect light. Black objects

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

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

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Additive Theory of Colour: any other colour Additive Theory of Colour: any other colour

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

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

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

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

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

n = c/vn = c/v Law of RefractionLaw of Refraction

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

opposite sides of the normalopposite sides of the normal

2020

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

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

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

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Total Internal Reflection TIRTotal Internal Reflection TIR The ray of light passes from one medium The ray of light passes from one medium

into another of lower optical density.into another of lower optical density. The angle of incidence is greater than the The angle of incidence is greater than the

critical angle.critical angle. When the angle of incidence is greater than When the angle of incidence is greater than

the critical angle, the refracted angle is 90the critical angle, the refracted angle is 90 ۫۫.. nn11sin sin ΘΘ11 = n = n22 sin sin ΘΘ22

Answer: Answer: P. 93, Q. 1-7P. 93, Q. 1-7

2222

Types of LensesTypes of Lenses

Converging:Converging: Causes light rays to come together Causes light rays to come together Examples: Magnifying glass, camera, eye Examples: Magnifying glass, camera, eye

glasses glasses

Diverging:Diverging: Causes light rays to spread apart Causes light rays to spread apart Examples: Microscope, telescope, eye Examples: Microscope, telescope, eye

glasses glasses

Principal RaysPrincipal Rays Just like curved mirrors, lenses are used to Just like curved mirrors, lenses are used to

produce imagesproduce images The images are found by using three principal The images are found by using three principal

rays:rays: Principal Ray 1: Principal Ray 1: A light ray parallel to the principal A light ray parallel to the principal

axis is refracted through the principal focal point.axis is refracted through the principal focal point. Principal Ray 2: Principal Ray 2: Passes through the optical center. Passes through the optical center.

No apparent refraction occurs but the ray is bent. No apparent refraction occurs but the ray is bent. The ray seems to pass straight through to the other The ray seems to pass straight through to the other side.side.

Principal Ray 3:Principal Ray 3: Passes through the secondary Passes through the secondary focal point and will be refracted. Will exit the lens focal point and will be refracted. Will exit the lens parallel to the principal axis.parallel to the principal axis.

Converging LensConverging Lens

LensLens

Diverging LensesDiverging Lenses

Principal ray 1Principal ray 1 (parallel to the principal axis, (parallel to the principal axis, PA) will be refracted away from the PA) will be refracted away from the PAPA. But the . But the prolongation of the ray will pass through the prolongation of the ray will pass through the principal focus.principal focus.

Principal ray 2Principal ray 2 (straight through the optical (straight through the optical center) has no apparent sign of refraction.center) has no apparent sign of refraction.

Principal ray 3Principal ray 3 (via the secondary focal point) (via the secondary focal point) is refracted parallel to the principal axis. is refracted parallel to the principal axis.

Diverging LensDiverging Lens

Answer: Page 98, Q. 1-3

Optical PowerOptical Power

The strength of a lens (optical power) is related The strength of a lens (optical power) is related to its focal length. A short focal length means to its focal length. A short focal length means the light rays are being refracted a lot. The the light rays are being refracted a lot. The optical power of the lens is strong.optical power of the lens is strong.

A long focal length means the light rays are not A long focal length means the light rays are not being refracted very much. The optical power being refracted very much. The optical power of the lens is weak.of the lens is weak.

The power of a lens is equal to the inverse of The power of a lens is equal to the inverse of the focal point. P = 1/fthe focal point. P = 1/f

Power unit: dioptres (d)Power unit: dioptres (d) Focal length: meters (m), negative if divergingFocal length: meters (m), negative if diverging

Lens CombinationsLens Combinations

By combining lenses, different optical powers can be By combining lenses, different optical powers can be obtained. This is quite useful for making telescopes, obtained. This is quite useful for making telescopes, microscopes or any other optical instrument that uses microscopes or any other optical instrument that uses more than one lens. more than one lens.

The The optical powersoptical powers can be added using the equation: can be added using the equation: PPTT = P = P11 + P + P22 + P + Pnn…… PPTT: Total optical power in dioptres: Total optical power in dioptres PP11, P, P22, P, Pnn,: Power of each lens in dioptres. ,: Power of each lens in dioptres. N.B. N.B. The focal lengths The focal lengths CAN NOTCAN NOT be added together to be added together to

solve optical power. You must use the equation solve optical power. You must use the equation P=1/fP=1/f to to obtain the obtain the PP value. value.

Lens CombinationsLens Combinations

What is the power, focal lengths?What is the power, focal lengths?

ActivitiesActivities

Page 112, Q. 2-3, 5Page 112, Q. 2-3, 5 Testing your knowledgeTesting your knowledge

Thin Lens Thin Lens EquationsEquations

ddoo is the distance to the object is the distance to the object

ddii is the distance to the image is the distance to the image

f is the focal lengthf is the focal length hhii is the image height is the image height

hhoo is the object height is the object height

N.B. the negative signN.B. the negative sign

Conventions for the Conventions for the EquationEquation Distances are measured from the vertex.Distances are measured from the vertex. Focal lengths are positive for converging lenses and Focal lengths are positive for converging lenses and

negative for diverging lensesnegative for diverging lenses Radii of curvature are positive for converging lenses and Radii of curvature are positive for converging lenses and

negative for diverging lenses.negative for diverging lenses. Image and object distances are positive for real images Image and object distances are positive for real images

and objects.and objects. Image and object distances are negative for virtual Image and object distances are negative for virtual

images and objects.images and objects. Image and object heights are positive when upright and Image and object heights are positive when upright and

negative when inverted.negative when inverted. Answer: Page 122, Q. 5,6,7,9Answer: Page 122, Q. 5,6,7,9

Devices that Use LensesDevices that Use Lenses

Many devices rely on lenses, but what is Many devices rely on lenses, but what is a lens? a lens?

Circular in shape Circular in shape Made of transparent material Made of transparent material When light rays pass through it, they are When light rays pass through it, they are

refracted (change direction) refracted (change direction)

Devices: CameraDevices: Camera

Eye BallEye Ball

SummarySummary Curved lens terms: optical centre, principal Curved lens terms: optical centre, principal

axis, principal focus, focal length, focal axis, principal focus, focal length, focal plane.plane.

A lens’ focal length is the distance between A lens’ focal length is the distance between the optical centre and the principal focus the optical centre and the principal focus measured along the principal axis.measured along the principal axis.

The rules for rays in curved lenses are:The rules for rays in curved lenses are: A ray parallel to the principal axis is refracted so A ray parallel to the principal axis is refracted so

that it passes thru’ (or appears to pass thru’ the that it passes thru’ (or appears to pass thru’ the principal focus. (F).principal focus. (F).

A ray passing thru’ (or appearing to pass thru’) A ray passing thru’ (or appearing to pass thru’) the secondary principal focus (F’) is refracted the secondary principal focus (F’) is refracted parallel to the principal axis.parallel to the principal axis.

A ray passing thru’ the optical centre goes A ray passing thru’ the optical centre goes straight thru’, without bending.straight thru’, without bending.

Converging lenses and their combinations Converging lenses and their combinations are used as magnifying glasses, spotlights, are used as magnifying glasses, spotlights, projectors, cameras, telescopes, binoculars, projectors, cameras, telescopes, binoculars, and microscopes, glasses.and microscopes, glasses.

Diverging lenses are used in glasses.Diverging lenses are used in glasses. Dispersion causes chromatic aberration in Dispersion causes chromatic aberration in

lenses. This defect can be partially lenses. This defect can be partially corrected by using 2 or more lenses with corrected by using 2 or more lenses with different optical densities. different optical densities.

P = 1/f, f in metres, P in dioptres.P = 1/f, f in metres, P in dioptres. Powers made be summed. PPowers made be summed. PT = P = P11+ P+ P22 +… +… Answer: Answer: Page 130, Q. 3`, 9Page 130, Q. 3`, 9