WR3

WR3Light

WR3-1Refraction

We have already seen this effect in S3, so the following experiment should be familiar to you. To help you remember what you found out about refraction copy and complete the note below in your Notes Jotter.

f

1

P

length

focal

1

Power

=

=

f

1

P

length

focal

1

Power

=

=

Wave Phenomena (note 2)

Refraction (phenomena 3)

Refraction is defined as the change in __________________ of waves (e.g. light) as they travel from one substance into another. If we consider light, refraction would occur when the light travels from one density of substance to another. If we consider water waves, refraction would occur when the waves travel from one depth of water to another.

Where ;θi= _____________________,

θr= _____________________.

EXPERIMENT WR 4A - Investigating Refraction

· Copy this table into your rough jotter.

θi

θr

· Set up the equipment as shown above

· Look at the path taken by the beam of light for four angles of incidence,

0o and any other angle greater than 0o (about 30o - 50o would make it easier to observe the path taken by the ray).

· Draw the path taken by these using accurate ray plotting techniques

Questions to consider before completing the note on the next page

· Where does the refraction take place, in the air, in the perspex/glass or as the light enters the perspex/glass?

· How does θi and θr compare?

· What happens to the speed of the light as it enters the block?

· What does this mean about the wavelength of the light as it enters the block?

· What happens to the frequency of light as it enters the block?

· What happens to the light’s direction, frequency and wavelength and it exits the block?

· Does the path taken by the light ray change if you reverse the direction of the light ray (i.e. does matter if the ray box is place at locations on side B of the block and the light ray is directed back along the paths it originally took when travelling out of the block)?

Add the following information onto the Refraction (phenomena 3) note;

When incident light travels from a less dense substance to a more dense substance, the light bends __________ the normal,

If the incident light travels along the normal the light ___________ ________________ ____________ _______________ as it enters the block.

When incident light travels from a more dense substance to a less dense substance, the light bends __________ the normal.

In all ray diagrams the direction (or path) taken by the light is reversible.

OPTIONAL EXPERIMENT WR 4B - Investigating Refraction

If you have time, repeat EXPERIMENT WR 4A, but use a triangular block instead.

Does the light still obey the rules you have just written about in the note above?

Real and Apparent Depth

The effect of refraction can be seen in many situations. You may have noticed it at the local swimming pool or looking into an aquarium from above the tank!

f

1

P

length

focal

1

Power

=

=

Your eyes assume information is coming from straight in front of you, as they (and your brain that interprets the information) are used to light travelling in straight lines; however that is not the case when light from a submerged object is travelling out of the swimming pool to your eyes. At the surface of the pool the light refracts (away from the normal) which means that the object appears to be at a shallower depth (apparent depth) compared with where it actually is (real depth). See the image below.

Question Practice on Refraction

1. For a ray of light travelling from air into glass, which of the following statements is/are correct?

IThe speed of light always changes.

IIThe speed of light sometimes changes.

IIIThe direction of light always changes.

IVThe direction of light sometimes changes.

A I only

B III only

C I and III only

D I and IV only

E II and IV

2.A ray of red light is incident on a glass block as shown.

Which row in the table shows the values of the angle of incidence and angle of refraction?

f

1

P

length

focal

1

Power

=

=

3.Visible light is part of a family of waves known as the electromagnetic spectrum.

(a) What is the speed of waves in the electromagnetic spectrum?

(b) A student notices that when white light passes through a glass of

lemonade it is split into different colours.

The student decides to reproduce this effect in a laboratory using the following equipment.

(i) State the name of the glass block that the student uses to split the

light into different colours.

(ii) When white light enters the glass block its speed and direction are

changed.

What name is given to this effect?

(iii)The colours appear on the screen in order of wavelength. The shortest

wavelength appears at Z.

State which of the colours green, blue or red would be seen at positions

X, Y and Z on the screen.

4. The diagram shows a ray of light P incident on a rectangular glass block.

Which of the following are refracted rays?

A Q and R

B R and S

C S and T

D Q and S

E R and T

5.(a) Refraction of light occurs in lenses.

What is meant by the term refraction?

(b) The following diagram shows a ray of light entering a glass block.

(i) Copy and complete the diagram to show the path of the ray of

light through the block and after it emerges from the block.

(ii) On your diagram indicate an angle of refraction, r.

6. A class investigates the effects of the following shapes of glass on rays of

white light.

The teacher sets up three experiments, covering the glass shape with card.

The paths of the light rays entering and leaving the different shapes of glass are shown.

For each of the three experiments, copy and complete the diagram to show the shape and position of the glass block that was used.

(a)

(b)

(c)

WR3-2Refraction and Lenses

We have already used lens shapes in S3. To help you remember what you found out about them copy the note below in your Notes Jotter.

Refraction - Lenses

There are two main shapes of lens

(i)A convex or converging lens - brings light to a focus

(ii)A concave or diverging lens - spreads light out (from a focus on the

opposite side of the lens)

More detail on Focal Length

Convex lenses bring light to a focus nearer to the lens than others. The position of the focus depends on where the rays come from. One way to indicate the amount of refraction is to give the focal length of the lens.

(i) Rays come from point close to lens - focus is far from lens.

(ii) Rays come from point further from lens - focus nearer to lens.

(iii) When the rays come from a point which is so far away that the rays seem parallel, the focus is even nearer to the lens.

In this case, the focus is called the principal focus.

The distance from the lens to the principal focus is called the focal length.

EXPERIMENT WR 5 - Measuring focal length

Collect

Metre Stick, convex lenses, wall and a distant object

Lens

Focal length

(metres)

1.Choose a convex lens and use it to produce a sharp image of parallel light

rays coming from a distant object (for example, an object outside of a

distant window) on the white card (or wall).

2.Measure the distance from the lens to the image. (This is the focal

length).

3.Repeat using lenses with different curvature.

Consider the following questions in your group as you do the experiment

1. In which way do lenses with a short focal length look different from

those with a long focal length?

2.Does the focal length affect the image in any way?

3.How does the image produced compare with the original image?

WR3-3Eyesight and its correction

Normal sight

An eye is quite a complex organ. A simple diagram of the human eye is shown below

When you look at something, light rays from the object pass into your eye through your cornea (the clear dome that covers your pupil) then through your lens and on towards the retina at the back of your eye. In a healthy eye, your lens and cornea bring the light to a focus on your retina so that you can see near or distant object clearly by adjusting the curvature of the lens (for simplicity the following diagrams only show the lens and retina).

To focus on near objects, the lens becomes thicker and more curved, bringing light rays from close objects into sharp focus on the retina.

To focus on objects in the distance, your lens becomes thinner and less curved, again bringing light rays from distant objects into sharp focus on the retina.

Long Sightedness (Hyperopia, Hypermetropia)

The long-sighted eye can successfully focus on distant objects but has a problem focussing on near/close objects. This means there is a problem in the way that your eye focuses light rays.

If an eye is long-sighted, light rays from near objects are focused behind your retina. This may be because your eyeball is too short in relation to the power of your eye lens, your cornea isn't curved enough or your lens cannot become curved (thick) enough for the length of your eyeball. Close objects appear fuzzy or blurred.

Distant objects won't look blurred because the light rays don't need to be refracted so far from their original path, and do not require such a curved lens so the lens can adjust enough to focus light on your retina properly.

Correcting Long Sight

To correct long sightedness we need a spectacle or contact lens that will take light travelling towards the eye at a steep angle and refract it to enter the eye straight on, as shown below.

ON A PERSONAL WHITEBOARD (OR YOUR ROUGH JOTTER), copy and complete the diagram below by inserting the correct type of lens to show how the vision of a long sighted person may be corrected. You can do this as an individual, as a pair or as a group; it may be helpful to discuss the solution before you submit your answer to your teacher on the whiteboard.

Short sightedness (Myopia)

The short sighted eye can focus on near objects successfully but has a problem focussing on distant objects.

If an eye is short sighted light rays from distant objects focus at a point between the lens and retina. This may be because your lens cannot reach a small enough curvature (become thin) enough for the length of your eyeball, your cornea is too curved or your eyeball is too long in relation to the power of your lens. This makes distant objects seem fuzzy or blurred.

Close up objects won't look fuzzy or out of focus because the light rays enter the eye at a steeper angle. This means the powerful lens in the short sighted eye can successfully refract light through large angles and focus it on the retina.

Correcting Short Sightedness

To correct short sightedness we need a spectacle or contact lens that will take light travelling straight towards the eye and refract it to enter the eye at a steep angle.

ON A PERSONAL WHITEBOARD (OR YOUR ROUGH JOTTER), copy and complete the diagram below by inserting the correct type of lens to show how the vision of a short sighted person may be corrected. You can do this as an individual, as a pair or as a group; it may be helpful to discuss the solution before you submit your answer to your teacher on the whiteboard.

Eye Defects and their Correction

Human eyes use the cornea and the lens to focus light onto the retina at the back of the eye. The lens is the only part of the eye that can adjust to ensure light can be in focus on the retina so we can see a clear image of what is in front of us, regardless of how far from our eye the object is.

There are two defects that can occur with human eyes.

1.Short Sight (myopia)

This occurs when people can focus on object that are close to them, however the lens in their eye is too powerful (too curved) to focus light from a distant object onto the retina

This defect can be aided using a spectacle lens or contact lens that is concave (diverging)

2.Long Sight (Hyperopia, Hypermetropia)

This occurs when people can focus on object that are far away from them, however the lens in their eye is too weak (cannot become curved enough) to focus light from a near object onto the retina

This defect can be aided using a spectacle lens or contact lens that is convex (converging).

Question Practice on Sight Defects

1.A student has an eye defect. An object close to the student’s eye appears focused but a distant object appears blurred.

(i) What name is given to this eye defect?

(ii) The diagram shows rays of light, from a

distant object, entering the student’s

eye.

Copy and complete the diagram to show

how the light rays reach the retina of

the student’s eye.

(iii) By referring to your completed diagram, explain why the image on the

retina of the student’s eye is blurred.

(iv) A lens is used to correct this eye defect.

Draw the shape of this lens and name the type of lens required.

2.A patient visits an ophthalmologist for an eye examination.

The ophthalmologist uses ultrasound to take measurements inside the eye.

(a)What name describes the shape of

the eye lens?

(b) The ophthalmologist has a graph obtained using measurements from a

person with normal eyesight. The graph shows times to receive reflected

ultrasound pulses from the front edge of the eye lens, the back edge of

the eye lens and from the retina.

2.(b)(i) Calculate the time taken for the ultrasound to travel from the

front edge to the back edge of the eye lens.

(ii)Ultrasound pulses travel at a speed of 1500 m/s inside the lens.

Calculate the thickness of the lens in the normal-sighted person.

(c) Another set of measurements indicates that a patient is long-sighted

and requires spectacles. Figure 1 shows rays of light from a book

entering an eye of this patient until the rays reach the retina.

(i) Copy Figure 2, replacing the dotted box with a drawing of the

shape of lens that would correct this eye defect.

(ii) In your copy of Figure 2, complete the path of the rays of light

from this lens until they reach the retina.

WR3-4 Power of Lenses [EXTENSION WORK]

People who have severe eye defects may need more powerful (stronger) lenses to correct their eyesight than those who have only slight defects. A more powerful lens is one which causes more refraction.

An optician must have a range of lenses to suit different individuals' needs.

The power of each lens is indicated by its focal lengths - powerful lenses have short focal lengths. Another way to give the amount of refraction caused by calculate its power from the following relationship:

NB: Be careful the letter ‘f’ is also used for frequency in some formulae!

If the focal length is measured in metres, then the power is given in dioptres (D).

· Converging (convex) lenses have positive powers (e.g. +10 D, +17 D).

This is because the focus for a converging lens is on the positive side of

the lens, i.e. the lens has a positive focal length and is on the opposite

side to where the light is coming from.

· Diverging (concave) lenses have negative powers (e.g. - 10 D, - 17 D).

This is because the focus for a diverging lens is on the negative side of

the lens i.e. the lens has a negative focal length, back on the same side

that the light is coming from.

i.e.

Example: a spherical convex lens has a focal length of +10 cm, find its power.

Question Practice on Lens Power

1. A lens has a focal length of +5 cm.

(a)What type of lens is it?

(b)What is its power?

2. A lens has a focal length of - 20 cm.

(a)What type of lens is it?

(b)What is its power?

3. A lens has a power of +3D.

(a)What type of lens is it?

(b)What is its focal length?

4. A lens has a power of - 14 D. What is its focal length?

(a)What type of lens is it?

(b)What is its focal length?

Power of Lenses

A MORE POWERFUL lens is one which causes MORE REFRACTION.

A LESS POWERFUL lens is one which causes LESS REFRACTION.

· Converging (convex) lenses have positive powers (e.g. +10 D, +17 D) and positive focal lengths.

· Diverging (concave) lenses have negative powers (e.g. - 10 D, - 17 D) and negative focal lengths.

Focal Length can be calculated using the following equation:

IMPORTANT: Be careful the letter ‘f’ is also used for frequency in some

formulae!

HWK WR2 -Medical use of lasers

Research the use of laser use in medicine.

Some applications are listed below, however other applications are possible, consult teacher to discuss

· Treatment of varicose veins (endovenous laser ablation).

· Corneal eye surgery to improve vision;

( LASIK (laser in situ keratomileusis),

( PRK (photorefractive keratectomy),

( LASEK (laser epithelial keratomileusis),

( Wavefront-guided LASIK.

· Repair of a retinal detachment.

· Treatment of diabetic eye disease (retinopathy).

· Removal of the prostate.

· Removal of kidney stones.

· Laser surgery (e.g. laser scalpel).

· Lasers for cancer treatment.

WR3-5 Refraction and Fibre Optics

EXPERIMENT WR 6 - Refraction, Critical Angle and Total Internal

Reflection (TIR)

· Set up the equipment as shown above with θP ≈ 10o and the ray directed at the centre of the straight side on the Perspex block.

· This experimental result is purely based on what you observe, so no table of results is required.

· Move the ray box slowly in the direction shown by the arrow, or rotate the paper with the block on it to create the same effect. MAKE SURE THE LIGHT RAY IS ALWAYS DIRECTED AT THE CENTRE OF THE STRAIGHT SIDE OF THE BLOCK.

· Use your observations to complete the diagrams in the following note.

Refraction, Critical Angle & Total Internal Reflection (TIR)

In the above diagrams θP is the angle inside the Perspex block, θc is a very special value called the c_______ a_______. At this c_______ a_______ the refracted ray travels almost parallel to the straight face of the block. At any value of θP larger than the c_______ a_______the straight face of the block acts like a mirror and the light is reflected back inside the block. This is called T_________ I_________________ R___________.

EXPERIMENT WR 7 - Application of Total Internal Reflection (TIR)

Your Teacher will demonstrate or display some items and uses of total internal reflection. Use the information you are given to complete the note that follows experiments 7A, 7B and passage below.

EXPERIMENT WR 7A -Transmission of information using fibre optic cables

Observe the demonstration of the use of fibre optics for telecommunications that has been set up using alpha kit

EXPERIMENT WR 7B -Fibre Optic Ornaments

Examine some fibre optic ornaments; examine how the lighting effect is produced if the ornament’s construction allows you to do this.

Read the following passage:

Medical Endoscopy, Customs Searches, Engineers, Builders and DIY.

In medical usage a fiberscope is called an endoscope (meaning to look inside). It is a device that looks inside the body. The endoscope is a long flexible tube which contains two fibre optic strands. They are very thin so they are flexible and can be steered by the surgeon or doctor around the twists and turns of human anatomy.

One bundle of strands carries light into the body by TOTAL INTERNAL REFLECTION. The inside of the body is a dark location and has to be illuminated before you can see anything!

The second bundle of fibres takes light reflected from the inside surfaces of the body back up to the observer, again this light travels along the fibres by total internal reflection. This second bundle of fibres can either be connected directly to a lens system for the doctor to observe the internal parts of the patient directly or could be attached to a camera and large screen or a computer for recording the operation.

Increasingly, special versions of fiberscope are used by Customs Officers to look inside imported packages and trucks/vans/vehicles/containers at UK borders.

In engineering fiberscopes are used to look inside machinery or other devices/ locations they are also referred to as borescopes.

Since becoming much less expensive due to mass production, cheaper fiberscopes are also being used by builders and plumbers, e.g. to look inside walls for routing cables, detecting problems with buildings and pipework, usually these have a small hand held camera attached - they are even available for sale in some supermarkets for DIY use!

Using Refraction - Fibre Optics

Optical fibres are very ____________ strands of glass, often grouped together to create a fibre optic _________.

These fibres and cables carry light by T________ I___________ R____________. Light enters the fibre and strikes the sides of the glass on it’s way through at an angle that is _________ than the C__________ A_______ , causing the light to ___________.

Homework WR 3 - Applications of Fibre optic note

In your notes put the heading ‘Application of Fibre Optics’.

Your task is to research machines or systems that make use of Fibre Optics and how they are used in society

You then must create a paragraph or two on what you have found to form part of your course notes - how many topics you want to research is up to you, however you must include one medical use (such as the endoscope in the passage above) and one telecommunications use in your note.

Possible topics you may want to research are;

Telecommunications (internet, telephone network, hifi/entertainment systems…etc)

Medical use

Use in construction industry (building, plumbing…etc)

Use in security

Use in espionage/military

Question Practice on Fibre Optics

1.Optical fibres are used to carry internet data using infra-red radiation.

(a) Is the wavelength of infra-red radiation greater than, the same as, or

less than the wavelength of visible light?

(b) The diagram shows a view of an infra-red ray entering the end of a

fibre.

Copy and complete the diagram to show the path of the infra-red ray as

it enters the glass from air.

Indicate on your diagram the normal, the angle of incidence and the

angle of refraction.

(c) The diagram shows the path of the infra-red ray as it passes through a

section of the fibre.

Name the effect shown.

2.The diagram shows the path of a ray of red light in a glass block.

A student makes the following statements.

IAngle x is equal to angle y.

IITotal internal reflection is taking place.

IIIAngle x is the critical angle for this glass.

Which of the following statements is/are correct?

A I only

B II only

C I and II only

D II and III only

E I, II and III

3. The diagram shows what happens to a ray of light when it strikes a glass block.

Which row in the table identifies the angle of incidence and the angle of refraction?

4.A DVD player contains a laser. Light from this laser enters a small glass prism as shown.

The glass has a critical angle of 40 °.

(a)Explain what is meant by the term “critical angle”.

(b)Copy and complete the diagram to show the path of the ray after it

strikes point P.

5. Different types of radiation can be used in medicine for both the diagnosis and treatment of a variety of illnesses. The table shows information on some of the types of radiation used in medicine.

(a) Copy and complete the table to show:

(i) the missing types of radiation;(ii) the missing uses of radiation.

(b) Lasers are also used in medicine for various treatments.

(i) State one use of lasers in medicine.

(ii) Copy and complete the diagram to show how light is transmitted

along an optical fibre.

6.When the sun shines during a shower of rain, a rainbow can sometimes be seen.

The diagram shows what happens to sunlight when it enters a raindrop.

(a) Name the wave effect that happens at point P.

(b) Name the wave effect that happens at point Q.

(c) Which colour of the rainbow has the longest wavelength?

7.Doctors can use an endoscope to examine internal organs of a patient.

The endoscope has two separate bundles of optical fibres that are flexible.

A section of optical fibre used in the endoscope is shown below.

(i) Copy and complete the diagram to show how light is transmitted along

the optical fibre.

(ii) Explain the purpose of each bundle of optical fibres in the endoscope.

(iii) The tip of the endoscope that is inside the patient is designed to be

very flexible. Suggest one reason for this.

?

?

?

?

θr

θi

NOTES

θa

θp

B

A

NOTES

NOTES

θC

θp

θC

θa

θp

When θP < θC

When θP = θC

When θP > θC

NOTES

Light from a near object

Light in focus on retina

Light in focus on retina

Light from a distant object

Light out of focus on retina

Focal point of lens

Light from a near object

Light from a distant object

Light in focus on retina

Light from a distant object

Light out of focus on retina

Focal point of lens

Light in focus on retina

Light from a near object

Light out of focus on retina

NOTES

piece of white card or a white wall

A long distance e.g. object outside classroom window

+ ve direction

- ve direction

Lens

metre stick

- ve direction

+ ve direction

focus

focus

� EMBED Equation.3 ���

IMPORTANT: Light rays must be parallel i.e. coming from a distant source or object

focus

focus

Light in focus on retina

Lens required

Light from a distant object

Spectacle Lens spreads light out

Light in focus on retina

Light from a distant object

Focal point of lens

Light out of focus on retina

Light from a near object

Focal point of lens

Lens required

Light from a near object

Light in focus on retina

Spectacle Lens brings light together

NOTES

� EMBED Equation.3 ���

NOTES

metres (m)

Dioptres (D)

metres (m)

Dioptres (D)

Real depth i.e. the correct depth of the object below the surface.

Apparent depth i.e. where the image of the object appears to be

SQA 2012 S Grade General Q18

SQA Nat 5 Specimen Question paper Q12 & 13

SQA 2012 Int 2 Q27

SQA 2011 Int 2 Q16

SQA 2011 Int 2 Q15

SQA 2009 Int 2 General Q16

SQA 2008 Int 2 Q26 (d)

SQA 2007 Int 2 Q27

SQA 2008 SG Credit Q6 (b)

SQA 2009 SG Credit Q6 (b) & (c)

SQA 2010 SG Credit Q5 (b) modified

SQA 2011 SG Credit Q5 modified

SQA 2007 SG General Q11

SQA 2012 SG General Q11

θi

Westhill Academy MJRWaves & Radiation (National 5)

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