Physics 504Physics 504

Reflections on LightReflections on Light

Types of ReflectionTypes of Reflection

• Specular (regular) reflection occurs Specular (regular) reflection occurs when the reflected rays of light when the reflected rays of light emerge parallel from the surface. emerge parallel from the surface.

• The surface is usually smooth and The surface is usually smooth and polished. polished.

• E.g. mirror, glassE.g. mirror, glass

Types of ReflectionTypes of Reflection

• Diffuse reflection occurs when the Diffuse reflection occurs when the reflected rays of light are not reflected rays of light are not parallel. parallel.

• The surface is usually rough and The surface is usually rough and uneven. uneven.

• E.g. paper, snowE.g. paper, snow

VocabularyVocabulary

• Reflection in a plane mirror, Reflection in a plane mirror,

• Incident ray – ray coming into surfaceIncident ray – ray coming into surface

• Point of incidence – where the ray hits Point of incidence – where the ray hits the surfacethe surface

• Reflected ray – the reflected rayReflected ray – the reflected ray

• Normal – line at 90 Normal – line at 90 ⁰⁰ to point of to point of incidence.incidence.

Mirrors:Mirrors:

• Mirrors are used in many ways Mirrors are used in many ways because they reflect light so well.because they reflect light so well.

• E.g. cameras (non-digital), E.g. cameras (non-digital), periscopes, reflecting telescopes, periscopes, reflecting telescopes, solar heaterssolar heaters

• P. 43, Q. 3-5P. 43, Q. 3-5

Angle of Incidence & Angle of Incidence & ReflectionReflection• The angles of incidence and reflection The angles of incidence and reflection

are measured relative to the NORMAL – are measured relative to the NORMAL –

• NOT THE SURFACE.NOT THE SURFACE.

• The angle of incidence equals the angle The angle of incidence equals the angle of reflection.of reflection.

• The incident ray, the reflected ray and The incident ray, the reflected ray and the normal are all in the same plane. the normal are all in the same plane.

RaysRays

Incident Rays Reflected RaysIncident Rays Reflected Rays

normal normal

Angle of incidence Angle of

reflection

Formation of Image by a Formation of Image by a Plane Mirror – Ray DiagramPlane Mirror – Ray Diagram• A ray of light incident on a plane mirror at 90A ray of light incident on a plane mirror at 90oo

gets reflected from the mirror along the same gets reflected from the mirror along the same path.path.

• A ray of light falling on a plane mirror at any A ray of light falling on a plane mirror at any angle gets reflected from the mirror such that the angle gets reflected from the mirror such that the angle of incidence is equal to the angle of angle of incidence is equal to the angle of reflection.reflection.

• P. 45, Q. 1, 3P. 45, Q. 1, 3

Images formed in a Plane Images formed in a Plane MirrorMirror• A plane mirror is a flat mirror.A plane mirror is a flat mirror.

• Images are the same attitude – upright or Images are the same attitude – upright or inverted.inverted.

• Images are the same distance “behind” Images are the same distance “behind” the mirror as the object is in front of it.the mirror as the object is in front of it.

• Images formed are virtual – they cannot be Images formed are virtual – they cannot be projected on a screen.projected on a screen.

• A line joining the image and object is A line joining the image and object is perpendicular to the plane mirror.perpendicular to the plane mirror.

QuestionQuestion

An object is placed in front of a plane mirror. Which of the following diagrams correctly represents the image formed in the plane mirror?

A)

C)

B)

D)

QuestionQuestion

An object is placed in front of a plane mirror. Which statement correctly describes the characteristics of the image?

A)

The image is real, upright, smaller than the object and located in front of the mirror.

B)

The image is virtual, inverted, larger than the object and located behind the mirror.

C)

The image is real, inverted, the same size as the object and located in front of the mirror.

D)

The image is virtual, upright, the same size as the object and located behind the mirror.

ExerciseExercise• Using a light beam, ruler and plane Using a light beam, ruler and plane

mirror, draw the incident, normal and mirror, draw the incident, normal and reflected ray coming off the mirror.reflected ray coming off the mirror.

• Compare the angles.Compare the angles.

• ____________________________________________________________________

Field of VisionField of Vision

• The field of vision from a mirror is The field of vision from a mirror is determined by the width of the determined by the width of the mirror, the distance of the observer mirror, the distance of the observer to the mirror and its curvature.to the mirror and its curvature.

• In stores curved mirrors are used to In stores curved mirrors are used to give a wider field of vision. give a wider field of vision.

FIELD OF VISIONFIELD OF VISION

normal normal

Field of Vision

Observer

ΘrΘi Θr Θi

Plane mirror

Field of VisionField of Vision

• Our view in a mirror depends on how we position ourselves in front or to the side of the mirror.

• The field of vision can be determined through ray diagrams:

• a. Draw two normals, one at either end of the mirror,• b. Draw an incident ray from your eye to each

normal.• c. Draw a reflected ray from each normal.• d. Remember the laws of reflection when drawing

both sets of rays (i = r)• Make a diagram – ask teacher.

Activity: Determine Field of Activity: Determine Field of Vision: Vision: for a Plane Mirrorfor a Plane Mirror ________MirrorMirror__________

OO

Activity: Determine Field of Activity: Determine Field of Vision: Vision: For a Curved MirrorFor a Curved Mirror

OO

QuestionQuestionAn observer is standing in front of a reflective window in which he can see the images of some shrubs.

REFLECTIVE WINDOW

I

II

III

IV

OBSERVER

Based on the above diagram, the images of which shrubs can be seen by the observer?

A)

III only

B)

II and III only

C)

III and IV only

D)

I, II, III and IV

ActivityActivity

• P. 47, Q. 1-3P. 47, Q. 1-3

2020

Concave MirrorConcave Mirror

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The Inside of A SpoonThe Inside of A Spoon

• Why is it when you look into a soup spoon that Why is it when you look into a soup spoon that your image turns upside down sometimes?your image turns upside down sometimes?

• The inside of a spoon is a concave mirror. The inside of a spoon is a concave mirror. • If the inside of the spoon is held close to the If the inside of the spoon is held close to the

eye, a magnified upright view of the eye will eye, a magnified upright view of the eye will be seen (in this case the eye is closer than the be seen (in this case the eye is closer than the focal point of the mirror). focal point of the mirror).

• If the spoon is moved farther away, a smaller If the spoon is moved farther away, a smaller upside-down view of the whole face will be upside-down view of the whole face will be seen. seen.

2222

Concave MirrorsConcave Mirrors

• The concave mirror has a reflection The concave mirror has a reflection surface that curves inward, like a surface that curves inward, like a portion of the interior of a sphere.portion of the interior of a sphere.

• When light rays that are parallel to When light rays that are parallel to the principal or optical axis reflect the principal or optical axis reflect from the surface of a concave mirror, from the surface of a concave mirror, they converge on the focal point they converge on the focal point (black dot) in front of the mirror. (black dot) in front of the mirror.

2323

Converging MirrorConverging Mirror

• Concave mirrors bring light rays to a Concave mirrors bring light rays to a focus and are called converging focus and are called converging mirrors.mirrors.

2424

Rules for Con/Di-Verging Rules for Con/Di-Verging MirrorsMirrors• A ray that is parallel to the principal axis is A ray that is parallel to the principal axis is

reflected through the principal focus – real or reflected through the principal focus – real or virtual.virtual.

• A ray passing through the principal focus is A ray passing through the principal focus is reflected parallel to the principal axis.reflected parallel to the principal axis.

• A ray passing through the centre of curvature A ray passing through the centre of curvature is reflected back along the same path.is reflected back along the same path.

• The centre of curvature or radius is twice the The centre of curvature or radius is twice the focal length.focal length.

• C = 2f.C = 2f.

Exam Question using the Exam Question using the 3Rs3Rs

Look at the diagram below. A 12 cm tall object is placed in front of a concave mirror. The focal length is 30 cm. The object is located 70 cm from the top of the mirror.

Object

C F

What will be the height of the image reflected by the mirror?

A) 5.1 cm

B)

9.0 cm

C)

16 cm

D)

28 cm

Concave MirrorsConcave Mirrors

Object’s Position

Image Characteristics

Type Orientation

Size Position

At ∞ Real Point Image At F

Beyond C

Real Inverted Smaller Between F and C

At C Real Inverted Same size At C

Between C and V

Real Inverted Bigger Beyond C

At F No Image

Between F and V

Virtual

Upright Larger than the object

Between F and V behind the mirror, farther from the mirror than the object.

2727

Convex MirrorsConvex Mirrors• The convex mirror has a reflecting surface The convex mirror has a reflecting surface

that curves outward like a portion of the that curves outward like a portion of the exterior of a sphere. exterior of a sphere.

• Light rays parallel to the optical axis are Light rays parallel to the optical axis are reflected from the surface in a manner reflected from the surface in a manner that diverges from the focal point, which is that diverges from the focal point, which is behind the mirror. behind the mirror.

• Images formed with convex mirrors are Images formed with convex mirrors are always right side up and reduced in size.always right side up and reduced in size.

• These images are also termed virtual These images are also termed virtual images, because they occur where images, because they occur where reflected rays appear to diverge from a reflected rays appear to diverge from a focal point behind the mirror.focal point behind the mirror.

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Diverging MirrorDiverging Mirror

• Diverging mirrors are convex.Diverging mirrors are convex.• Regardless of the position of the object Regardless of the position of the object

reflected by a convex mirror, the image reflected by a convex mirror, the image formed is always virtual, upright, and formed is always virtual, upright, and reduced in size. reduced in size.

• When the brain retraces the rays they When the brain retraces the rays they appear to come from behind the mirror appear to come from behind the mirror where they would converge, producing a where they would converge, producing a smaller upright imagesmaller upright image

• The image is upright since the virtual image The image is upright since the virtual image is formed before the rays have crossed the is formed before the rays have crossed the focal point. focal point.

Convex MirrorsConvex Mirrors

Object’s Position

Image Characteristics

Type Orientation

Size Position

Any position

Virtual

Upright Smaller than the object

Between F and V behind the mirror, closer to the mirror than the object.

Exam Question – on boardExam Question – on board

 A)

 + 1.60 

B) + 0.63 

C) - 0.63 

D)

 - 1.63

The teacher will draw this on the board, please.

Alex places a 5.00 cm tall candle 30.0 cm in front of a mirror that has a focal length of ‑50.0 cm.What is the magnification of the candle?

ActivityActivity

• Page 54, Q. 4, 7, 9Page 54, Q. 4, 7, 9

Mirror EquationsMirror Equations

• 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 EquationsEquations• Distances are measured from the vertex.Distances are measured from the vertex.

• Focal lengths are positive for converging Focal lengths are positive for converging mirrors and negative for diverging mirrors.mirrors and negative for diverging mirrors.

• Radii of curvature are positive for converging Radii of curvature are positive for converging mirrors and negative for d.m.mirrors and negative for d.m.

• Image and object distances are positive for real Image and object distances are positive for real images and objects.images and objects.

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

• Image and object heights are positive when Image and object heights are positive when upright and negative when inverted.upright and negative when inverted.

Exam QuestionExam Question

 A)

 20.0 cm

 C)

 90.0 cm

 B)

 60.0 cm

 D)

 120.0 cm

An object, 45.0 cm high, is placed in front of a convex mirror. A virtual image, 15.0 cm high, is formed.The focal length of the mirror is -30.0 cm.

At what distance is the object from the mirror?

ActivityActivity

• Page 70, Q. 1-4Page 70, Q. 1-4

• Page 76, Q. 1,2Page 76, Q. 1,2

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Uses for Concave MirrorsUses for Concave Mirrors

• Concave mirrors are used in optical Concave mirrors are used in optical telescopes to collect the faint light emitted telescopes to collect the faint light emitted from very distant stars.from very distant stars.

• The curved surface concentrates parallel The curved surface concentrates parallel rays from a great distance into a single rays from a great distance into a single point for enhanced intensity.point for enhanced intensity.

• This mirror design is also commonly found This mirror design is also commonly found in shaving or cosmetic mirrors where the in shaving or cosmetic mirrors where the reflected light produces a magnified image reflected light produces a magnified image of the face.of the face.

3737

Uses for Convex MirrorsUses for Convex Mirrors• Convex mirrors are often used in automobile Convex mirrors are often used in automobile

right-hand rear-view applications where the right-hand rear-view applications where the outward mirror curvature produces a smaller, outward mirror curvature produces a smaller, more panoramic view of events occurring more panoramic view of events occurring behind the vehicle. behind the vehicle.

• Convex mirrors are also used as wide-angle Convex mirrors are also used as wide-angle mirrors in hallways and businesses for mirrors in hallways and businesses for security and safety.security and safety.

• Carnival Fun House mirrors often incorporate Carnival Fun House mirrors often incorporate a mixture of concave and convex surfaces, or a mixture of concave and convex surfaces, or surfaces that gently change curvature, to surfaces that gently change curvature, to produce bizarre, distorted reflections when produce bizarre, distorted reflections when people observe themselves. people observe themselves.

Other ReflectionsOther Reflections

• Non-optical reflection can occur as:Non-optical reflection can occur as:

• Echoes – sounds reflected from Echoes – sounds reflected from surface;surface;

• Radio and microwaves;Radio and microwaves;

• Ocean waves when they recede;Ocean waves when they recede;

• Strings/Springs – as in guitar strings;Strings/Springs – as in guitar strings;

SummarySummary

• A normal is a line drawn at right A normal is a line drawn at right angles to a reflective surface at the angles to a reflective surface at the point of incidence.point of incidence.

• The Laws of Reflection are:The Laws of Reflection are:– The incident ray, the reflected ray & the The incident ray, the reflected ray & the

normal are all in the same plane;normal are all in the same plane;– The angle of incidence equals the angle The angle of incidence equals the angle

of reflectionof reflection

• Specular reflection is the reflection of Specular reflection is the reflection of light from smooth plane surfaces. Rays light from smooth plane surfaces. Rays parallel.parallel.

• Diffuse reflection is the reflection of light Diffuse reflection is the reflection of light from irregular surfaces. Rays not parallel.from irregular surfaces. Rays not parallel.

• The field of vision using a plane mirror The field of vision using a plane mirror depends on the size of the mirror and the depends on the size of the mirror and the distance of the observer and the object-distance of the observer and the object-to-be-seen from the mirror.to-be-seen from the mirror.

• Curved mirror terms – centre of Curved mirror terms – centre of curvature, principal axis, vertex, radius of curvature, principal axis, vertex, radius of curvature, principal focus, focal plane & curvature, principal focus, focal plane & focal length.focal length.

• All rays parallel to each other and All rays parallel to each other and striking a converging mirror meet at striking a converging mirror meet at the focal point.the focal point.

• The focal length is the distance The focal length is the distance between the principal focus and the between the principal focus and the vertex.vertex.

• In a curved mirror, the focal length is In a curved mirror, the focal length is ½ the radius.½ the radius.

• Rays from distant objects are Rays from distant objects are considered parallel. considered parallel.

• Spherical aberration occurs in curved Spherical aberration occurs in curved mirrors when parallel rays do not mirrors when parallel rays do not meet at the focal point. A parabola meet at the focal point. A parabola can solve this.can solve this.

• The Laws of Reflection apply to non-The Laws of Reflection apply to non-optical phenomena, like sound.optical phenomena, like sound.