Law of Reflection

Law of Reflection Point source

Incident ray

Incident ray is reflected right at the point where it intersects the mirror.

Angle of incident

Reflected ray

Angle of reflection

Angle of incidence = Angle of reflection

A Point source radiates many light rays Point source

Angle of incidence = Angle of reflection Note arrow direction!

Image in a mirror

Image seems to locate behind the mirror!

Four methods to determine the location of an image 1. 1 2. 3 3. 4.

By the Law of Reflection By different lines of sight of the image By parallax method By properties of image

Method 1

Theoretical Argument: g Formation of image g Point source

Point image Angle of incidence = Angle of reflection Note arrow direction!

Method 1 This is true even when the point object is at a “strange” position Point source

Point image

Method 1

Using Law of Reflection to locate the image off a point i t source Point source

Pick up any ray from the point source that will hit the mirror

Method 1

Using Law of Reflection to locate the image off a point i t source Point source

Draw th D the reflected fl t d ray with ith a protractor: t t Angle of incidence = Angle of reflection

Method 1

Using Law of Reflection to locate the image off a point i t source Point source

Pick up another ray from the point source that will hit the mirror (These two rays should be far apart for higher accuracy)

Method 1

Using Law of Reflection to locate the image off a point i t source Point source

Draw th D the secondd reflected fl t d ray with ith a protractor: t t Angle of incidence = Angle of reflection

Method 1

Using Law of Reflection to locate the image off a point i t source Point source

Extendd the E h first fi reflected fl d ray as a straight i h line li behind the mirror (make sure it is long enough)

Method 1

Using Law of Reflection to locate the image off a point i t source Point source

Extendd the E h secondd reflected fl d ray as a straight i h line li behind the mirror (make sure it is long enough)

Method 1

Using Law of Reflection to locate the image off a point i t source Point source

Point image The image is at the intersection of the two extended line behind the mirror.

Method 2

Use ppins to track the reflected rays y

Method 2

Use ppins to track the reflected rays y Point source

Point image

When viewing the point image at a certain direction, direction align two pins with the point image.

Method 2

Use ppins to track the reflected rays y Point source

Point image

When viewing the point image at another direction, direction align two pins with the point image.

Method 2

Use ppins to track the reflected rays y Point source

Point image

Join the first two pins with a straight line and extend it behind the mirror. Make sure it is long enough.

Method 2

Use ppins to track the reflected rays y Point source

Point image

Join the other two pins with a straight line and extend it behind the mirror. The two lines intersect at the point image.

Method 2

Use ppins to track the reflected rays y Point source

Point image

Th pins The i are actually ll tracking ki the h reflected fl d rays.

Method 3

Which Object is closer?

Method 3

Method 3

Parallax method

Method 3

Parallax method Note position of further object relative to the nearer one

Moving your eye to the right

Method 3

Parallax method Note N t position iti off further f th object relative to the nearer one

Moving your eye to the left

Method 3

Which Object is closer?

Moving my eye towards right

Method 3

Which Object is closer?

Moving my eye back to original position

Method 3

Which Object is closer?

Moving my eye towards left

Method 3

Which Object is closer?

Moving my eye back to original position

Method 3

Which Object is closer?

Method 3

Method 3

It two objects j always y stay together, no matter which way and how far you move your eye, then these two objects must be at the same place!

Method 3

Using parallax to locate the untouchable image Placing marker too far away.

Method 3

Using parallax to locate the untouchable image

Placing marker too close to the mirror.

Method 3

Using parallax to locate the untouchable image

Marker at the right place.

Method 4

Location of an image g Point source

Point image Angle of incidence = Angle of reflection Note arrow direction!

Method 4

Location of an image g Point source

Equal distance

Line joining point source and point image is perpendicular to the mirror.

90o

Point image A real object j is always y in front of the mirror,, and its image is always behind the mirror.

Method 4 This is true even when the point object is not “directly” in front of the mirror: Point source

90o Equal distance

Point image

Method 4

Location of an image g Point source

90o

Drop a perpendicular from the point source to the mirror. mirror Extend the perpendicular behind the mirror. Make sure it is long enough.

Method 4

Location of image g (another ( method)) Point source

90o

Measure the distance between the ppoint source and the mirror, along the perpendicular.

Method 4

Location of image g (another ( method)) Point source

Equal distance

90o

Copy the same distance behind the mirror

Method 4

Location of image g (another ( method)) Point source

Equal distance

90o

Point image This is where the point image is.

Extensive object

Where is the image and how does it look like?

D

Extensive object

C

A B

Choose some “extreme extreme points” outlining the object. j

D

Extensive object

C

A B

B A C D

Locate the point image of these extreme points.

D

Extensive object

C

A B

B A C D

Image (note that it has the same size as the object).

Use of Extreme Rays – where you can see the image Point source Region g where yyou can see the point object in the mirror (i.e. the point image)

Use of Extreme Rays – where you can see the image Point source Region g where yyou can see the point object in the mirror (i.e. the point image)

Point image

D

Extreme rays y

C

A B

Region g where yyou can see the right head light (B) of the object car in the mirror.

D

Extreme rays y

C

A B

B A C D

Region g where yyou can see the right head light (B) of the object car in the mirror.

D

Extreme rays y

C

A B

Region where you can see only the right head light (B) of the object car in the mirror. Region where you can see only the left head light (A) of the object car in the mirror. Region where you can see both head lights (A and B) of the object car in the mirror.

D

Extreme rays y

C

A B

Region where you can see only the right head light (B) of the object car in the mirror.

B A C D

Region where you can see only the left head light (A) of the object car in the mirror. Region where you can see both head lights (A and B) of the object car in the mirror.