Reflection of light

1 You can have a view of yourself if you look into

a mirror.

calm water surface.

a piece of shiny metal wall.

a piece of window glass.

2 Now you look into a mirror and see the image of yourself.

In front of the mirror.

On the surface of the mirror.

Behind the mirror.

3 Alex says, ‘Your mirror image is smaller than yourself’. Do you agree?Yes, the image is really smaller.

No, your image and you are always of the same size. It looks smaller just because it is far from you.

No, it depends on how far you stand from the mirror.

Others: ________.

A ray of light bounces off a plane mirror.

mirror

This is an example of reflection of light.

1 Laws of reflection

useful words to describe the reflection of light

reflected rayincident ray

normal

mirror

angle of incidence

angle of reflection

1 Laws of reflection

Laws of reflection

Video Simulation

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

normal

incident ray reflected ray

mirror

normal

incident ray reflected ray

mirror

Laws of reflection

normal

incident ray reflected ray

mirror

• angle of incidence angle of reflection =

Laws of reflection

Regular reflection

parallel incident rays

parallel reflected rays

• on a flat, smooth surface• e.g. mirror

2 Regular and diffuse reflection

Diffuse reflection

• on a rough, not perfectly smooth surface

• e.g. cover of a book

parallel incident rays

reflected rays in different directions

2 Regular and diffuse reflection

Q1 The angle between the…

The angle between the incident ray and the _______ is called the angle of incidence.normal

The laws of reflection state that it is equal to the angle of ___________.reflection

Q2 The angle between an…

The angle between an incident ray and the mirror is 20.The angles of incidence and reflection are ____ and ____.

The mirror turns the ray by ____ .

70 70

140

2070 70

Q3 True or false: Reflection of…

True of false: Reflection of light by a rough surface does not obey the laws of reflection.

(T/F)

Images formed by a plane mirror

Video

Rays from object O become divergent after reflection.

3 How a plane mirror forms an image

mirror

O

• I is a virtual image.– cannot be picked up on a screen

Reflected rays appear to come from I behind the mirror.

mirror

O

3 How a plane mirror forms an image

I

• by geometric construction

O I

virtual rays shown by dotted lines

Locating the position of the imageLocating the position of the image

3 How a plane mirror forms an image

• OI⊥MN

N

M

• ON = NI

O IThe ray diagram shows that

Locating the position of the imageLocating the position of the image

3 How a plane mirror forms an image

Locating the position of the imageLocating the position of the image

3 How a plane mirror forms an image

Simulation

3 virtual

1 behind the mirror

2 of same size as the object

4 laterally inverted

3 How a plane mirror forms an image

Properties of the image formed by a plane mirror

mirror

object

Fimage

3 How a plane mirror forms an image

Properties of the image formed by a plane mirror

Simulation

Example 1A device consists of 2 mirrors at right angles to each other.Complete the path of the ray.

M1

45

M2

4545

The ray is turned back .

parallel but

opposite in direction

parallel but

opposite in direction

Example 2A girl can just see her feet at the bottom edge of the mirror.

Her eyes are 10 cm below the top of her head.

150 m

150 m

(a) What is the distance between the girl and her image in the mirror?

Example 2(a) Image distance

= Distance = 150 2 = 300 cm

Object distance

Example 2(b) How high above the floor is the

bottom edge of the mirror?

h1

2

h1

2

Example 2(b) How high above the floor is the

bottom edge of the mirror?By laws of reflection, 1 = 2

= 70 cm

The bottom edge of the mirror is 70 cm above the floor.

h = 140

12

Example 2

(c) What is the minimum height of the top edge of the mirror if she is able to see the top of her head?

(c) What is the minimum height of the top edge of the mirror if she is able to see the top of her head?

Example 2

3

4

h1

2

10 cma

b

By laws of reflection, 3 = 4

= 5 cm

Minimum height of top edge of the mirror

b = a = 1012

= b + 140

=145 cm above the floor

140 cm

Q1 Which of the following…Which of the following descriptions about the image formed by a plane mirror is INCORRECT ?

A Light rays come from the image to our eyes.

B The image cannot be projected on a screen.

C It is as far away from mirror as the object is in front.

D It is virtual.

Q2 Leon and Partick look in a...Leon and Patrick look in a plane mirror placed directly in front of Leon.Can they see themselves and each other in the mirror?

Leon can see himself but Patrick cannot. Both of

them can see each other

Q3 True or false: If the boy...

True or false: If the boy walks closer to the plane mirror, the image position of the candle will change.

(T/F)

Q4 A boy of height 1.5 m stands...

A boy of height 1.5 m stands 5 m in front of a plane mirror.

His image is ______ m tall and ______ m from him.

1.5 10

Q5 The ray diagram shows the...The ray diagram shows the extension of one of the reflected rays.Locate the image by completing the ray diagram.

Q6 Jenny views herself in a...Jenny views herself in a full-length mirror.

(a) Construct lines from the images of Jenny’s feet and top of her head to her eyes.

Q6 Jenny views herself in a...Jenny views herself in a full-length mirror.

(b) Mark on the mirror to indicate minimum region Jenny uses to see her full image.

Q6 Jenny views herself in a...

(c) Does this region of the mirror depend on Jenny’s distance from the mirror? (Yes/No)

Jenny views herself in a full-length mirror.