LIGHT

WHAT IS LIGHT?

Light is a form of energy that travels away from the source producing it at a speed of 3 x 108 m s-1

Transparent: allows light to pass through it, and can see clearly through it e.g. glass

Translucent: allows light to pass through it, but cannot see clearly through it e.g. frosted glass

Opaque: does not allow light to pass through it e.g. wood

Light Travels in Straight Lines

This can be seen in the following examples:

LaserBeam of light from a searchlightIt can also be shown using pieces of

cardboard with a small hole in the middle and a length of thread

i r

Normal

Angle ofincidenc

e

Angle ofreflectio

n

Plane Mirror

Reflected ray

Incident ray

Plane Mirror

LAWS OF REFLECTION OF LIGHT

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

2. The angle of incidence is equal to the angle of reflection (i = r)

HOW IS AN IMAGE FORMED IN A PLANE MIRROR

Properties of an image in a plane mirror

Laterally invertedE.g. your right hand appears as a left

handThe “ambulance” sign

Erect (right way up)VirtualSame size as the object

Uses of Plane Mirrors

Make up mirrorThe periscope

A virtual image cannot be formed on a screen

A real image can be formed on a screen

Experiment to prove the angle of incidence equals the angle of reflection

i r

Plane mirror

Pins

Sheet of paper

Diagram

Experiment to prove the angle of incidence equals the angle of reflection

Method1. Set up the apparatus as in the

diagram.2. Mark the incident ray3. Mark the reflected ray4. Draw in the normal5. Measure angles i and r6. Repeat for different angles

Conclusion

Angle i = angle r

Precaution

Make sure the mirror is perpendicular to the page

Mark the back of the mirror on the paper

Use a sharp pencil

Reflection of light is when light bounces off a surface

Experiment to find the position of an image in a plane mirror

(Not a mandatory experiment)(Write up should be in homework copy)

Object pin

Plane mirror

Image (Object pin)

O M I

Method

1. Set up the apparatus as in the diagram

2. Move the tall finder pin in and out behind the mirror until there is no parallax between the finder pin and the image of the object pin in the mirror

3. Measure the distance from the object pin to the mirror (OM), and the distance from the mirror to the finder pin (MI)

Result

OM and MI are equal

Conclusion

The image is as far behind the mirror as the object is in front of it

Spherical Mirrors

Convex mirrors and concave mirrors

CONVEX CONCAVE

Radius of curvature

Focal lengthCentre of curvature

Pole

Focus point

Diagram of concave mirror

The line from the centre of curvature to the pole is called the principal axis

Rules for Ray Diagrams for Concave Mirror

1. A ray travelling parallel to the principal axis is reflected through the focus

2. A ray travelling through the focus is reflected parallel to the principal axis

3. For a ray which strikes the pole, angle i will be equal to angle r

“In parallel, out through the focus”

“In through the focus, out parallel”

Uses of concave mirrors

SpotlightsReflectors in car headlightsShaving and make-up mirrors

Pole

Diagram of convex mirror

Centre of curvature

Focus pointSHINY SIDE

Ray Diagrams for Convex Mirrors

Uses of convex mirrors

Shops (to deter shoplifters)BusesDangerous bends in roads

They give a wide field of view

The Mirror FormulaeThe focal length of a spherical mirror may be found using the formula:

fvu

111

fvu

111

fvu

111

u = distance from object to mirrorv = distance from image to mirrorf = focal length

Example 1

An object is placed 15cm in front of a concave mirror, of focal length 12cm. Find the position and nature of the image

12

11

15

1v

15

1

12

11

v

60

451 vfvu

111

60

11v

v = 60 cm

It is a real image since the object is outside f

Example 2

When an object is placed 16 cm in front of a concave mirror of focal length 8 cm, an image is formed. Find the distance of the image from the mirror and say whether it is real or virtual.

8

11

16

1v

16

1

8

11

v

16

121 vfvu

111

16

11v

v = 16 cm

It is a real image since the object is outside f

(HL)

Magnification

m =

m =

u

v

object ofheight

image ofheight

Example 3 (HL)

An object is placed 20 cm from a concave mirror of focal length 25 cm. Find the position, magnification and nature of the image.

25

11

20

1v

20

1

25

11

v

100

541 vfvu

111

100

11 v

v = 100 cm

It is a virtual image since the object is inside f

m =

m =

m = 5

u

v

u

v

Example 4 (HL)

A concave mirror of focal length 10 cm forms an erect image four times the size of the object. Calculate the object distance and its nature.

10

111vu

4u

vM

fvu

111

u = 7.5 cm

1

4

u

v

10

1

4

11

uu

10

1

4

14

u

10

1

4

3

uuv 4

304 u

It is a virtual image since the object is inside f

Experiment to Measure the Focal Length of a Concave Mirror

RAY BOX

CONCAVE MIRROR

SCREEN

CROSS THREADS

Method

(An approximate value for the focal length can be found by focusing the image of a distant object on a sheet of paper (e.g. a tree or window))

Set up the apparatus as in the diagram

Move the screen in and out until the image of the cross threads is in sharpest focus

Measure object distance (u) and image distance (v)

Repeat and calculate an average value of f using

fvu

111

Precautions

Measure v when the cross threads is in sharpest focus