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# LIGHT - PowerPoint PPT Presentation

LIGHT. Everything written in black has to go into your notebook Everything written in blue should already be in there. WHAT IS LIGHT?. Light is a form of energy that travels away from the source producing it at a speed of 3 x 10 8 m s -1.

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## PowerPoint Slideshow about ' LIGHT' - eli

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Presentation Transcript

### LIGHT

Everything written in black has to go into your notebook

Everything written in blue should already be in there

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

Light Travels in Straight Lines clearly through it e.g.

Light travels in straight lines. This can be seen in the following examples

• Laser

• Beam of light from a searchlight

It can also be shown using pieces of cardboard with a small hole in the middle and a length of thread

Plane Mirror (diagram on page 1) clearly through it e.g.

Normal

Incident ray

Reflected ray

Angle ofincidence

Angle ofreflection

i

r

Plane Mirror

LAWS OF REFLECTION OF LIGHT clearly through it e.g.

• 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)

Properties of an image in a plane mirror clearly through it e.g.

The image is:

• Laterally inverted

• E.g. your right hand appears as a left hand

• The “ambulance” sign

• Erect

• Virtual

• Same size as object

Uses of Plane Mirrors clearly through it e.g.

• Make up mirror

• The periscope

Diagram page 3 clearly through it e.g.

Diagram on page 26 of reflection (page 26)

Plane mirror

Sheet of paper

r

i

Pins

Diagram (in homework copy) of reflection (written up in homework copy)

Finder pin

Plane mirror

Object pin

O

M

I

The following goes in your homework copy of reflection (written up in homework copy)

• Method

• Set up the apparatus as in the diagram

• Move the finder pin in and out behind the mirror until there is no parallax between the object and its image in the mirror

3. Measure the distance from the object to the mirror (OM), and the distance from the mirror to the image 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 (page 4) and the distance from the mirror to the image pin (MI)

CONVEX

CONCAVE

Rules for Ray Diagrams for Concave Mirror the principal axis

• 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

Top of page 5 the principal axis

• “In parallel, out through the focus”

• “In through the focus, out parallel”

Uses of concave mirrors the principal axis

• Spotlights

• Shaving and make-up mirrors

Uses of convex mirrors the principal axis

• Shops (to deter shoplifters)

• Buses

• They give a wide field of view

The Mirror Formulae the principal axis

u = distance from object to mirror

v = distance from image to mirror

f = focal length

Example 2 the principal axis

• 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.

v = 16 cm the principal axis

Magnification the principal axis

• m =

• m =

Example 3 (HL) the principal axis

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

v = 100 cm the principal axis

• m = the principal axis

• m =

• m = 5

Example 4 (HL) the principal axis

• 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.

u = 7.5 cm the principal axis

RAY BOX

CONCAVE MIRROR

SCREEN

Diagram page 30

### Light (2) Refraction and Lenses (page 30)

Incident ray one optical medium to another

i

r

Refracted ray

Glass block

(Page 12, under diagram) one optical medium to another

• Less dense to more dense: bends towards normal

• More dense to less dense: bends away from normal

The Laws of Refraction of Light one optical medium to another

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

• 2. where n is a constant

• This is called Snell’s Law

Experiment to Verify Snell’s Law and determine the refractive index of glass (diagram page 27)

Pins

Glass Block

Sheet of paper

Your graph in page 28 should look like this the corresponding graph underneath

Sin i

Sin r

Real and Apparent Depth (page 12) the corresponding graph underneath

• A swimming pool appears to be less deep than it actually is, due to refraction at the surface of the water

• We can calculate the refractive index of a liquid by using

n =

Critical angle the corresponding graph underneath

• The critical angle is the angle of incidence in the denser medium when the angle of refraction is 90˚

Total Internal Reflection the corresponding graph underneath

• This occurs when the angle of incidence in the denser medium exceed the critical angle

• The ray of light is refracted away from the normal

• As i is increased so is r

• Eventually r = 90˚

• At this point i has reached the ‘critical angle’

• If i is increased beyond the critical angle, the ray does not enter the second medium

• It is reflected back into the first medium

C = critical angle

Example the corresponding graph underneath

The critical angle of glass is 41.81˚

Find the refractive index of glass

• n =

• n = 1/0.666

• n = 1.5

n = the corresponding graph underneath

Applications of Total Internal Reflection the corresponding graph underneath

• Periscopes (using a prism)

• Diamonds and bicycle reflectors

• Optical fibres – in telecommunications and by doctors

PERISCOPE (diagram page 14) the corresponding graph underneath

Total internal reflection in a prism the corresponding graph underneath

Total internal reflection in a prism the corresponding graph underneath

Total internal reflection in a prism the corresponding graph underneath

A the corresponding graph underneath

AIR

GLASS

B

• Remember that rays are path-reversible

Example the corresponding graph underneath

• The refractive index of glass is 1.5

• This value is for a ray of light travelling from air into glass

• So = = 1.5 =

• Or = =

Mirages the corresponding graph underneath

• Mirages are caused by the refraction of light in air due to temperature variations

SKY the corresponding graph underneath

LENSES the corresponding graph underneath

• Convex lens (converging)

Ray diagrams for lenses the corresponding graph underneath

• 1. Ray incident parallel to principal axis is refracted out through focus

• 2. Ray incident through focus is reflected out parallel to axis

• 3. Ray incident through optic centre continues in straight line

Lens formulae the corresponding graph underneath

u = distance from object to lens

v = distance from image to lens

f = focal length

Magnification the corresponding graph underneath

• m =

• Or m =

RAY BOX (page 29)

CONVEX LENS

SCREEN

(Diagram page 29)

Two Lenses in Contact (page 29)

Where F = focal length of combination

f1 and f2 are the focal lengths of the two lenses

Spectrum of Visible Light constituent colours

R

O

Y

G

B

I

V

Red is deviated the least and has the longest wavelength

Violet is deviated the most and has the shortest wavelength

Uses of lenses constituent colours

• Magnifying glass

• Spectacles

• Binoculars

• Compound microscope

• Astronomical telescope

F constituent colours

F

Magnifying glass/Simple Microscope

• Is simply a convex lens, with the object placed inside the focus point

• Image is magnified, erect and virtual

The Compound Microscope constituent colours

Eyepiece

Objective lens

Fo

Fe

The compound microscope constituent colours

• Consists of 2 convex lenses

• The first image is formed at the focal point of the eyepiece

• The final image is formed at infinity so we view it with a relaxed eye

• This is called ‘normal adjustment’

• The image formed is inverted

The Astronomical Telescope constituent colours

Objective lens

Eyepiece

Fe

Fo