LIGHT

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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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### 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 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. aluminium
Light Travels in Straight Lines

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)

Normal

Incident ray

Reflected ray

Angle ofincidence

Angle ofreflection

i

r

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)
Properties of an image in a plane mirror

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
• Make up mirror
• The periscope
A virtual image cannot be formed on a screen
• A real image can be formed on a screen

Diagram on page 26

Plane mirror

Sheet of paper

r

i

Pins

Experiment to prove the angle of incidence equals the angle of reflection (written up in homework copy)

Diagram (in homework copy)

Finder pin

Plane mirror

Object pin

O

M

I

The following goes in your 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

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
Top of page 5
• “In parallel, out through the focus”
• “In through the focus, out parallel”
Uses of concave mirrors
• Spotlights
• Shaving and make-up mirrors
Uses of convex mirrors
• Shops (to deter shoplifters)
• Buses
• They give a wide field of view
The Mirror Formulae

u = distance from object to mirror

v = distance from image to mirror

f = focal length

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.
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.
m =
• m =
• m = 5
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.

RAY BOX

CONCAVE MIRROR

SCREEN

Diagram page 30

### Light (2) Refraction and Lenses

Refraction of light is the bending of light as it goes from one optical medium to another
• A medium is a substance; e.g. glass, air etc.

Incident ray

i

r

Refracted ray

Glass block

(Page 12, under diagram)
• Less dense to more dense: bends towards normal
• More dense to less dense: bends away from normal
The Laws of Refraction of Light
• 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

Real and Apparent Depth (page 12)
• 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 critical angle is the angle of incidence in the denser medium when the angle of refraction is 90˚
Total Internal Reflection
• 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 critical angle of glass is 41.81˚

Find the refractive index of glass

• n =
• n = 1/0.666
• n = 1.5
Applications of Total Internal Reflection
• Periscopes (using a prism)
• Diamonds and bicycle reflectors
• Optical fibres – in telecommunications and by doctors

A

AIR

GLASS

B

• Remember that rays are path-reversible
Example
• 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
• Mirages are caused by the refraction of light in air due to temperature variations
LENSES
• Convex lens (converging)
Ray diagrams for lenses
• 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

u = distance from object to lens

v = distance from image to lens

f = focal length

Magnification
• m =
• Or m =

RAY BOX

CONVEX LENS

SCREEN

(Diagram page 29)

Two Lenses in Contact

Where F = focal length of combination

f1 and f2 are the focal lengths of the two lenses

Spectrum of Visible Light

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
• Magnifying glass
• Spectacles
• Binoculars
• Compound microscope
• Astronomical telescope

F

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

Eyepiece

Objective lens

Fo

Fe

The compound microscope
• 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

Objective lens

Eyepiece

Fe

Fo