LightReflection and Refraction

Learning OutcomesOP33 understand that light is a form of energy

and that it can be converted to other forms

OP34 show that light travels in straight lines and explain how shadows are formed

OP35 understand that luminous objects are a source of light while non-luminous objects are seen as a result of light reflected from them

Lessons

Learning Outcomes

OP36 Recall that white light is made up of different colours which can be separated by dispersion

OP37 Produce a spectrum of white light using appropriate apparatus, and list the colours of the spectrum.

Lessons

Learning OutcomesOP38 investigate the reflection of light by plane

mirrors, and illustrate this using ray diagrams; demonstrate and explain the operation of a simple periscope

OP39 show the refraction of light as it passes from: air to glass, air to water, glass to air, water to air; show refraction of light through a lens; demonstrate the operation of a magnifying glass

Lessons

Light is a Form of Energy

• ‘the’ source of energy for plants

• Indirect source of energy for animals

Light Energy

Conversion of Light Energy

Light kinetic energy

Crookes Radiometer

Light Electrical Energy

Luminous/Non-Luminous Bodies

• An object is seen when light from it reaches eyes

• Luminous objects give out light

• Non-luminous objects don’t give out light

• How can we see non-luminous objects?

• Sources of light – luminous objects– Sun, stars, candle, lamp, fire and other chemical reactions

Seeing Non-Luminous Objects

luminous object

Non-luminous

object

Transmission of Light

• Light travels in straight lines

• Can you give examples?

Grand Central Station, New York

To Show That Light Travels in Straight Lines

Mandatory Practical

light bulb

cardboard

thread/string

To Show the Formation of Shadows

Mandatory Practical

Torch

White Sheet/Cardboard

Object

This also shows that light travels in straight lines.

Light Speed

• 300 million metres per second (3x108 m/s)

• Sun Earth (150million miles)– takes ~8 minutes

• Light year – distance light travels in 1 year– How many seconds in a year?

• Answer: 9,460,000,000,000km

Reflection

• Practically all surfaces reflect light– Shiny smooth surfaces have regular reflection

– Matt (dull) surfaces have scattered reflection

Uses of Reflection

• What are some uses of reflection?– Mirror, periscope (used in submarines)

mirror at 45o angle

Periscope

To show reflection of light by a plane mirror and to demonstrate the operation of a periscope

Mandatory Practical

light box

bulb inside box

sheet

mirror

light box

From the top

mirror

pencil mark

Refraction of Light

Refraction is the bending of light from one medium to another

• Why do objects appear contorted under water? • Why does a swimming pool look shallower than it really is?

• Light travels in straight lines through air, glass, water etc.• BUT• When it passes from one to the other it changes direction.

Refraction Demonstration

light box

sheet

glass block

refraction fromair to glass occurs here

Plan view of experiment to demonstrate refraction

Lenses

• Simple optical devices• Curved surfaces use refraction to focus• Convex/converging lens (fat in the middle)

• Concave/diverging lens (thin in the middle)

Example: magnifying glass

Example: lenses of short sighted people

Spectrum of White Light

Spectrum of Light

• Where do the colours of the rainbow come from?• Are they always the same colours?• YES!

• The name given to the splitting up of white light into its separate colours is dispersion

Colour SpectrumRed, Orange, Yellow, Green, Blue, Indigo and Violet

Producing a Colour Spectrum

light box

bulb inside box

paper or card

prism

screen

Colour SpectrumRed, Orange, Yellow, Green, Blue, Indigo,

Violet

Homework Suggestions• Why is the sky blue?• Why are sunsets orange?• Why is sunburn more likely to occur if you are up a high mountain or at the

beach?• How do photosensitive sunglasses work?• Why are soap bubbles coloured in white light?• Why are compact discs coloured?• Why are sunglasses of particular benefit to fishermen?• How would a rainbow appear to passengers in an aeroplane when there is

rain in the air above and below them?• Why do materials purchased in a shop appear a different colour in daylight?• How does the eye see colour?• What causes after images and what colour are they?• If black is a good absorber of heat, why do Bedouins wear black robes in

the hot desert? • Why is the filament of an electric light bulb coiled?• Why is a firefly a more efficient light producer than a filament lamp?

Light Cloze Test A

• Light is a form of ______. In a piece of apparatus called a ______, light energy is converted to ______ energy, and in the process of photosynthesis it is converted to ______ energy. Animals get energy ______ from this process. Light travels in ______ lines, but it can be made to change direction in two ways by ______ and ______. When light strikes a mirror it is ______ and when it enters a glass block it is ______.

Light Cloze Test B

• Light, on being passed through a ______, is split up into its component colours. This process is known as ______ and the band of colours which is formed is called a ______. The colours are red, ______, ______, ______, ______, ______, ______. When all of these colours are recombined, ______ light is produced. This property of light was discovered by a scientist named ______. An example of real-life dispersion is a ______.

Light Cloze Test C• An experiment to show that light travels in ______ lines

includes a light box, ______ sheets with a ______ in the centre, a piece of ______ used to align the sheets and a ______ as a source of light. If the sheets are not-aligned the viewer sees ______ and if they are aligned the viewer sees ______.

• To see the effect of shadows you require:______, ______ and a ______

When the torch is turned on in a darkened environment the dark______ of the object appears on the ______.

• A light box, sheet, screen and a ______ ______ are required to show refraction from ______ to glass. To indicate ______, a triangular glass block known as a ______ is required which disperses the light and produces a ______ ______.

Light Cloze Test D

• A ______ glass is an example of a convex or ______ lens. This causes _____ rays to ______ at a point. The opposite type of lens is ______ in the middle and is know as a ______ lens. This causes light rays to ______ and is used in the glasses of ______ sighted people.

• If confronted by a tall wall, a ______ would enable you to see over it. They are more commonly found in ______ to see ______ the water surface. This works by placing ______ mirrors at an angle of ______.

Resources

• Images• http://acept.la.asu.edu/PiN/mod/light/reflection/pattLight1Obj1.html• http://science-education.pppl.gov/SummerInst/auroraborealis.jpg

• Information– Diagram & Info on Crooks Radiometer

• http://www.teachersparadise.com/ency/en/wikipedia/c/cr/crookes_radiometer.html

– Did you know questions for light• www.juniorscience.ie/jsss/Main/3B3R.htm

– Syllabus information• www.education.ie

– Info on light, reflection & refraction• http://acept.la.asu.edu/PiN/mod/light/reflection/pattLight1.html • Science Today, Physics Textbook

• Sounds– Farm noises

• http://www.grsites.com/sounds/farm001.shtml