LIGHT: What Is It? Light Energy –Atoms As atoms absorb energy, electrons jump out to a higher energy level. Electrons release light when falling down to the lower energy level. –Photons - bundles/packets of energy released when the electrons fall. Light: Stream of Photons © 2000 Microsoft Clip Gallery

Electromagnetic Waves Speed in Vacuum –300,000 km/sec –186,000 mi/sec Speed in Other Materials –Slower in Air, Water, Glass © 2000 Microsoft Clip Gallery

Transverse Waves Transverse Waves Energy is perpendicular to direction of motion Moving photon creates electric & magnetic field –Light has BOTH Electric & Magnetic fields at right angles! © 2000 Microsoft Clip Gallery

Part 1 – Properties of Light Light travels in straight lines: Laser

Light travels VERY FAST – around 300,000 kilometres per second. 3x10^8 m/s At this speed it can go around the world 8 times in one second.

Light travels much faster than sound. For example: 1)Thunder and lightning start at the same time, but we will see the lightning first. 2) When a starting pistol is fired we see the smoke first and then hear the bang.

We see things because they reflect light into our eyes: Homework

Luminous and non-luminous objects A luminous object is one that produces light. A non-luminous object is one that reflects light. Luminous objectsReflectors

Shadows Shadows are places where light is “ blocked ” : Rays of light

Properties of Light summary 1)Light travels in straight lines 2)Light travels much faster than sound 3)We see things because they reflect light into our eyes 4)Shadows are formed when light is blocked by an object

Part 2 - Reflection Reflection from a mirror: Incident ray Normal Reflected ray Angle of incidence Angle of reflection Mirror

The Law of Reflection Angle of incidence = Angle of reflection In other words, light gets reflected from a surface at ____ _____ angle it hits it. The same !!!

Using mirrors Two examples: 1) A periscope 2) A car headlight

Colour White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow. We can demonstrate this by splitting white light with a prism: This is how rainbows are formed: sunlight is “ split up ” by raindrops.

The colours of the rainbow: Red Orange Yellow Green Blue Indigo Violet

Seeing colour The colour an object appears depends on the colours of light it reflects. For example, a red book only reflects red light: White light Only red light is reflected

A white hat would reflect all seven colours: A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue): Purple light White light

Using coloured light If we look at a coloured object in coloured light we see something different. For example, consider a football kit: White light Shorts look blue Shirt looks red

In different colours of light this kit would look different: Red light Shirt looks red Shorts look black Blue light Shirt looks black Shorts look blue

Refraction Refraction is when waves bend or slow down due to travelling in a different medium. A medium is something that waves will travel through. When a pen is placed in water it looks like this: In this case the light rays are slowed down by the water and are bent, causing the pen to look odd. The two mediums in this example are water and air. Words – speed up, water, air, bent

Corpuscular Theory of Light (1704) Isaac Newton proposed that light consists of a stream of small particles, because it –travels in straight lines at great speeds –is reflected from mirrors in a predictable way Newton observed that the reflection of light from a mirror resembles the rebound of a steel ball from a steel plate

Wave Theory of Light (1802) Thomas Young showed that light is a wave, because it –undergoes diffraction and interference (Young ’ s double-slit experiment) Thomas Young ( )

Waves versus Particles A particle is localized in space, and has discrete physical properties such as mass A wave is spread out over many wave-lengths in space, and could have amplitudes in a continuous range Waves superpose and pass through each other, while particles collide and bounce off each other

26 Whether light displays wave or particle nature depends on the object it is interacting with, and also on the experimental set-up to observe it If an experiment is set-up to observe the wave nature (such as in interference or diffraction experiment), it displays wave nature If the experimental set-up has a scale that is corresponding to the quantum nature of radiation, then light will displays particle behaviour, such as in Compton scatterings When is light wave and when is it particle?

Diffraction

Interference applet