Light In this unit: 1)Properties of light 2)Reflection 3)Refraction 4)Absorption 5)Colors

Properties of Light Light travels in straight lines called rays: Laser

Light travels VERY FAST – around 300,000 kilometres per second. 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 gun is fired we see the light first and then hear the bang.

Transmission Occurs when waves pass through a given point or medium. example: Sound waves transmitted thru solids, liquids and gases. Radio waves are transmitted through one electron through another.

Light waves are transmitted – 3 ways light and matter can pass through material. A. Transparent material: Only a small amount of light is reflected or absorbed (may be clear or colored material such as filters, windows). B. Translucent materials: Transmit some light, but cause it to be scattered so no clear image is seen. C. Opaque material: allows no light waves to be transmitted through them.

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

Reflection Light rays bounce off a surface. 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 The same !!!

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

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

Clear vs. Diffuse Reflection Smooth, shiny surfaces have a clear reflection such as a mirror Rough, dull surfaces have a diffuse reflection. Diffuse reflection is when light is scattered in different directions. To a piece of paper, light is reflecting diffusely

Reflection and mirrors Objects that are in front of a mirror appear as if they are behind the mirror. This is because light rays are reflected by the mirror. Your brain perceives the light as if it always traveled in a straight line.

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

Refraction Refraction is when waves speed up or slow down due to travelling in a different mediums. 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.

How do prisms separate visible light? When light shines through a prism, the glass changes the behavior of each light wave according to the wave's wavelength. This property of glass is known as its refractive index. The prism changes the direction the light is traveling. As a result, what goes in as ordinary white light comes out separated into a spectrum of different colors. That's because white light is really all those wavelengths mixed together.

The Rainbow The rainbow takes the concept of dispersion and multiples it through the atmosphere The sun shines on water droplets in a cloud or when it is raining The light is dispersed by the raindrop into its spectral colors

The Rainbow

Atmospheric Refraction On hot days there may be a layer of very hot air in contact with the ground, the light will travel faster through this air and will bend, creating a mirage When you watch the sun set, you can still see the sun for several minutes after it has sunk below the horizon, because light is refracted by Earth’s atmosphere

Atmospheric Refraction

Absorption - Occurs when the energy is not transferred through or reflected by the given medium Disappearance of an electromagnetic wave into a medium. –Opposite of reflection We see colors because of the selective absorption of visible light. –Objects absorb certain wavelengths and we see what is leftover. –The colors we see depend on the wavelength absorbed.

Absorption Orange YellowGreen Blue Violet Red The apple absorbs all wavelengths except red. The apple reflects mostly red. Therefore, it looks like red.

Absorption A material that absorbs all wavelengths of visible light will appear black. In contrast, a material that reflects all wavelengths will appear white. Objects or substances that absorb any wavelength of EM radiation become warmer and convert the absorbed energy to infrared radiation. (GREENHOUSE EFFECT)

Color White light is not a single color; it is made up of a mixture of the seven colors 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 colors of the rainbow: Red Orange Yellow Green Blue Indigo Violet

Adding colors White light can be split up to make separate colours. These colors can be added together again. The primary colors of light are red, blue and green: Adding blue and red makes magenta (purple) Adding blue and green makes cyan (light blue) Adding all three makes white again Adding red and green makes yellow

Color and Vision We can think of different colors of light like balls with different kinetic energies. Blue light has a higher energy than green light, like the balls that make it into the top window. Red light has the lowest energy, like the balls that can only make it to the lowest window.

Each of these colors actually corresponds to a different wavelength of light. Visible Light

Seeing color The color an object appears depends on the colors 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 colored light If we look at a colored object in colored light we see something different. For example, consider a football kit: White light Shorts look blue Shirt looks red

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

Some further examples: ObjectColor of light Color object seems to be Red socks Red BlueBlack GreenBlack Blue teddy RedBlack Blue Green Green camel Red Blue Green Magenta book Red Blue Green

Using filters Filters can be used to “block” out different colours of light: Red Filter Magenta Filter

Investigating filters Colour of filterColours that could be “seen” Red Green Blue Cyan Magenta Yellow

Red Magenta White Yellow BlueGreen Cyan