Light

How fast does light travel?The SUN emits huge energy (solar). Light Energy: can be converted into electrical energy for businesses, homes etc. Sunlight (visible light) passes through a vacuum (void of matter) as electromagnetic waves to reach us.

Wave: disturbance that transfers energy from one point to another without transferring energy.

Wavelength: distance from similar points on a wave

Amplitude: wave height from rest position. Smaller amplitude = less energy

Frequency: rate of repetition of a wave. Higher frequency = more energy. Measured in hertz (Hz) – cycles/second.

Frequency and Wavelength:

v = f x λ

-As frequency increases, wavelength decreases-As frequency decreases, wavelength increases

Ex: if wavelength is 10 cm and frequency is 5 cycles/sec then the speed is 50 cm/s.

Electromagnetic Radiation: wave pattern of electric and magnetic fields travelling through empty space.

Spectrum includes shortest gamma rays to longest radio waves.

Radio waves: longest wavelength and lowest frequency-Used for radio, TV signals, cellphones, MRI, and satellite communications.

Microwaves: water molecules in food vibrate and heat up food. Also in radars for speed, through Earth’s atm. to a satellite.

Infrared: any heat. Thermograms produce images. Used in burglar alarms, night vision goggles etc.

UV rays: Sun. Also used to disinfect drinking water and waste water. DNA analysis for investigations.

X-rays: medical imaging, scan luggage at airports

Gamma rays: extremely high radiation that can penetrate human tissues. Sterilize medical equipment, kill cancer cells etc.

Wave Model of Light

Visible Spectrum: light separates into colours of the rainbow.

Wave model of light can be explained using wave model. Each colour has a different wavelength and frequency.

Additive Colour of Light

• White light is composed of different colours of light

• Primary colours can be mixed to form all other colours (yellow, cyan, magenta = secondary colours) and white light.

• Complementary colours of light are the two colours that mix together to form white light.

•Magenta + green

•Yellow + blue

•Red + cyan

Subtractive Colour Theory

The pigment absorbs (subtracts) certain colours of light and reflect/transmit other colours (i.e. blue jeans?)-Black objects absorb all light.-Colours are opposite those of the ADDITIVE colour theory

Primary subtractive colours:Magenta, cyan and yellow are primary colours and red, green and blue are secondary colours – formed by mixing two primary colours of pigment.

PIGMENT: powder used to colour substances

Objects in coloured light:

Uses for subtractive colour theory: traffic lights, colour printing – what cartridges do you use?

Visible Light

Man-made (light bulb, TV screen etc.) or natural (sun, fire, fireflies etc.).

Luminous: produce and emit light

1. Incandescence: process where substances produce light when heated to very high temperatures. Very inefficient: only 5% of energy is light

2. Fluorescence: light emitted by substance when exposed to electromagnetic radiation.

– Ex: fluorescent light bulbs: coated glass (with phosphor – glows after exposure to energy) tube filled with Hg and Ar gas. When turned on, current goes through tube = invisible UV light. Phosphor absorbs UV light and converts it to visible light.

• Still only 20% efficient

3. Phosphorescent: can store energy from radiation. Ex: glow in the dark toys

4. Chemiluminescence: light stick = production of light from a reaction between two chemicals. Investigators use chem. Luminol to detect blood because it reacts with iron.

Bioluminescence: when chemiluminescence occurs in organisms (fireflies).

5. Triboluminescence: light from friction. Ex: wintogreen mints

6. Electric Discharge: current passes through gas or air. Ex: Neon sign.

7. LED: when electric current passes through LED = lightSave energy, cooler, last longer than any other. Faster.OLEDs: organic molecules are used.

8. Plasma Display: each colour is a tiny fluorescent light where an electrical signal causes a gas (Ne) to release UV radiation that is absorbed by phosphors = light.

9. Liquid Crystal Display: laptops, iPods etc. A white light shines behind a liquid crystal (a solid that can change like liquid with energy).

Human Vision

• At the back of the eye – RETINA– Has two light-sensitive cells

1. Rods: detect low levels of light (night vision). Once adapted can see shapes/movement but light interferes (NOT red light).

2. Cones: 3 types to see colour (red, green, blue). Yellow light is brightest (fog lights).

• Colour blindness? Pg. 436

•Digital cameras, computer monitors, TV use this info – pixels are tiny luminous dots – more pixels = better image.

11.5 The Colour of Objects

• Light transmission: process of light traveling through a medium.

1. Transparent: allows light to pass through (window, air)

2. Opaque: no light can pass (door etc.) Gather light energy via absorption. If object is smooth, shiny (i.e. mirror) light bounces off = reflection.

3. Translucent: transmit some light and reflect rest. Allows light through but scatters it.

• Pigments absorb some colours of light and reflect others (the colours your eyes see). Ex: plants have pigment chlorophyll etc. Inks, dyes, paints etc. for many uses.

The Reflection of Light – Ch. 12

Ex: Mirrors on the moon…

Light Rays and Reflection

Ray model of light states a RAY is represented by a single straight arrow of light. BEAM is many.

Plane mirror: flat mirror with 2 layers (front transparent, back opaque). A ray diagram shows incident ray (strikes the mirror) and reflected ray. Normal allows measurement of angles formed by rays.

Recall geometry… perpendicular, parallel, 90 and 360 degree angles.

Plane Mirrors

Laws of Reflection:

1. Angle of reflection is always equal to the angle of incidence (fig 2a on pg. 466).

2. The incident ray, normal, and reflected ray all lie in the same plane (fig 2b on pg. 466).

REGULAR REFECTION: when light strikes a smooth surface.

Light is also reflected off of rough surfaces (i.e. concrete, paper, clothing etc.) but the rays strike the bumps at different angles of incidence = DIFFUSE REFLECTION.

What about a wet road?

Applications: reflective clothing, bumpy windshields etc.

Image: A picture of an object produced by light. Described by Size, Attitude (upright or upside down), Location and Type (Real: light rays reach the image or Virtual: Light rays don’t reach). SALT

Plane mirror images are always virtual images – looks like they are behind the mirror

WHY?

Images in plane mirrors are always virtual, the same size as the object, upright, located behind the mirror. Fig. 10 on page 471

Converging Mirrors

2 types of Curved Mirrors:

1. Converging: shaped like inside of sphere (concave)

• Shape determines how rays are reflected from it

• Small or large

• Centre of curvature (C): point at centre of sphere with same curvature as curved mirror

• Vertex: point where a line (principal axis) from the centre of curvature meets the centre of the mirror

• Solar cookers (Fig. 5 on pg. 475).

1. Diverging: shaped like outside of sphere (convex)