What do you know about light? Light carries energy Travels in straight lines Does not need a medium to travel through Exists in a wide variety of forms, defined as the Electromagnetic spectrum. Nothing can move faster than particles of light!

The Speed of Light How long does it take for light to fill a classroom once the light switch is flipped? Almost instantly! This is because the speed of light is 3.0 x 10^8 m/s in a vacuum! This is very fast!! How long does it take for light to reach Earth from the Sun? Try it! The Earth is 1.5 x 10^11 meters from the Sun

Speed of light: constant? Does light always travel at “c”, or 3.0 x 10^8 m/s? Light travels very close to c in air, so this is a good approximation What happens when light hits an object? Objects can be Transparent, Translucent or Opaque If light passes into an object, it slows down (for example, glass) When light slows down, we get refraction, but that’s getting ahead of ourselves!

Propagation of light Unlike sound, light does not require a medium (“something”) to travel through. It used to be thought that, in order to travel through space (a vacuum), light required the existence of an “ether”. An “ether” was a medium that existed in outer space, through which light could travel. We know now that there is no such thing as an “ether” – space really is a vacuum!

The Electromagnetic Spectrum You may be familiar with X-rays and Ultra-violet rays from dentist visits, broken bones or worries about sun burns. Did you realize they were forms of light?

The ElectroMagnetic Spectrum Notice how small the visible light section is! We only see a small portion of reality! Notice energy increases to the right of the visible spectrum.

The EM Spectrum Using instruments, we can “see” other areas of the EM Spectrum and understand the universe better.

Chandra: looking at the X-rays of the Universe Chandra, a space telescope, looks at the x-ray emissions of galaxies such as this one, to see features undetectable at the visible wavelengths. In this case, this is a Pulsar in the Crab nebula, shooting out streams of matter and anti-matter.

Infra-red The section of the EM spectrum given off as “heat”

Spitzer Space Telescope: Infrared SST examines the universe through an infrared lens. In this case, it is looking at the heat (IR) radiation given off by the Fireworks Galaxy

SST IR viewpoint reveals a large new ring of Saturn!

Microwaves In addition to being useful for cooking, microwaves have allowed astronomers to examine the very beginning of the universe, which carries a residual microwave “glow”.

Fill in a chart for the EM Spectrum! Include each area of the spectrum (Radio, Micro, IR, Visible, UV, X-ray, Gamma) Also include where each type of light is found (examples)

Sources of light Can you distinguish between “luminous” and “non- luminous” objects? Which is the Moon? What sources of light can you think of? We can divide sources of light into artificial and natural – what are examples of each?

Types of light sources Incandescent Is caused by heating up a material

Florescent Light Sources Florescent lights are more efficient than incandescent They are, however, more complex and do not render colour well. They also create a “beat effect”

How florescent lights work

Florescent Light Sources

How Florescent Light Sources Work

Phosphorescent Light Sources Similar to florescence, but is slower Many glow-in-the-dark items are examples – they need to “charge up” or absorb light energy for a while before emitting again.

Chemiluminescent Light Source Artificial example is glow sticks Bioluminescence is a natural form

Electric Discharge

Light Emitting Diodes Much more efficient than incandescent or florescent lights! However, they are more expensive and complicated, relying on semiconductors.

Lasers How are lasers different from other light sources? Word is an acronym/description: Light Amplification by Stimulated Emission of Radiation They are a single frequency of light Much more powerful How can we use a laser to calculate the distance to the Moon? If we know... From experiment.. That a laser pulse takes 2.56 seconds to return to us, after bouncing off the moon.

Lasers Applied to make Holograms!

Calculations with light c = 3.0 x 10^8 m/s OR c = m/s Velocity = distance/time c = d/t for light in a vacuum

What is light? Particle Theory: light is made of individual particles called photons. These photons carry energy. Particle theory explains the photoelectric effect – release of electrons dependant on frequency of light, not intensity. Wave Theory: light moves like a wave, and can bend around corners in some circumstances Wave theory explained double-slit experiment The Truth: it is both at once (particle-wave duality)

Prisms & Colours White sunlight can be separated by a prism into 7 colours. Can you name all 7 colours of the rainbow? What colour is found below? Why is it so difficult for people to recognize indigo? What colour do people perceive the best? Why might this be?

Prisms & Colours! How do prisms work? Notice which colour is on top... (bend least while in the prism)

Ray Diagrams A “light ray” is a line and arrow representing the direction and straight-line path of light. Combinations of light rays can be used to construct a “ray diagram”. Such ray diagrams describe the behaviour of light in a certain circumstance (e.g. Being emitted from a light bulb, reflected off a mirror, or refracted into water)

Reflection Reflection occurs when light bounces off an object. This follows two laws of reflection: The angle of incidence is equal to the angle of reflection The incident ray, the reflected ray, and the normal line all lie in the same plane.

Reflection: surfaces Smooth surfaces create a mirror-like effect called “Spectacular reflection”:

Reflection off rough surfaces Diffuse reflection results from the reflection of light off an irregular surface.

Reflection: surfaces Reflection is how we see the world Example of laser: can’t see it until use dust! Examine light ray diagrams of smooth and rough surfaces: All individual incident rays will still follow the laws of reflection

Images in Mirrors The image you see in the mirror isn’t real.... It’s “VIRTUAL” How far away does the “virtual” you appear to be compared to the real you and the mirror? Try creating a light ray diagram of yourself looking in the mirror.