Light as a Wave Part 2 SNC2D

The Speed(s) of Light Light will travel slower in denser media (e.g. glass), but the speed in air is still effectively 3.0 x 108 m/s and

The Spectrum The wavelength (and therefore the frequency) of a light wave will depend of the scale of the disturbance in the EM fields.

The Spectrum Visible light is actually a very small part of the EM spectrum:

The Spectrum Very short (i.e. gamma ray) wavelengths would be generated by very small-scale transitions (i.e. in the nucleus).

Wavelength and Frequency Short wavelengths are associated with high frequencies:

Wavelength and Frequency Short wavelengths are associated with high frequencies:

X-rays Still-short wavelength x-rays are useful for diagnostic imaging.

Ultraviolet Ultraviolet light rays, originally called deoxidizing or chemical rays for their chemical reactivity, can cause skin cancer.

Infrared Infrared light is felt as heat. You are emitting infrared radiation.

Microwaves and Radio Waves We use longer-wavelength microwaves to cook food and still-longer-wavelength radio waves to communicate.

Visible wavelengths In the visible spectrum, wavelength translates to colour.

Visible wavelengths Red light has the longest l, ~700 nm. Violet light has the shortest l, ~400 nm.

Visible wavelengths Red light has the longest l, ~700 nm (700 x 10-9 m) Violet light has the shortest l, ~400 nm (400 x 10-9 m)

Visible wavelengths Red light has the longest l, ~700 nm (700 x 10-9 m) Violet light has the shortest l, ~400 nm (400 x 10-9 m) Practice Question: What is the frequency of red light?

Visible wavelengths Practice Question: What is the frequency of red light?

Visible wavelengths Practice Question: What is the frequency of red light?

Visible wavelengths Practice Question: What is the frequency of red light?

Visible wavelengths Practice Question: What is the frequency of red light?

Visible wavelengths Practice Question: What is the frequency of red light?

Resonance Different atoms and molecules have different natural frequencies of vibration and will emit and absorb different frequencies of light. (When light is absorbed, the wave energy is converted to the thermal energy of the particles.)

Resonance e.g. hydrogen atoms, when excited, will emit the frequencies:

Resonance e.g. hydrogen atoms, when excited, will emit the frequencies: so when light is reflected from or transmitted through hydrogen, it will absorb those same frequencies:

These lines are created by electrons jumping between the second and higher orbital shells. (An electron falling from the higher to the second emits light; an electron jumps up when light is absorbed.)

Resonance Gas discharge tubes produce these characteristic frequencies by exciting the gas/vapour with an electric current. The frequencies can be viewed using a spectroscope.

White Light “White” light is the sum of all frequencies (ROYGBIV), which can be seen by viewing white light through a spectroscope or, similarly, by refracting light through a prism.

Colour The perceived colour of an object illuminated with white light therefore depends on which frequencies are reflected. Example:

Colour The perceived colour of an object illuminated with white light therefore depends on which frequencies are reflected. Example: Reflects GREEN and Appears GREEN

Black = Hot A shirt that appears black is absorbing all colours (and therefore all that energy of the light waves). A shirt that appears white is reflecting all colours.

Black = Hot A shirt that appears black is absorbing all colours (and therefore all that energy of the light waves). A shirt that appears white is reflecting all colours. So wear white to stay cool.

Primary Colours White light can also be produced by combining only three colours. These colours are called primary colours, and in physics are

Primary Colours White light can also be produced by combining only three colours. These colours are called primary colours, and in physics are Red, Blue, and Green.

Secondary Colours Secondary colours are formed by the addition of 2 primary colours:

Secondary Colours Secondary colours are formed by the addition of 2 primary colours: Blue + Red =

Secondary Colours Secondary colours are formed by the addition of 2 primary colours: Blue + Red = Magenta

Secondary Colours Secondary colours are formed by the addition of 2 primary colours: Blue + Red = Magenta Blue + Green =

Secondary Colours Secondary colours are formed by the addition of 2 primary colours: Blue + Red = Magenta Blue + Green = Cyan

Secondary Colours Secondary colours are formed by the addition of 2 primary colours: Blue + Red = Magenta Blue + Green = Cyan Red + Green =

Secondary Colours Secondary colours are formed by the addition of 2 primary colours: Blue + Red = Magenta Blue + Green = Cyan Red + Green = Yellow

More Practice p. 391 #3 – 5, 7 – 13 Tomorrow: Reflection