More about Light“Let there be Light”—Gen.1:3

SPH4U – Grade 12 Physics

Unit 2

Tribal Challenge!

In a ripple tank, a point on the third nodal line from the center is 35.0cm from one source and 42.0cm from another. The sources are 11.2cm apart and vibrate in phase at 10.5Hz. Calculate the wavelength and the speed of the waves.

3 points – first team correct

1 point – every correct answer after that

Tribal Challenge!

In a ripple tank, a point on the third nodal line from the center is 35.0cm from one source and

42.0cm from another. The sources are 11.2cm apart and vibrate in phase at 10.5Hz. Calculate

the wavelength and the speed of the waves. Solution:

Review – True or False

1) All the different colours of light have the same wavelength, frequency, and speed.

Review – True or False

1) All the different colours of light have the same wavelength, frequency, and speed.

FALSE!As you saw in thesimulation, the different colours of light have different wavelengths. Since the speed of light is constant for all light, the frequency for each of these colours is also different.

Review – True or False

2) When light is shone on to atoms, sometimes the atoms aborb the light. When this happens their electrons gain energy.

Review – True or False

2) When light is shone on to atoms, sometimes the atoms aborb the light. When this happens their electrons gain energy.

True!As you saw in thesimulation, when a photon pulse is shot at the atom, the electrons are able to move to a higher energy level.

Review – True or False

3) The critical angle is the largest angle of incidence before total internal reflection happens. It is only possible to have when going from a less dense medium to a more dense medium.

Review – True or False

3) The critical angle is the largest angle of incidence before total internal reflection happens. It is only possible to have when going from a less dense medium to a more dense medium.

False!As you saw in thesimulation, the critical angle is possible going from a more dense medium into a less dense medium.

Review – True or False

4) If you add green light to red light, you get yellow light.

Review – True or False

4) If you add green light to red light, you get yellow light.

True!As you saw in thesimulation, addition of colours of light does not work the same as the addition of colour pigments in paint. When you add all the colours of light together you get white light. (When you add all the paint colours together you get black/brown).

Review – True or False

5) Newton was a believer in the particle theory of light. Huygens was a believer in the wave theory of light.

Review – True or False

5) Newton was a believer in the particle theory of light. Huygens was a believer in the wave theory of light.

True!

More About Light

The history of the study of light is very interesting. Originally people thought light was made up of tiny particles that objects sent out. Later, people thought that light was a kind of wave.

More About Light

Scientists today use the quantum model to explain light. This model says that light is an electromagnetic wave, made up of microscopic (very tiny) particles called photons. So light is both a particle and a wave.

More About Light

We now define light as a form of radiant energy that the human eye can detect. It sometimes acts like waves and sometimes acts like particles.

The Electromagnetic Spectrum

This diagram shows different wavelengths of electromagnetic waves. We can only see the waves when the wavelengths are around 10-7 meters. Other types of wavelengths make radio waves, microwaves, infrared, and X-rays.

The Electromagnetic Spectrum

Last semester, we learned that sound waves cannot travel through a vacuum. But Light waves can! This is because they are electromagnetic waves. Electromagnetic waves are produced by vibrating electric charges.

An electromagnetic wave is a transverse wave which has both an electric and a magnetic part.

The Speed of Light

The speed of light is such an important quantity in physics that it is given its own symbol, c. It takes light only one billionth of a second to travel the length of a 30-cm ruler. It takes light 8.3 minutes to travel from the Sun to Earth. The speed of light in a vacuum is defined as c = 299 792 458 m/s or c = 3.00 x 108 m/s (to three significant digits)

The Speed of Light

From what we have already learned about waves, we remember the universal wave equation, v = fλ. Use v = c (the speed of light) in the following problem…

Example 1

A laser gives off a light with a wavelength of 1064 nm. Calculate the frequency of the laser.

Example 1

A laser gives off a light with a wavelength of 1064 nm. Calculate the frequency of the laser.

Propagation of Light

Light carries energy outwards at very fast speeds in all directions from its source.

Rectilinear propagation is the term used to describe the fact that light appears to travel in straight lines through a uniform medium. It will travel in a straight line until it hits another medium.

Propagation of Light

In physics a straight line is often used to show the direction of the light energy, because it is difficult to draw all the light. These straight lines are called rays. A group of these lines is called a beam.

ray

beamsun

Sources of Light

Luminous Sources Non-Luminous

Produce their own light Do not produce their own light. Reflect light from a luminous source.

Eg. Candle Eg. Moon

There are two sources of light. Luminous sources and Non-luminous sources.

The Transmission of Light

When light hits an object, it may be transmitted, absorbed, or reflected.

Some materials let certain frequencies of light to move right through them. Materials that light can travel through easily are called Transparent. These are materials you can see through very clearly, like glass and water.

The Transmission of Light

Translucent materials are materials that light moves though, but the light is scattered so you can not see through them very clearly, like tissue paper.

Opaque materials do not let any light move through them. All light hitting opaque materials is either absorbed or reflected. For example, the table absorbs light.

Polarization of Light

The Polarization of Light provides more evidence that light behaves as a wave – specifically, as a transverse wave.

Natural sunlight transmits light waves whose electric field vectors vibrate equally in all planes perpendicular to the direction in which the light is moving. When their electric field vectors are restricted to a single plane by filtration, however, then the light is polarized.

Homework

Begin your end of Unit 2 Review Assignment