# Physics Unit 13: the nature of Light

## Learning Objective:

Understanding the basic nature of light

PH 5

## Warm up:

{Predictive, Diagnostic} What is Light? Is light a wave or a particle? If light is a wave, what do you think is oscillating?

## Introduction to light

This video is simple overview – you should watch this on your own after class….

Historical backgrounds – myths and measurements.

What is an electromagnetic wave?

Q: if the electric and magnetic fields are in the y and z direction, what is the axis of propagation of the wave?

The Electromagnetic Spectrum:
Key length scales: meters, nanometers, Angstroms
1 nm = 10^-9m, 1 A = 10^-10m

Visible orange light is 6,000A wavelength. What is this wavelength in nanometers? in meters? (use scientific notation).

The Electromagnetic Spectrum – courtesy of Monkey See

The speed of light is known to be 3×10^8 m/s in free space.
Q; Calculate the frequency of visible orange light.
Ans.

## Challenge problem:

Alpha Century is actually a binary star with Alpha Century A and B orbiting each other at a distance of 200 AU.  Alpha century a is similar to our sun but slightly larger.  Assume Alpha century A has the same luminous flux as our sun, calculate its apparent brightness as seen from Earth.

A light year is the distance light travels in one year – it is equal to 9.46 x 1012 km. Alpha Centauri A & B are roughly 4.35 light years away from us. Our sun has a luminous flux of 3.846×10^26 J/s and is 149,600,000 km away. source: Cosmic Distance Scales

Assume Alpha century A has the same luminous flux as our sun, calculate the apparent brightness of as seen from Earth.

## Exit Assessment:

How would you use the apparent brightness of a star to measure its distance from Earth?

This is a great video, includes light, astronomy, Mars, remote sensing. Watch it now, I’ll be posting questions later…..

Calculations: Formulas you need to know;

Luminous Flux P {lumens}, Illuminance E {lux} read pg 440-441

Luminous Intensity {candela}

Point Source Illuminance pg 443

GP: Example Problem 1 pg 444

IP: Practice Problems 1,3,4 pg 444