Category Archives: Physics Unit 13: Light

Unit 13: Light – Essential learning goals

Textbook: Glencoe Ch 16
1: You are responsible to read the entire chapter pg 441-437
2: Packet due next monday:
pg 436 practice problems #1-6
pg 447 practice problems# 14-17

Key concepts:

  1. Wave nature of light, E&M fields, E&M spectrum (see post )
  2. Luminous flux, illuminance (be able to calculate), illumination
  3. Relativistic Doppler effect

There will be a short quiz next Monday – Tues.


Part 2: Chapter 17 – to be assigned later

Learning objectives:

  1. Reflection, Huygen’s principle of wavelets.
    1. Excel project due following monday by email
  2. Specular and Lambertian reflectance, scattering
    1. effect of surface roughness on reflectance


Part 3: The peculiar properties of light

  1. Refraction (Huygen’s model)
  2. Diffraction
    1. single slit (drawing and calculating)
    2. double slit
    3. Fraunhofer interference





The Doppler Effect – formulas

Lesson Objective: How to calculate the Doppler Effect  for moving sources and detectors. How to solve the Doppler equation for the speed of a galaxy based on its  Red or Blue shift.

Warm Up: {FM} Why is the Doppler Shift formula for light different than the Doppler Shift formula for sound?

What you need to know:

For Sound: fd = fs (v-vd) / (v-vs)
Derive expressions for fd where vd=0 and where vbs=0 (see pg 415).

New material:
For light: f observer = f (1 +/- v/c) for v<<c
And the “red shift” DL =Lobserved – L = +/-(v/c) L
Where D means delta and L means Lambda.

Do Practice Problems pg 456 # 16-19, you will be called on to present your work at the board.

Relativistic Doppler Effect: Now the problem is that the speed of light is a  constant and we are stuck never being able to exceed the speed of light.

Q: What is the speed of light in m/s? in miles/sec?

One of Einstein’s colleagues, Lorenz, came up with a transformation that allowed one to create  mapping between points in space-time. Using the Lorentz transformation, they derived the relativistic Doppler Effect:

nu’ = nu [  (1-v/c) / (1+v/c) ]^1/2

This is the Relativistic Doppler Effect which applies when v gets close to c.

This post from Wikipedia has the derivation of this equation – but it’s really complicated.  Diagrams 1 and 2 will give you the general idea.
The Relativistic Doppler Effect

Now calculate the non-relativistic and relativistic Doppler “Red Shifts” for a star moving away from you at;
Assume the star emits 500nm light.

Include your results in a table and in the third column, calculate the % difference between the non-relativistic and the relativistic Doppler red shifts.

Exit Assessment: What would 500nm light look like of you were traveling faster than the speed of light?

The speed of light

Learning Objective: Understanding the speed of light and how it has been measured.
Standard: PH5
Warm Up: {FM} Sketch the electromagnetic spectrum and label Gamma Rays, X rays, ultra-violet, the visible spectrum, infrared, microwaves and radio waves.

Quick Quiz 25 pts
Measuring the speed of light:read Roemer’s experiment pgs 445-446. Pair share and discuss if his approach was valid. Class discussion.

Read Michelson’s experiments pg 446.

Is the speed of light constant? Does its speed depend on a luminiferous Aether?

Now that we know the speed is constant in free space, and there is no Luminiferous Aether, how can the laws of physics be the same as seen in all possible inertial reference frames?

The relativistic Doppler effect: read pg 453-454.

Exit Assessment: How can you use the relativistic Doppler effect to determine if a distant star or galaxy is moving towards or away from you?

Quick Quiz: Light

EM Spectrum

5 question quick quiz on light and the EM spectrum

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.

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