Diffraction of a red Laser beam with a  diffraction grating

red laser beam passing through a diffraction grating. image: en.academic.ru

Diffraction is a test for wave behaviour.  When a ray of light passes through a diffraction grating, the energy of the incident beam is split into a series of interference fringes.  Constructive interference is occurring at each location where a fringe (or spot) is observed because the rays are in phase when they arrive at these points.


diffraction spots projected on to a wall.  image: microscopy uk

Find out about diffraction gratings here and here.


image: laserpointerforums.com

We can measure the relative positions of the fringes in a diffraction pattern to determine the wavelength of the light used.  The diffraction grating equation is

m lambda = d sin theta


  • m is the diffracted order  –  some resources may use n instead of m
  • λ is the wavelength
  • d is the line spacing.

Here is an infrared diffraction experiment you can try at home to calculate the wavelength of the infrared LED in a remote control.

I’ve attached a set of pdf notes and questions on diffraction.  These notes use n rather than m for the diffracted order.

the photoelectric effect

We learned about the photoelectric effect this week.  This video has a similar demonstration to the gold leaf electroscope experiment I showed you in class and includes an explanation of the process.

Click on the picture below to download the simulation we used to investigate the effect of irradiance on frequency on photocurrent.  You’ll be prompted to install Java if you don’t have it already.

Once the animation is running, you can;

  • change the metal under investigation (we used zinc in class)
  • vary the wavelength of the incident light
  • vary the irradiance of the incident light.

Notice that below the threshold frequency you can’t get any photoelectrons, even if you set the light to its brightest setting.

Compare your results to the graphs provided in your notes.

I have attached some notes & questions on the photoelectric effect. Click on the link below to download a copy.

pp chain fusion in stars

Twinkle, twinkle little star,
How I wonder what you are.
Giant thermonuclear reaction;
Held by gravitational attraction.
Twinkle, twinkle little star,
You look so small ’cause you’re so far.

As you burn through constant fusion,
Your twinkle’s just an optical illusion.
That happens when your light gets near;
distorted by our atmosphere.
Twinkle, twinkle little star,
spreading light and heat so far.

As you use up fuel you’ll grow,
and give off a scarlet glow;
Maybe you’ll go supernova,
exploding elements all over.
Now I know just what you are;
and I know I’m made of stars.

Where does the Sun get its energy?  A straightforward question but physicists struggled to find an answer until the 1920s, when Eddington suggested that nuclear fusion might be responsible.

A star is drawing on some vast reservoir of energy by means unknown to us. This reservoir can scarcely be other than the subatomic energy which, it is known exists abundantly in all matter; we sometimes dream that man will one day learn how to release it and use it for his service. The store is well nigh inexhaustible, if only it could be tapped. There is sufficient in the Sun to maintain its output of heat for 15 billion years. — Sir Arthur Stanley Eddington

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the Hertzsprung-Russell diagram

Our Sun is a typical yellow star, so its emission would be represented by the middle star in this image.

Screen Shot 2015-11-22 at 18.39.13

image courtesy of kstars, kde.org – colour is exaggerated

The colour of a star also tells us something about the expected behaviour of a star, it’s lifetime, and destiny.  This is achieved by plotting stars on a Hertzsprung-Russell diagram.  More about HR diagrams here.

This video clip looks at how the stars are arranged on the HR diagram.

Hertzsprung Russell diagram from mr mackenzie on Vimeo.

While some HR diagrams use temperature along the x-axis, others use star classification. 

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introduction to particle physics

There’s a nice guide to particle physics on The Particle Adventure web site.  The site has it’s own free apps for android & apple devices that are worth installing.

Here’s Prof Cox with a two part summary of particle physics.


and a short video with just the quarks

The video below was made before the Higgs boson was confirmed, so please bear that in mind.  It’s still a nice video though.


Learn about the world’s largest neutrino detector in Antarctica.

100 years of general relativity


It seems general relativity is 100 years old today. We’ve been struggling a bit with this topic, so I thought these videos might help.

Why Einstein is such a big deal from Fusion Media Network on Vimeo.

General relativity explained in under 3 minutes from Fusion Media Network on Vimeo.

Science Museum – Einstein’s Cosmos from ORDER Productions on Vimeo.

#Einstein100 – General Relativity from Eoin Duffy on Vimeo.

cosmic microwave background radiation

The cosmic microwave background radiation (CMB) is radiation left over from the big bang.  When the universe was very young, just as space became transparent to light, electromagnetic energy would have propagated through space at a much shorter wavelength.  Nowadays, the temperature of space has fallen to approximately 2.7 K (that’s 2.7 K above absolute zero!) and, using Wien’s Law, we can confirm that the peak wavelength of the electromagnetic radiation is so long that the background radiation lies in the microwave portion of the em spectrum.

The CMB was first detected in 1964 by Richard Woodrow Wilson and Arno Allan Penzias, who worked at Bell Laboratories in the USA.

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star colours

Astronomers often refer to the colour of a star, which seems a bit odd because we mostly see stars as white twinkly objects.  However, even with the naked eye, we can look closely at certain stars and detect a hint of colour – just look at this image of the Orion constellation.  As we view him, the left shoulder has a red coloured star, while the right shoulder and right foot appear to be blue.


image: Orion 3008 huge.jpg, Wikipedia

Now click on the image to see the same view at much higher resolution.  In the hi-res photo, look at the stars in the background.  They’re not all white!

What can the colour of a star tell us?

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