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.

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.

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.

Read more

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?

Read more

what to do if the website is busy

You might have noticed that the server has struggled to cope with so many people trying to view the resources as the Physics exams approach.

If you can’t get in, you can still access some of the video files and PDF notes through iTunes.

There are iTunes podcasts set up for Advanced Higher, Higher and National 5 Physics.  Those of you sitting National 5 might also find useful resources in the old Standard Grade and Intermediate 2 podcasts.

Advanced Higher podcast

Higher podcast – includes some past papers

National 5 podcast – includes specimen question paper and the 2014 exam, plus marking instructions

Standard Grade podcast

Intermediate 2 podcast

Subscribe to the appropriate podcast and download the pdf/mp3/mp4 files.  I’ve run some tests and this seems to work even when though the site won’t let you in.