Monthly Archives: April 2011

An experiment to investigate light in the style of a pirate

I have had a lovely weekend in the sunshine, much of which has been spent outside in our garden, under the beautiful cherry tree, on our new garden furniture. I’ve been pottering around, weeding the vegetables, planting more seeds, and watching the cats chase motes of dust.

In between, I’ve been studying hard – book 3, Energy and Light. I’m almost there; I’ve completed most of TMA03; all I needed to do was Activity 11.1 – Investigating Light.

Joe and I liberated a cardboard document box from his offices, and I planned my experiment. It is set out below, just as it is in my folder (with perhaps just one or two embellishments, and an extra instructive illustration). Some of the details have been changed and the first-person voice has been used because the write up was part of the assessment, and so cannot be made public for fear someone may plagiarise me. So some of this may or may not be true!

Investigating light: determining the wavelengths of spectral lines from an energy-saving light bulb.


  • Diffraction grating (300 lines per mm)
  • Tungsten filament light bulb (40 W)
  • Energy-saving light bulb (11 W)
  • Lovely stripy table lamp (for the tungsten bulb)
  • Tall standard lamp (for the energy-saving bulb)
  • Large cardboard document box
  • Pieces of Amazon book cardboard
  • Gaffer tape
  • Sharp knife
  • Paper protractor
  • Blu-Tack
  • Black cotton thread
  • Red drawing pin
  • Dressmaker’s pin
  • Eye-patch
  • Table
  • Dressing gown


An experimental measurement of the angles of diffraction of blue, green and red spectral lines from an energy-saving light bulb was undertaken using a diffraction grating, a protractor, and household items. The wavelengths of each spectral line were calculated. The value obtained for the blue spectral line was 450 nm; the value obtained for the green spectral line was 550 nm; the value obtained for the red spectral line was 600 nm (all to two significant figures).


To determine the wavelengths of blue, green and red spectral lines from an energy-saving light bulb.


Having liberated the box from Joe’s work in a ninja-style midnight operation, I cut a thin slit in it using the sharp knife. Joe took this off me, and did it properly with a minimum of blood spilled. I tidied the edges using gaffer tape. Gaffer tape can do anything: FACT. The table lamp containing the tungsten bulb was placed within, and Amazon cardboard was cobbled around the edges, in an attempt to prevent too much light from escaping and having a party where my spectral lines were supposed to be.

I had a gander through the diffraction grating, and this is what I saw – a continuous spectrum:

This is an actual photograph I took *proud*

Professional pirate dark-room. Eye patch provided.

Next, I needed to take a look at the spectrum produced by the energy-saving bulb. So I undid my gaffer-taped masterpiece, and fumbled the floor lamp with the energy-saving bulb under there. I couldn’t quite manage to see the spectrum this time, so I created a dark-room, thus:

This created the ideal conditions to observe and photograph my diffraction spectrum – which was not continuous, and was in fact a line spectrum. Again, I photographed it:

Line spectra from an energy-saving bulb

All this was very pretty, but had to be interrupted by a trip to Charlie’s to make me a longbow. You see, my marvellous husband (he of the fabulous presents) bought me A Big Piece of Wood for my birthday. Not just any piece of wood, mind; a piece of yew, laminated with maple and lemon wood. He, Charlie and I began the shaping of a (very) long bow. It’s going to be grand!

Back to science.

Later, when darkness had fallen, I continued my experiment and set up my equipment. Leaving the box with the energy-saving bulb where it was, I stuck it down to prevent any disastrous movement, and placed a paper protractor about 50cm away. A drawing pin pierced the protractor, to provide an anchor point for the thread. The diffraction grating was placed upon the protractor at the axis. Thread was tied to the drawing pin and the dressmaker’s pin, and all was ready. See:

Experimental set up.

This is where the eye-patch comes in. To measure the angle of diffraction for each spectral line, you have to line up the spectral line itself with the line on the grating and the thread upon the protractor. This is to be done with one eye, to prevent parallax error. I found myself unable to do this, and so had to use an eye patch.

Of course, it naturally followed that I had to conduct the rest of the experiment in the manner of a pirate. Grog was acquired, and duly consumed. Tables were swabbed, angles were swashed, and the thread was buckled. Much like my knees.

The experiment was a success! The wavelength of the blue, green and red spectral lines from the energy-saving light bulb were calculated as: 450 nm,  550 nm and 600 nm respectively. This isn’t far off the actual wavelengths of light emitted by an energy-saving light bulb. Go and google it if you don’t believe me.

This has been Science, by Vicky. I’ve enjoyed it; all that remains to be seen is how well my tutor likes the write-up… I do think that the eye patch was relevant. And the dressing gown.

Energy and problem-solving

So, I am doing pretty well so far in S104: Exploring Science. My iCMA (interactive computer marked assignment) scores are: 80% and 100% (and the 80 was me not understanding how the system worked!) while my TMA (tutor marked assignments) scores are: 96% and 92%. I’m not so happy with the 92%… but I understand where I dropped marks!

I am two-thirds of the way through Book 3: Energy and Light and I’m really enjoying it. I’m (re)discovering a love of mathematics and equations (with the help of the fabulous ScaryCalc). However, I’m struggling a little with problem-solving.

Activity 7.1 asked us to solve a problem, using the problem-solving skills we developed in chapter 4, and using the information provided as well as relevant equations taught throughout the book.

Imagine that you are stuck in your car in bad weather, trying to boil water to make yourself a hot drink.

A cup contains 150 g of water at 19 °C. A small heater is immersed in the water and connected to a 12 V car battery. An electric current of 13 A flows through the heating element. Find the time taken to heat all of the water to 100 °C and for one-third of the water to vaporise.

The specific heat capacity of water is 4.2 ×103 J kg-1 °C-1 and the specific latent heat of vaporisation is 2.3 × 106 J kg-1. When answering the question, assume that the cup is very well insulated, i.e. that all the electrical energy supplied is transferred to internal energy in the water. Will this assumption lead to an answer that is an overestimate or an underestimate of the time taken?

We are advised to plan the problem out before doing the calculations – so I start by writing exactly what I’ve been asked to find out: How long does it take to heat 150g of water to 100°C and to vaporise 50g of water?

Then I make a note of all the information I have been given; followed by a list of the equations I may want to use. All well and good – I wrote down a couple of equations that turned out to be superfluous, but better too many than too few, I always say.

I attempted this last night, in a fug of exhaustion and desperation brought on by the realisation that I am falling behind a little. It did not go well, so I retired to the sofa and watched Kurt Russell attempt to save America from the terrists in 1980something.

Today, at lunchtime, the activity went better. I did achieve the correct answer, to the correct number of decimal places, and everything.

This would appear, on the face of it, to be good news – but wait! I had solved this problem in a particularly long-winded way. You may think this doesn’t matter; however, part of what this chapter is trying to get us to do is rearrange and combine equations where necessary. I only succeeded partly in this quest.

I think I understand how the book got to the result; I’ll have a proper look when I get home. I’ll probably ask The Husband to try to explain it to me too.

Today I am not feeling quite so confident about things. I am hoping that with practise I will begin to find this aspect of the course easier, because at the moment it is not coming naturally.

The information itself is terribly interesting and about to get more so; and what’s more, I have two new books to read, which are bang on topic:

“Six Easy Pieces” by Richard Feynman and “We Need to Talk About Kelvin”.

Bring on the E = mc2!