Home made spectroscope

Friday 4th March 2016

Pretty much everything we can observe about the universe comes from the light we can see from stars. I've been doing an Astronomy course over the last year and I've come to realise just how much information can be obtained from light!

One thing that's really impressed me is how it's possible to find out all kinds of things about the composition of a star from looking at its spectrum. For example helium was observed on the Sun before it was discovered on Earth.

For a few quid I bought some diffraction gratings which smear out an object's light into a wide spectrum. 

Making a spectroscope

Apart from the diffraction grating, it's all simple stuff. I bought ten gratings from Amazon for £3 if you'd like me to post you one, and I also have some spare razor blades.

You'll need:

  • Black electrical tape
  • Cardboard
  • A long cardboard tube, about 10cm diameter
  • A razor blade
  • A 500 lines/mm diffraction grating

We need to block out almost all the light except for a very thin slit at one end of the tube. This makes for clearer lines in the image.

First of all, cut a circle of carboard that's about the same size as the tube diameter. Cut a rectangle from the middle of the circle that the razor blade will cover. Now carefully break the razor blade in two (obviously it's very sharp) and tape one part to each side of the circle, making sure the sharp bit of the blade protrudes from the semi-circle, like the top right image below.

Now tape the blades onto the card leaving about a 1mm gap between the two halves of the razor blade - like the bottom left image below. A smaller gap gives a clearer image, but needs more light to show a spectrum. 1mm seems to work for me.

Now tape the disc to one end of the cardboard tube, making sure that you still have a 1mm gap when you've finished. When you're done it should look like the bottom right image below. You can check it's okay by looking down the other end of the tube and you should see a very thin slit of light.

Now you need to attach the diffraction grating to the other end of the tube, like this. Make sure that the grating is mounted in the same orientation as the slit, that is when the slit is vertical, the text on the grating is the right way up.

The easiest way to do this is to hold the tube up to a CFL-type bulb in your home and place the grating at one end.

Rotate it around until you can see clear lines in the spectrum - if it's the wrong way round you'll just see a single unbroken line spectrum.

Have a play with it and you'll see what I mean.

Once you've got it in the right orientation, tape the grating to the other end of the tube and you're done! Yes, I taped this one upside down.


So what am I looking at?

Good question. White light isn't one colour, it's a bunch of different wavelengths. When you look through the diffraction grating at a light source, you'll see rainbows off to either side of the slit. The grating spreads the light out into different wavelengths like a prism.

At the blue end (normally depicted on the left, but you might have yours upside down!) is light with a wavelength of 390nm, and at the red end most people can see up to about 700nm. Most lights produce radiation beyond these wavelengths, but you can't see into the ultraviolet or infra-red so it just looks black.

Different light sources display very different spectra. I took some rather beautiful photos so you can see what it looks like.


Incandescent lights

Old school incandescent tungsten bulbs, and halogen lights, produce their light by becoming incredibly hot and emitting radiation as visible light. When you look through a spectroscope at one of these lights, you see a continuous spectrum of light like this:

These kind of lights actually emit more radiation in the infra-red than in the visible part of the spectrum, but my camera and your eyes can't see this, so the red just fades out as the wavelength gets longer on the right side. This is why these lights are really inefficient and also why they're really hot to touch.


Compact fluorescent light

About a decade ago everyone switched to compact fluorescent light (CFL) bulbs in their homes. These bulbs work in a totally different way to incandescent lights. They contain mercury atoms which emit energy in the ultraviolet. We can't see ultraviolet so the inside of the tube is coated in phosphors using various metals including europium and terbium which absorb the UV energy and re-emit it at very specific visible wavelengths.

The mixture is chosen carefully to give a particular colour of light which is how you get warm, cool and daylight CFL bulbs. But unlike incandescent bulbs, they only give off a very specific set of wavelengths of light, as you can see above.  With the help of my good friend Wikipedia, you can identify exactly which atoms are emitting which parts of the spectrum shown.

Mercury's pretty toxic so we'd ideally like to move away from CFLs over time. And also as you can see, the light isn't actually that good compared to incandescent bulbs, which is why a lot of people don't like them very much.



Enter the Light Emitting Diode. These are lower power still than CFL bulbs, and their spectrum is much more like tungsten bulbs.

Don't be fooled though, LEDs also only emit a very narrow set of wavelengths (I'm not sure I fully understand why) so they are also covered in a layer of phosphors. Most household LED lights gives off a blue light which the phosphors absorb and re-emit as other colours.

Because the mix of phosphors is so clever it gives the appearance of a full spectrum, but you can see a dip in the blue part of the spectrum, and less at the red end than an incandescent light.

The horizontal stripes in the image are due to the shape of the LED torch that I used. Good LEDs produce a very nice colour, use a tiny amount of energy and last a really long time. I'm gradually moving all our lights over to LEDs, starting with the halogen bulbs.



You'd think that the cleanest, most complete spectrum would be sunlight, but that's not the case. The inside of the sun is rather hot, and so it radiates light as a broad spectrum. But as the light passes through the outer parts of the sun, and through the Earth's atmosphere, cooler atoms absorb specific wavelengths of light and the resulting spectrum has characteristic dark bands, which you can clearly see in the image below.

These are called Fraunhofer lines after the person who discovered them, and each one is caused by a specific atom. This is how we know the composition of the Sun, and also how helium was discovered - absorption lines were seen in the Sun's spectrum that didn't correspond to any elements that scientists knew about on the Earth.



Recent posts

Happy Christmas

Friday 25th December 2015

Happy Christmas from Lost In Thought!

This looks best full screen.

This is 45 hours of time-lapse photography compressed into a minute and a half. We used three of these magic snowmen from eBay (I love eBay), which work in the same way as those crystal trees that were so popular when I  was a kid.

It's a saturated solution of potassium phosphate, which is drawn up into the paper snowman and then forms crystals as the water evaporates from the snowman's extremities.

See you in the new year!



Recent posts

Lostwithiel then and now

Friday 20th November 2015

This is something I've been working on for a while, but it's really hard to get perfect. We have access to a whole bunch of old photos of Lostwithiel, so I thought it would be interesting to see how the view has changed over the years.

There's something quite eerie about finding the exact same spot that someone stood in a century ago to take a picture.

Interactive version

Here's an interactive version that lets you fade between the old and new images - although it's a bit fiddly on a  mobile.



Bodmin Hill




Duke Street



Fore Street



North Street



Pascal's / Watts Trading



Quay St and the river


Wedding on Melville Terrace, 1916

According to Barry West who kindly supplied these photographs, the couple marrying are Frederick George Thompson and Helen Johns. He says "One of the children at the front below the groom is Doris Evelyn Nicholls of St Austell and Lostwithiel throughout her life. The little girl in Brown is Winnifred Johns. "



Recent posts

How old can I look?

Friday 8th May 2015

I must admit I'm not a fan of the selfie, but in the interests of science I took a bunch of photos of myself to see how Microsoft's how-old.net website reacts to different expressions. How old can I make myself look?


From above

Fair play to Microsoft, they got this one spot on - I am indeed 39. Lucky guess?



I've read articles saying that the best way to get a young score is to pull a neutral expression that minimise any lines on your face. I can add a decade by looking angry.



Result! An extra 16 years by looking confused.

Kat always says I look like Beaker from the Muppets when I pull this face. Sadly the website couldn't tell me how old Beaker looks:



I can only hope I look this good at 48.



I'm pretty sure I've got a face. Maybe it's too old to detect.


Tongue out

Bit surprised by this one, is this what 52 year old men do?


Black and white

This is the same picture that scored me 39 at the top of the page, now scores 49 in black and white. What's going on there? Maybe it thinks that the world used to be in black and white and so anyone with a black and white must be old?



I wondered how Photoshop could change my age, if black and white adds ten years. Going from least to most saturated (left to right) takes me from 48 to 44, but at the extremes it couldn't detect my face. Young people are more colourful, obviously.


Me too

Finally, I thought I'd try putting multiple copies of myself in the picture and see if it gave the same answer:

I think we've learned something important here: how-old.net needs more work. In a row of six identical colour photos of me it can only spot faces in two of them and gives them ages of 44 and 51, and in black & white there's an 8-year age difference between six identical photos, from 40 to 48.

To look as old as possible, I need to look confused. Have a go yourself and see how old a score you can get, then leave a comment with your tips for looking old!


Recent posts

Post boxes of Lostwithiel

Friday 10th April 2015

When I started writing this blog, I always wanted to feature posts by other people. Here's the first one by John Pegg, a quirky gallery of all of the post boxes in Lostwithiel.

If you've got an idea you'd like to share, get in touch!

A rather tasteful post box incorporated into the main sorting office window of the main post office on Queen street, I'd say circa 1935.
Quite a nice GVR box located on the main road into lostwithiel opposite St Winnow School.
All the family frozen in awe at the amazing and interesting GVR box located at the bottom end of Fore street, by the Co-op.
A jaw droppingly unbelievably rare EVIIR box (1902 -11) cemented into a wall up Bodmin hill. You just don't realise how rare these are . It should be listed.
Well situated GVR box on a wall of the 'Old Post Office' on Grenville road, by the Boer War peace memorial.
This is a fairly common GVR (1911 - 35) located near the entrance to Scrations lane, on Edgcumbe road.
A standard EIIR box situated just below Lostwithiel School, Bodmin Hill.
Another standard EIIR box located on the corner of Coffeelake, on the Lerryn road.
Bonjour... another EIIR box situated on Cott Road, adjacent to the entrance to Barn Park.
Rose Hill / Summmer Street - a standard GVR box.
Standard EIIR at Maudlin, which is within the borough of Lostwithiel so counts as a post box in this category.


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A hundred places I've weed

Friday 23rd January 2015

I'm very good at starting things, and very bad indeed at finishing them. The main reason I started this blog is to make myself complete projects so I can write about them. I figure if I start off with small challenges, I'll finish them and train myself to pay attention for a bit longer each time.

A friend recently completed a project of taking a photo every day for a year. I love the idea of doing this, I tried once but gave up in mid-August.

So I thought I'd set myself a smaller challenge that I could achieve. Don't ask why, but I decided to take photos of a hundred different places that I've had a wee.

Initially I just started photographing places where I'd been to the toilet, so far so good. But pretty quickly I started holding on until I found somewhere attractive to relieve myself.

Towards the end it I discovered that I'd trained myself to need a wee whenever I see a nice view, a bit like Pavlov's dogs. The nicest place I had a wee was on Corfe Castle, #36.

Anyone whose house I visited between November and January will now have a horrified look on their face.

Where's the nicest place you've had a wee?

Leave a comment below:

Recent posts

What's this about?

Hi I'm Mat and I'm addicted to new hobbies. I used to think this was a bad thing but now I'm embracing it.

Writing them all up in this blog encourages me to finish projects, and helps me keep track of which ones I've tried.

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Happy Christmas

Happy Christmas

Happy Christmas from Lost In Thought! This looks best full screen...

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Home made spectroscope

Home made spectroscope

Pretty much everything we can observe about the universe comes from the light we can see from stars...