How to… make a DIY spec­tro­graph

Read into the se­crets of as­tro­nom­i­cal light

All About Space - - Contents -

You’ll need:

✔ Ce­real box

✔ Old CD disc ✔ Sticky tape ✔ Mod­el­ling knife As you may know, light can be bro­ken up into the colours of the rain­bow, known as a spec­trum.

This can be done at home us­ing the prop­er­ties of an old CD to break up the light. The spec­trum will show you light and dark lines, which can also tell you what the sun­light, or other light source, is made up from.

As­tronomers use sim­i­lar de­vices, although some­what more so­phis­ti­cated, in their stud­ies of the stars. The CD acts as a dif­frac­tion grat­ing, that is, the cir­cu­lar tracks on the CD are so close to­gether that they split up the light and spread each dif­fer­ent wave­length to a dif­fer­ing po­si­tion; this is the spec­trum.

This spec­trum is spread per­pen­dic­u­lar to the CD – this is why the slit and the view­ing holes need to be at 90° to each other. Each colour bends at a par­tic­u­lar an­gle. For you to see the spec­trum, the light must dif­fract off the CD and re­flect into your eye. Ad­just­ing the tilt of the CD al­lows you to prop­erly bounce the spec­trum into your eye.

The qual­ity of the spec­trum you get de­pends on how well you make the spec­trom­e­ter. The slit which lets the light into the box needs to be thin, but not too thin, oth­er­wise the im­age will be too dark. Con­versely, if the slit is too wide, the spec­trum lines will ap­pear to be blurred.

If done right it is easy to use and a great way of get­ting into spec­troscopy. It's a great tool for chil­dren, too, and is per­fectly safe to use. You can also take pic­tures of the var­i­ous spec­tra that you can see. This makes it eas­ier to com­pare dif­fer­ent kinds of light.

It's easy to make, although care should be taken, as you would us­ing any sharp tool, when cut­ting the slit. You can use the spec­trom­e­ter to ex­am­ine light from other sources too, such as tung­sten lights, strip lights and even the Moon. If you have a tele­scope, you could try us­ing it on a bright star by hold­ing the de­vice over the eye­piece. You'll be able to spot the bright lines in the spec­trum cor­re­spond­ing to var­i­ous chem­i­cal el­e­ments in the light source. See what you can dis­cover.

"The qual­ity of the spec­trum you get de­pends on how well you make the spec­trom­e­ter”

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