10 Lab 5 – Spectroscopy
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Acknowledgment
This project is made possible with funding by the Government of Ontario and through eCampusOntario’s support of the Virtual Learning Strategy. To learn more about the Virtual Learning Strategy visit: https://vls.ecampusontario.ca.
PHY137 Stay-At-home Experiment 5
Spectroscopy
What you will do
Use a prism and a diffraction grating to observe the spectra of various light sources
What you will need
- A prism
- A diffraction grating
- A green LED, a red LED, and an incandescent light bulb
- A lens with a focal length shorter than 10cm
- Battery pack and alligator wires
- A non-transparent cardboard box with at least one dimension between 10~20 cm
- A lamp or some other bright object that is not the sun
- Some tape
Demonstration Video: https://youtu.be/pZLZn6rVJgA
Procedures
1- Hold the prism close to (but not immediately next to) the camera of your phone. Point your camera to a light source. Adjust the orientation of the prism so that in the same photo, you can see the same light source twice: once directly by the camera, another one indirectly through the refractions of the prism. (An example is attached below. If you have difficulty finding the orientation, recall how light is refracted by a prism.)
2- Notice that in your photo, the object seen through the prism has two colored edges, one red and one blue (i.e., “dispersion”). Draw a light ray diagram and determine which color has the higher refractive index through the prism. Note that our eyes were seeing a virtual image of the object through the prism.
3- Hold the diffraction grating directly next to the camera of your phone. Point your phone to the same light source that you looked at in Step 1. Take a photo of the same light source through the grating. You should see the diffracted spectral bands on both sides of the light source. (If not, try rotating the grating or finding a smaller light source.)
Part I – Prism-based Spectroscope
4- Pick two opposing surfaces on your box that are between 10~20 cm apart. Use a knife or a pair of scissors to cut open a small 3cm x 3cm window near the bottom edge on one of the surfaces. Then on the opposite surface at approximately the same location, cut open a 3mm x 4cm narrow vertical slit. (You can simply reuse the box from the previous lab.)
5- Tape the lens to the window of the box (if you bought the experimental kit, you should tape the thicker lens to the box). Try not to let the tape cover too much area of the lens. Place a prism in front of the lens so that the light from the slit will be first focused by the lens then refracted twice by the prism. Now you have a prism-based spectroscope.
6- Fix a green LED, a red LED, and an incandescent light bulb together into the slit of the box. Tape them if they are not secure. Connect the three lights to the batteries in parallel. (Tip: you can use the batteries as a power switch. Install them to turn lights on, then take out one to turn lights off.) Important not