Detecting IR light using a thermometer

Light coming from the sun is made up of a large range of wavelengths. It contains light from the ultraviolet, visible, and infrared regions


  • Prism
  • Thermometer
  • Black tape


Cover the tip (metal part) of the thermometer with a piece of black tape. This will help it absorb light. Put the prism in the path of the sunlight and use it to create a rainbow pattern on a flat, white surface (The distance between the prism and the flat surface will determine the width of the rainbow spectrum. Larger distance will allow it to spread out more, but intensity will be lower.) Place the tip of the thermometer in the blue region and leave it for two minutes, then record the temperature. Do this again for the green and the red. Now put the thermometer in the region just past the red, where there appears to be no light, and record the temperature again. Where is the temperature highest?


Much of the UV is filtered out by our atmosphere, leaving mostly visible and infrared radiation. We can see the visible with our eyes, but what about the infrared? When light is absorbed by a material, it can heat up. We can use this property of light to detect light that we cannot see.