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Curriculum Resources

Prism Spectrum – Why can you get sunburnt on cool days?

A refracting triangular prism is made from clear material like glass or plastic. When a beam of light is passed through it in a certain way, the light is refracted or split into different wavelengths. This reveals the colours of the spectrum and raises some interesting questions. Developing an awareness of these concepts helps students understand why you can get sunburnt on cool days.

Learning Intentions:

  • Introduce the concept refraction and reflection
  • Illustrate that heat and UV are different
  • Introduce the Latin words infra and ultra
  • Demonstrate the visible spectrum.

Key messages:

  • Visible spectrum (ROYGBIV)
  • Skin damage is caused by ultraviolet (UV) radiation, not temperature. A cool or overcast day can have similar UV levels to a warm, sunny day
  • UV and heat are different, they are not the same thing
  • When the UV Index is 3 or above, sun protection is required
  • Best sun protection practices are using a combination of slip, slop, slap, seek and slide


  • A prism – these can be purchased online at Mad About Science or visit the SciTech shop
  • A white screen. This can be a sheet of paper or card (or a white wall or projector screen etc)
  • A variety of torches – some LED and some incandescent is best
  • A sheet of thin card
  • Scissors
  • A darkish room helps this lesson greatly
  • Screen – wall, large white paper, whiteboard or projector screen


  1. The prism works best when a narrow beam of light is passed through it. You can use a piece of card to achieve this
  2. Cut a short 2mm wide slit in the card
  3. Fold card so that it stands on the table
  4. Turn torch on and place it behind the slit in the card
  5. Move the torch closer to or further away from the card until you achieve a beam of light with parallel sides coming from the slit
  6. Place your prism into the beam of light. Experiment with the position of the prism until you see the beam of light bend ~ 90 degrees
  7. Place your screen in the path of the beam that is coming out of the prism. You should see a spectrum on the screen. If you are using a wall or projector screen, you will need to move the prism and beam of light until you see the spectrum
  8. Using your prism, ask students to experiment with the positions of the prism, the torch and the screen to achieve a good spectrum. The spectrum is brightest when the screen is close to the prism and fainter as the prism and screen distance become further apart.

What’s going on?

Light travels at different speeds in different media. When light enters the prism, it slows down. This speed change causes the light to be bent (refracted) and to travel through the prism at a different angle. The degree of bending of the light’s path depends on the angle that the incident beam of light makes with the surface of the prism and on the refractive index of the prism.

The refractive index of a material describes the degree to which the material bends light). Since different wavelengths – or colours – of light refract at different angles, the prism causes each colour in the white light to be refracted differently and to leave the prism at a different angle, creating an effect like a rainbow.

With accuracy care, precision students should be able to produce a full spectrum from red though to violet. This represents the full range of wavelengths that can be seen by the human eye. But there is MUCH more light energy here that cannot be seen.

Ultraviolet radiation

Just beyond the red is the infrared. The word ‘infra’ is a Latin word, and it means ‘beneath or below’. This is the part of the spectrum that we feel as heat. Infrared light can be seen with night vision goggles and some cameras. Infra redlight was discovered by William Herschell.

At the other side of the visible spectrum just past the violet light is ultraviolet. (Ultra is Latin for ‘above or beyond’). Ultraviolet light is the part of the light that causes sunburn and skin damage.
You cannot feel it. This demonstration shows us that heat radiation, infrared (IR) radiation and
sunburning radiation, ultraviolet (UV) radiation are separate.

Key questions

  • Do you put sunscreen or wear a hat on when it is cold outside? Why/why not?
  • What do you see and feel from the sun?
  • What do you notice when the light shines onto the prism?
  • Is there more to light than we can see at first?
  • Why can you get sunburnt on cool days?
  • Why do rainbows happen?