Bending Light

Felicia Cherry
Product Manager for Physical Science, Physics, and Earth Science

Total internal reflection is a phenomenon that occurs when light enters a medium at an angle greater than what is required for refraction into a second medium, where the first medium has a higher index of refraction than the second. Instead of passing through the boundary into the second medium, the light gets reflected back into the first medium.

Students with a strong understanding of light wave properties, reflection, refraction, and Snell’s Law know that light travels in a straight path. So how is it possible for light to “bend” and follow a curved stream of water?

Materials

• Clear Plastic Bottle with a Cap, 1 L (or larger)
• Small Nail or Screw
• Red Laser Pointer
• Water
• Sink or Container (to catch the water)
• Clear Tape

Safety

Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules.

Know and follow all school district guidelines for classroom laser use. Avoid direct eye exposure to laser light. Never look directly at a working laser. Never direct or deflect a laser at oneself or another person. This activity does not require the use of mirrors or highly reflective surfaces.

Teacher preparation

• Using a small nail, poke a hole in the side of the bottle 2 to 3” from the bottom. To help get smooth edges, place a piece of clear tape over the area before making the hole.
• Place a finger over the hole. Fill the bottle with water and cap it. Remove your finger. The water will stay in the bottle. 

Procedure

1. Darken the room (complete darkness is not necessary).
2. Check to be sure the stream of water will be directed into the sink or catch container.
3. Remove the cap; the water will start to flow.
4. Aim the laser at the water in such a way that the stream of water appears to glow.

This demonstration can be a quick way to assess what students already know before beginning a unit on light or to assess the level of student understanding at the completion of a unit.

Ask students to apply what they know about light, light waves, or the properties of light to explain what they saw. Responses may include:

• Light bends when it is in moving water.
• Water breaks the light into smaller pieces that follow the water then go out when they hit something.
• The light is bouncing around in the stream.
• The laser is a special light that bends.

Explanation

What occurs is the total internal reflection of the laser light shining through the water at an angle greater than the one required for refraction into the air. Instead of the light passing from water to the air and refracting, all of the light gets reflected back into the water stream flowing out of the bottle. The stream appears to glow red.

More specific to this activity, we have a laser light and a water/air interface.

The known critical angle for this system is 48.6°. Light traveling towards the boundary:

< 48.6°     refracts into the air
= 48.6°     travels along the boundary between the 2 media
> 48.6°     gets reflected back into the first water

The water is bending but the light is refracting in a sequence of straight paths bouncing off the sides of the stream. The angle of refraction between the 2 media is too great for the beam to overcome. The bouncing beams are in essence trapped within the stream.