Refraction of Laser Light

# Refraction of Laser Light

Introduction

CONSTRUCTION DEMONSTRATION

Look at the picture above. Sometimes when we place a straw in a cup of water, it appears to look disjointed. This is something we see everyday and never question. But this is impossible, isn't it? We know that the straw is in one piece, not two. The reason this happens is refraction. As light travels from one substance to another, it actually bends. This is why, in the example above, the pencil does not appear to be connected. Refraction is a fascinating albeit perhaps counterintuitive idea. How and why does light bend? Which direction does light bend?

This experiment uses laser lights and acrylic lenses to demonstrate refraction across different media and shapes. This experiment acts as a clear, visual representation of the concepts and equations learned in class. The idea being: it is much easier to understand something if you can see it with your own eyes.

This experiment assumes a general background in refraction. Students should be familiar with the following terms: normal line, medium, incident ray, refracted ray.

Construction

Materials Used:

30 inch x 14 inch Piece of Plexiglas (1/2 inch thickness)

30 inch x 14 inch Thin, White Board (1/4 inch thickness)

30 inch x 8 inch Piece of Plexiglas (1/2 inch thickness)

1 Circular Magnet (1 inch diameter)

Laser Box (design attached below)*

5 Laser Lights, varying in colour (below, left)

Acrylic Lenses, varying in shape (below, right)

Thin, Metal Sheet

White Poster Paper

Drill

Drill Press

Small Screws

Pen

Scissors

Epoxy Glue

*Note: The Laser Holder design was a collaborative effort between me and the Student Machine Shop (SMS) at UBC. The Laser Holder is composed of a metallic base with five circular holes for each of the lasers. At the end of each hole is a cylindrical lens. When the lasers are put through the holes and turned on, their light hits the cylindrical lens and it is projected downwards. This effect creates a visible line of light on the surface on which the Laser Holder is placed.

1 - Using a drill press, drill a one inch in diameter circular hole into the middle of the thin, white board.

2 - Using a drill and screws, attach the 30 inch piece of Plexiglas to the thin, white board so that they are overlapping. There should be at least one screw in each corner. This is the base of the experiment.

Note: make sure that the length of the screws does not exceed the thickness of the Plexiglas and the white board.

3 - On the left edge of the base (in step 1) and in the middle vertically, use a drill to screw on the Laser Box, one screw for each corner.

4 - Using epoxy glue, place and glue the magnet into the small circular hole in the base. Allow 5 minutes for this to dry.

Note: the magnet should be level with the rest of the white board.

5 - Place the 30 inch x 8 inch Plexiglas piece flat on a sturdy surface. Place a generous amount of epoxy glue along the 30 inch edge of the base. Place the edge of the base in the middle of the 8 inch Plexiglas piece to create a stand.

Note: Make sure you manually hold this in place for at least 30 minutes to allow the glue to dry properly and evenly.

6 - Using a pen, trace out the shapes of the acrylic lenses on the metallic sheet and the white poster paper. Cut out these shapes using scissors.

7 - Using expose glue to attach these pieces together so that it goes:

Acrylic Lens/Poster Paper/Metallic Sheet

Allow 5 minutes for this to dry.

Demonstration

Now, we can attach the acrylic lenses to the base via magnets. When the lasers are added into the Laser Box, we are able to see the path of the lasers as the approach the lens and begin to refract.

Lesson Plan/Pedagogy

Light and Optics of a part of the BC science curriculum that is introduced in grade 8. It is further expanded upon in grade 11.

The following lesson plan is designed for grade 8 students, however, I will include some adaptions for presenting to older students. The main difference being: the older students will be familiar with Snell's Law whereas the grade 8 students will work in more generic terms (the light will bend away/toward the normal).

Learning Outcomes:

Students will be able to:

• to briefly describe refraction and how it occurs
• to give at least 2 examples of refraction in everyday life
• to identify the normal line within a system consisting of an incident ray and two media
• to define the relationship between the density of a substance and the speed of light (Grade 8 only)
• to define the relationship between the index of refraction of a substance and the speed of light (Grade 11 only)
• to predict the path of a refracted light ray by determining the normal line and the direction in which light bends (Grade 8 only)
• to use Snell's law to calculate the path of a refracted light ray (Grade 11 only)

Breakdown of Lesson:

Introduction to Refraction/Background Information (10 mins)

Slides 1-9

Discussion of Basic Principles

• when light is travelling from one substance to another, it bends
• light changes speed depending on the density of the substance
• density refers to how closely the particles in a substance are packed together
• a normal line is perpendicular to the boundary between two substance and intersects the incident ray

Classroom Discussion - Making Predictions

• if a substance has a high density are the particles packed closely together or farther apart?
• if a substance has a high density, will light be able to move faster or slower within it?

Examples of Refraction - Analysis

Straw in a cup of water

• the straw appears disjointed at the boundary between the air and water
• water has a higher density than air
• light travels faster in air than in water
• at the boundary light changes speed and bends

Making Predictions - Where is the Normal Line? (10 minutes)

Slides 10-13

Think Pair Share

Students are paired in groups of 2 and are given a system with an incident light ray and two media. The students must discuss where the normal line will be and draw a brief diagram in their notes.

Results and Classroom Discussion

More Background Information (5 minutes)

Slides 14-16

• light bends towards the normal if travelling from a low density to high density substance
• light bends away from the normal if travelling from a high density to low density substance

Making Predictions - Laser Demonstration(25 minutes)

Slides 17-35

Students are given a system featuring an incident ray and two media. This system will resemble the laser light passing through an acrylic lens. Working in small groups of 2-3, they must determine

• 1. which substance has a higher density
• 2. the position of the normal line
• 3. if the light bends towards of away from the normal

Note that: this is done in a step-by-step process. Students are asked to discuss one aspect at a time (e.g. which substance has a higher density?). This is followed by a class discussion. Then, they are asked to discuss the second aspect (e.g. where is the normal line?). This is followed by another class discussion. So on and so forth.

After all three aspects have been discussed both in groups and as a class, the experiment is turned on to replicate this system. Students will be able to compare and contrast their predictions with what actually happened as they observe the path of refracted light. This is followed by a brief class discussion of results.

This system is repeated 5 more times with different lenses and their configurations.

This is a worksheet for Grade 11s and incorporates Snell's Law and other more advanced concepts. This worksheet would replace the Think Pair Share activities above and is meant to be completed in pairs.