# Optical Inversion

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Activity Language
Time Needed for Activity
Subjects

Students will be introduced to a refraction - when light bends it passes from one material to another.

## What You Need

Materials:

• A clear glass/cup for the class
• Water to fill the glass/cup
• 1 notecard/paper per student
• 1 marker/pen per student

## What To Do

Demonstrating the experiment:

1. Fill your glass of water
2. Draw a horizontal arrow on a note card
3. Put the note card behind the glass of water and slowly move the note card back
4. Look through the glass from the front and observe the arrow changing direction

Applying the experiment:

1. Distribute the note cards and the markers. Make sure each student gets one note card and one marker
2. Let each student draw something on the card, as long as the right-hand side differs than the left-hand side (as long as the design is NOT symmetric). Alternatively, they can draw two different things beside each other (e.g., a circle and a triangle)
3. Let each student try placing their card behind the glass of water, moving it slowly backwards and observe the inversion of their drawing

## Discovery

#### What's Happening?

When light that is traveling through a material reaches a second material, some of it may enter this second material. At the point at which the light enters the second material, the light will bend and travel in a different direction than the incident light. This is called refraction. Refraction happens because the speed of light is different in different materials though always less than the speed of light in a vacuum.

In this experiment, light first travels through the air - which is easy to move through - and then through the water, where it slows down. This change in speed causes the light to bend, or refract.

The round sides of the glass forces the water into a rounded shape, which acts as a convex lens. This lens bends the incoming light towards the middle. Here, the light rays meet at what is called the focal point. Past the focal point, the image is inverted because the light rays overlap.

#### Investigate Further

For a wider audience, more than one glass can be used.
A greater amount of water might be needed, depending on the number of glasses being used.
However, it is important that each and every student gets the chance to watch and observe the inversion.
If time does not permit, the drawings on the note cards can be done ahead of time or each group instead of each student can have one card with a drawing on it.

#### Resources

A similar experiment can be found here: https://ingeniumcanada.org/scitech/education/try-this-out-broken-pencil-illusion.php

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