Student Exploration: Basic Prism
Vocabulary: angle of incidence, angle of refraction, critical angle, dispersion, index of refraction, medium, prism, refract, Snell’s law, total internal reflection, visible spectrum, wavelength
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
A prism is a transparent object that can be used to bend, or refract, light. The photo below shows what happens when sunlight passes through a prism.
What do you see?
What does this indicate about the composition of sunlight?
The Basic Prism Gizmo™ allows you to investigate how a prism refracts light. The Gizmo shows a laser emitting a beam of light through a triangular prism.
To begin, check that Single color beam is selected, λ is 500 nm, n is 1.50, w is 2.0, and the angle (θ) is 0°.
What do you notice about the path of light when it passes through the prism?
Move the λ slider to increase and then to decrease the wavelength of the beam of light.
How does this affect the color of the light ray?
How does this affect the direction of the light ray?
Refraction and dispersion
Get the Gizmo ready:
Set λ to 500 nm and n to 1.00.
Check that w is 2.0 and θ is 0°.
Introduction: When light passes from a vacuum into a medium such as glass, it slows down. The index of refraction (n) is equal to the ratio of the speed of light in a vacuum to the speed of light in a medium. The index of refraction of air is very close to 1.00.
Question: What factors affect the refraction of light through a prism?
Observe: What do you notice about the beam of light when the n is 1.00? The beam of light travels in a straight line through the prism.
Predict: How do you think increasing the index of refraction will change the path of the light?
Observe: Slowly increase n to 1.50, close to the value for glass. What happens?
Observe: Increase n to 1.53. What happens now?
In this situation, the light cannot move from the prism into the air. Instead, it reflects off the surface, a phenomenon called total internal reflection.
Gather data: Set n to 1.50. The Gizmo also allows you to change the width of the prism (w), the angle of the prism (θ), and the wavelength of the light (λ). Describe the effect of each action listed below. Return the Gizmo to its starting conditions after each trial.