Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Snell's Law and Conservation of Energy
Name: Leo
Status: other
Age: N/A
Location: N/A
Country: N/A
Date: N/A

Assume the follow condition: A Prism in a vacuum Now shot a ray of light through the prism. We know through observation that the light will slow relative to the refractive index. However, when the ray of light leaves the prism it speed back up to 3.0x 10^8 m/s when no additional energy is added to the system. How can this be? I know that the conservation of energy must be true, so what am I missing?

A photon is massless and hence its energy cannot be equated to mass*(velocity*velocity)/2. Instead it is described by Planck's constant times the frequency. The frequency (and hence the energy) does not change when the light goes from vacuum into glass.

Greg Bradburn

Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory