Passage VI

A physics teacher asked 3 students to predict the changes, if any, to light's energy, E; frequency, f(the number of wave peaks passing a given point each second); wavelength, λ (the distance between adjacent peaks of a light wave); and speed, v_L, that occur when light travels from a vacuum into clear glass and then from the glass back into the vacuum. The teacher asked the students tobase their predictions on one or both of the following equations:

E = h x f, where h always has the same value

v_L = f x λ

Student 1

When light enters the glass, f and E are unchanged. This occurs because light loses no energy when it collides with atoms of the glass. However, v_L becomes less than c (the speed of light in a vacuum, 3 x 10⁸ m/sec) due to these collisions, so λ must also decrease.

As the light leaves the glass, both f and E are still unchanged. But upon reentering the vacuum, v_L once again becomes c, so λ must increase.

Student 2

When light enters the glass, both f and E decrease, because light loses energy when it collides with atoms of the glass. In addition, v_L decreases due to these collisions, so the product f x λ must also decrease. However, λ can either decrease or increase, so long as any increase in λ is not so great as to cause f x λ to increase.

When the light leaves the glass, neither f nor E changes, because there is nothing present in a vacuum that will cause for E to increase. But v_L increases to c, the speed of light in a vacuum, so λ must also increase.

Student 3

When light enters the glass, both f and E decrease, because light loses energy when it collides with atoms of the glass. However, v_L becomes greater than c due to the gravitational attraction between the glass atoms and the light, so the product f x λ must also increase. Thus, λ must increase, and its increase must be great enough to overcome the decrease in f.

As the light leaves the glass, f and E will have the same values as they had inside the glass, because there is nothing present in a vacuum that will cause f and E to change. However, because of the gravitational attraction between the glass atoms and the light, v_L decreases to c, so λ must also decrease.