Modern Physics

Assessment Problems

• 1. Explain how spectral lines in a line spectrum are produced.

Answer: When the atoms of a gas like neon absorb energy and “jump” to higher energy levels, they exist in an “excited state”. These electrons then fall back down to lower energy levels, emitting energy with specific frequencies determined by the size of the downward jump. These frequencies appear in various portions of the electromagnetic spectrum, depending on the size of the jump. Some jumps downward emit light of frequencies that appear in the visible portion of the spectrum. These are the spectral lines we see.

• 2. What is the difference between the visible spectrum produced by the sun and the line spectrum of hydrogen gas?

Answer: The sun produces all frequencies in the visible spectrum. These frequencies range from the red to the violet. Hydrogen gas, however, produces a line spectrum, each line representing the frequency of the radiation released when the electron of hydrogen falls from a higher energy level to a lower one.

• 3. List the four fundamental forces and define their purpose.

Answer:  1. Gravitational force: A weak force but acts over a long range. It’s an attractive force that acts

between any two pieces of matter.

2. Weak force: A very weak force that acts over very short distances.

It is responsible for radioactive decay.

3.  Electromagnetic force: A long-ranged force that can be either repulsive or attractive.

It acts between electrically charged pieces of matter.

It is responsible for magnetic and electric effects.

4. Strong force: A force holding the nucleus of atoms together. Operates over very short distances.