3.2: Measuring Light

1.1 Inverse Square Law of Light

Model

The model below shows how light spreads out with distance. Each single grid square is the same size.

Light source illuminating expanding square regions at increasing distances from the source.

Image Credit: OpenStax Astronomy 2e (https://openstax.org/details/books/astronomy-2e)

1. Identify answers to the following:
   1. How should the total amount of light that is spread across the full square at each letter compare?
   2. As distance from the source increases, does this light spread over a smaller or larger area?
2. Construct a CER: How should the amount of light that is spread across a single grid square at each distance?
3. The amount of light in a single grid square is the brightness / intensity of light. As you get farther from a source, does that brightness / intensity increase or decrease? Explain your reasoning.

1.2 Absolute and Relative Brightness

Model

On the road you see a number of identical cars with their headlights on.

Identical cars with headlights at different distances demonstrating apparent brightness.

Image Credit: Astronomy Interactives from the Astronomy Education Department at the University of Nebraska-Lincoln (https://astro.unl.edu/interactives/)

1. How do you think the amount of light given off by the headlights (Luminosity) compares for the different cars? Explain.
2. How do you think the brightness / intensity of the light as interpreted by your eye compares for the different cars? Explain.
3. If the road is really dark and you cannot see which car is farther and which car is closer, how could you figure out which car is farthest away?

1.3 Measuring Distance Using Brightness

Model

The image below shows the apparent brightness of 5 stars from Earth. The darker shades represent dimmer stars.

Image Credit: Astronomy Interactives from the Astronomy Education Department at the University of Nebraska-Lincoln (https://astro.unl.edu/interactives/)

The image below shows the absolute magnitude (luminosity) of the different stars. Lower numbers (including more negative values) mean greater luminosity.

Image Credit: Astronomy Interactives from the Astronomy Education Department at the University of Nebraska-Lincoln (https://astro.unl.edu/interactives/)

1. Which star appears the brightest?
2. Which star has the greatest luminosity?
3. Which star appears the dimmest?
4. Which star has the least luminosity?
5. Rank the stars from closest to farthest away. Explain your reasoning.

Summary

If an object has a lower (more negative) absolute magnitude but appears dimmer, that means the object is ____________________ away.

1. Fill in the blanks in the table to identify the fraction of light received at each distance. Explain your reasoning below the table.

   Activity Credit: Astronomy Interactives from the Astronomy Education Department at the University of Nebraska-Lincoln (https://astro.unl.edu/interactives/)

2. Fill in the blanks in the table to identify how distance, luminosity, and apparent brightness are related.

   Activity Credit: Astronomy Interactives from the Astronomy Education Department at the University of Nebraska-Lincoln (https://astro.unl.edu/interactives/)