Section 1 review
Light travels in straight lines at 3×108 m/s. Light carries energy and information and can vary in intensity. The intensity of light has units of power divided by area, or watts per square meter. Light has both wave and particle properties, and its energy determines its color. Light rays can be diverted when they interact with matter. The properties of light explain the formation of shadows, why Earth has cycles of night and day, why there are eclipses of the Sun and Moon, and why there are phases of the Moon. Light intensity decreases with distance following an inverse square law. Gravitational and electrical forces are other examples of inverse square laws. Read the text aloud
ray diagram, inverse square law, object distance, image distance

I= P A

Review problems and questions

  1. The distance between the Earth and the Sun is one astronomical unit (AU), which is 1.50×108 km. A light year (ly) is the distance light travels in one year.
    1. How many minutes does it take light to travel from the Sun to the Earth?
    2. How many kilometers is a light year?
    3. How many AU is a light year? Read the text aloud Show
    1. What is the difference between a solar and a lunar eclipse?
    2. Which body’s shadow projects onto another in a solar eclipse? In a lunar eclipse?
    3. Which shadow is larger? Why? Read the text aloud Show
  1. The dwarf planet Makemake has an average orbital distance of 45.8 AU from the Sun. If sunlight reaching the Earth has an intensity of 1,365 W/m2, what is the intensity of sunlight reaching Makemake? Read the text aloud Show
  1. Where is the lens of a pinhole camera located? Read the text aloud Show
  1. The light intensity at a spot 1.0 m away from a bright lightbulb is 30 W/m2.
    1. What is the intensity at a distance of 2.0 m?
    2. What is the intensity at a distance of 0.5 m?
    3. At what distance from the bulb will the intensity be 15 W/m2? Read the text aloud Show
  1. Sometimes near midday on a sunny day, you can see many images of the Sun projected onto the ground underneath a leafy tree. What creates the images? Read the text aloud Show

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