Focus

Definition

Focus is a point where rays of light or other radiation converge or from which they appear to diverge. In astronomy, it often refers to the points in an elliptical orbit around which celestial bodies revolve.

5 Must Know Facts For Your Next Test

1. In Kepler's First Law of Planetary Motion, each planet orbits the Sun in an ellipse with the Sun at one focus.
2. The concept of focus is crucial for understanding how telescopes gather and concentrate light.
3. For elliptical orbits, there are two foci; one of these is occupied by the central body being orbited.
4. The distance between the foci determines the eccentricity of an ellipse, affecting orbital shape and behavior.
5. In reflecting telescopes, mirrors are shaped to bring incoming light to a focus, forming a clear image.

Review Questions

• What role does the focus play in Kepler's First Law of Planetary Motion?
• How do telescopes use the concept of focus to form images?
• Why is understanding the distance between foci important for determining an orbit's eccentricity?

Related terms

Ellipse: An oval-shaped curve that describes the path of celestial bodies in orbit under gravity.

Eccentricity:

A parameter that measures how much an orbit deviates from being circular.

Reflecting Telescope:

A type of telescope that uses mirrors to gather and focus light.