In your own words, explain what a focal length is. Try not to use any equations or refer to any specific type of mirror or lens.

Estimate the focal length of a typical bathroom "magnifying mirror".

Images formed by spherical lenses can be real or virtual. What does it mean for an image to be virtual? Can such an image be seen?

Estimate the focal length of a typical magnifying glass.

What is the focal length of a clear, flat, piece of glass, such as a normal household window?