One may conceive light to spread successively, by spherical waves. –Christiaan Huygens, 1600s
What theory was Huygens writing about?
the particle theory of light
the reflection of light off objects
the way objects refract light
the wave theory of light

DE = 4cm.

Step-by-step explanation:

The Isoscles Triangle Theorem says that:

"If two sides of a triangle are congruent, then angles opposite those sides are congruent."

The converse of this theorem is

"If angles opposite those sides are congruent, then two sides of a triangle are congruent."

Therefore, since ∠F is congruent to ∠D and are opposite sides ED and EF

while EF = 4 cm and DF = 5 cm.

DE = 4cm.