# Lesson Plan: Thin Film Interference Physics • 9th Grade

This lesson plan includes the objectives, prerequisites, and exclusions of the lesson teaching students how to analyze the interference of light reflected from parallel layers of substances with different refractive indices.

#### Objectives

Students will be able to

• diagrammatically describe the mechanism of production of thin film interference,
• for a thin film of refractive index that is resting on a substance of greater refractive index than , relate the separation, , between the upper and lower layers of the thin film to the order, , of the interference maximum produced by light that has wavelength in a vacuum when it reflects from the upper and lower layers of the thin film, using the equation , where is the angle of incidence at the upper layer,
• for a thin film of refractive index that is resting on a substance of greater refractive index than , relate the separation, , between the upper and lower layers of the thin film to the order, , of the interference minimum produced by light that has wavelength in a vacuum when it reflects from the upper and lower layers of the thin film, using the equation , where is the angle of incidence at the upper layer,
• for a thin film of refractive index that is resting on a substance of smaller refractive index than , relate the separation, , between the upper and lower layers of the thin film to the order, , of the interference maximum produced by light that has wavelength in a vacuum when it reflects from the upper and lower layers of the thin film, using the equation , where is the angle of incidence at the upper layer,
• for a thin film of refractive index that is resting on a substance of smaller refractive index than , relate the separation, , between the upper and lower layers of the thin film to the order, , of the interference minimum produced by light that has wavelength in a vacuum when it reflects from the upper and lower layers of the thin film, using the equation , where is the angle of incidence at the upper layer,
• relate the thickness, , of antireflection coating of refractive index to the wavelength, , in a vacuum of light that the coating prevents reflection of, using the formula .

#### Prerequisites

Students should already be familiar with

• total internal reflection,
• the phase change of a reflected wave,
• the fact that a path difference change of one wavelength corresponds to a phase difference change of rad,
• sine and cosine functions.

#### Exclusions

Students will not cover

• films of variable thickness.