AP Physics 2- 13.1 Reflection- Study Notes- New Syllabus
AP Physics 2- 13.1 Reflection – Study Notes
AP Physics 2- 13.1 Reflection – Study Notes – per latest Syllabus.
Key Concepts:
- Light as a Ray
- Reflection of Light from a Surface
Light as a Ray
Light rays are an idealized model used to represent the straight-line propagation of light in a medium. This model is part of geometric optics, which simplifies the behavior of light by ignoring its wave nature and diffraction effects.
Key Concepts:
- Straight-line motion: In a homogeneous medium, light travels in straight lines. These lines are represented as rays with arrows indicating the direction of energy transfer.
- Rectilinear propagation: The tendency of light to travel in straight lines explains phenomena such as sharp shadows and the apparent straight-line path of vision.
- Boundaries and interactions: When light rays encounter boundaries between media, they follow rules of reflection and refraction.
- Ray diagrams: Useful tools for analyzing the formation of images by mirrors and lenses. Rays are drawn to represent the path light takes through an optical system.
- Limitations: The ray model does not account for diffraction and interference, which require the wave model of light.
Example:
Why do objects cast sharp shadows when illuminated by a point source of light?
▶️ Answer/Explanation
Because light travels in straight lines, when a point source is blocked by an opaque object, no rays bend around the edges. This results in a sharply defined shadow. If the source is extended, overlapping rays produce both an umbra (full shadow) and penumbra (partial shadow).
Reflection of Light from a Surface
Reflection is the phenomenon in which light rays bounce back from the boundary of two media, instead of passing through. The law of reflection governs how light interacts with reflective surfaces such as mirrors.
Laws of Reflection:
- The angle of incidence (\(\mathrm{\theta_i}\)) is equal to the angle of reflection (\(\mathrm{\theta_r}\)).
- Both the incident ray, reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
Types of Reflection:
- Specular Reflection: Specular reflection is the reflection of light from a smooth surface and results in light uniformly reflected from the surface, because the line normal to the surface has an approximately constant direction over the area the light strikes.
- Diffuse Reflection: Diffuse reflection is the reflection of light from a rough surface and results in light reflected in many different directions, because the line normal to the surface varies over the area over which the light is incident.
Image formation by plane mirrors:
- The image is upright, virtual (cannot be projected on a screen), and the same size as the object.
- The image appears behind the mirror at the same distance as the object is in front.
Example:
A light ray strikes a plane mirror at an angle of incidence of \(\mathrm{40^\circ}\). What is the angle of reflection?
▶️ Answer/Explanation
By the law of reflection, \(\mathrm{\theta_i = \theta_r}\). So, if \(\mathrm{\theta_i = 40^\circ}\), then \(\mathrm{\theta_r = 40^\circ}\).