Light – Reflection and Refraction

Chapter Notes: Light – Reflection and Refraction

Light is a form of energy that makes objects visible. It travels in straight lines and can be reflected or refracted in various media. Understanding light's behavior is crucial for various optical phenomena and technologies.

Light's Behavior and Properties

• Visibility: Objects are visible when they reflect light into our eyes. In a dark environment, light sources are necessary for us to see.
• Ray Model of Light: Light is often modeled as rays traveling in straight lines. The path of a ray can be represented by an arrow indicating direction.
• Diffraction: When light encounters a very small obstacle, it bends around it, known as diffraction. This explains some anomalies in the strict ray model.

Reflection of Light

Reflection occurs when light bounces off a surface. The primary laws of reflection are:

1. Angle of Incidence = Angle of Reflection: The angle between the incoming ray and the normal is equal to the angle between the reflected ray and the normal.
2. Planarity: The incident ray, normal, and reflected ray all lie in the same plane.

Types of Mirrors:

• Plane Mirror: Produces a virtual, erect image that is the same size as the object.
• Concave Mirror: Curved inward. Can produce both real (inverted) and virtual images depending on object distance.
• Convex Mirror: Curved outward. Always produces virtual images that are smaller and erect.

Image Characteristics with Spherical Mirrors:

• The nature (virtual or real), position (location relative to the mirror), and size (comparison to the object) of the image depend on the object’s position.
• Table of Image Formation by Concave Mirror:
  • At Infinity: Real and diminished image at the focus F.
  • Beyond C: Real and diminished between F and C.
  • At C: Real image of the same size at C.
  • Between F and C: Real and enlarged image beyond C.
  • At F: Real image formed at infinity.
  • Between P and F: Virtual, enlarged image behind the mirror.

Refraction of Light

Refraction is the bending of light as it crosses the boundary between two different media. Its behavior is governed by:

1. Continuity: The incident ray, refracted ray, and normal all lie in the same plane.
2. Snell's Law: The sine of the angle of incidence (i) to the sine of the angle of refraction (r) is a constant. This constant is known as the refractive index (n) of the second medium concerning the first:
   [ n = \frac{\sin(i)}{\sin(r)} ]

The refractive index also relates to the speed of light in different media: [ n = \frac{c}{v} ]
Where c is the speed of light in vacuum and v is the speed in the medium.

Laws of Refraction:

• Light travels fastest in a vacuum and slows down when entering denser media (lower refractive index to higher).
• Common experiences of refraction include the apparent bending of objects seen in water.

Lenses and Their Formation of Images

Lenses are transparent materials that refract light. They can be:

• Convex Lens (converging): Thickest in the middle.
• Concave Lens (diverging): Thickest at the edges.

Image Formation by Lenses: Similar principles apply to lenses as to mirrors, summarized in tables for various object positions:

• Convex Lens: Forms real images when objects are beyond F; virtual images are formed when the object is between F and O.
• Concave Lens: Always forms virtual images, always smaller than the object, regardless of the object's position.

Ray Diagrams: Used to illustrate image formation. Depending on the lens type:

1. Parallel rays converge at the focus in convex lenses.
2. Parallel rays appear to diverge from the focus in concave lenses.

Sign Convention and Formulas

• Sign Convention: Similar conventions for mirrors and lenses where distances are measured from the optical center for lenses.
• Mirror Formula: [ \frac{1}{f} = \frac{1}{v} + \frac{1}{u} ]
  Where f is the focal length, v the image distance, and u the object distance.
• Lens Formula: Similar to the mirror formula, but with different signs for focal lengths, [ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} ]
• Magnification: Ratio of image height (h') to object height (h): [ m = \frac{h'}{h} = - \frac{v}{u} ]

Key Concepts in Refraction and Lenses

• Power of a Lens: The ability to converge (convex) or diverge (concave) light. Given by: [ P = \frac{1}{f} ]
• Power is measured in diopters (D). A positive value indicates a converging lens, and a negative value indicates a diverging lens.

1. Light travels in straight lines, making objects visible by reflection and refraction.
2. Laws of Reflection: Angle of incidence equals angle of reflection.
3. Types of Mirrors: Concave mirrors can produce real and virtual images; convex mirrors only produce virtual images.
4. Refraction: Bending of light occurs when entering different media, following Snell's law.
5. Refractive Index: Determines how much light bends, calculated as the ratio of light speeds in different media.
6. Lenses: Convex lenses converge light and can form real and virtual images; concave lenses always produce virtual images.
7. Sign Conventions: Distances and focal lengths are categorized based on directions from mirrors and lenses.
8. Mirror and Lens Formulas: Used for calculating image and object distances; magnification ratios.
9. Power of Lens: Indicates ability to bend light, expressed in diopters (D). A convex lens has positive power; a concave lens has negative power.