Physical science 7

Reflection

  • Reflection is defined as the change in direction of a wave when it strikes and rebounds from a surface or the boundary between two media.

  • Can be conceptualized as light "bouncing off" a surface; however, this phenomenon is more complex than just bouncing.

  • Two types of reflection:

    • Regular (specular) reflection: Occurs from very smooth surfaces, like mirrors.

    • Irregular (diffuse) reflection: Occurs from relatively rough surfaces.

Reflection & Ray Diagrams

  • Ray: A straight line that represents the path of light.

  • A group of parallel rays represents a beam of light.

  • Law of Reflection: The angle of reflection is equal to the angle of incidence. Additionally,

    • The reflected and incident rays are in the same plane.

Specular & Diffuse Reflection

  • Regular reflection is from smooth surfaces.

  • Diffuse reflection results from rough surfaces.

Ray Diagrams

  • Ray diagrams are used to determine the apparent location of an image formed by a plane mirror.

    • Components:

    • Mirror: Reflective surface.

    • Normal Line (0): Perpendicular to the surface at the point of incidence.

    • Image (a): The perceived location of the object as viewed in the mirror.

    • Object (b): Actual object being reflected.

    • Eye: The observer's perspective; where the light rays converge.

Height Requirement for Full Reflection

  • To see their complete figure in a plane mirror, a person's mirror must be at least half the height of the person.

Refraction of Light

  • Refraction: The bending of light waves caused by a speed change as light transitions from one medium to another.

Indexes of Refraction for Common Substances

Higher indices of refraction result in greater slowing of light.

Substance

Index of Refraction (n)


Water

1.33


Crown glass

1.52


Diamond

2.42


Air (0°C, 1 atm)

1.00029


Vacuum

1.00000

Reflection and Refraction

  • When light passes into a denser material, the angle of refraction is less than the angle of incidence and bends toward the normal line.

Examples of Light Refraction

  • Stars that Twinkle: Variation of index of refraction with density/temperature causes twinkling as starlight passes through different atmospheric layers.

Internal Reflection

  • When light passes into a less dense material, the angle of refraction exceeds the angle of incidence and bends away from the normal line.

Reflection by Refraction

  • At a certain critical angle, the angle of refraction becomes 90 to the normal. If incidence exceeds the critical angle, internal reflection occurs.

Internal Reflection Exceeding the Critical Angle

  • Diamonds and other gemstones are designed to enhance internal reflection, which contributes to their sparkle.

  • Fiber optics: Technology utilizing internal reflection to transmit light efficiently through glass/plastic fibers.

Dispersion

  • The index of refraction, changes slightly according to the wavelength of light.

Dispersion in Various Media

  • Dispersion occurs in materials like diamonds, gemstones, and leaded glass, contributing to their visual characteristics (e.g., "fire").

    • The sparkle of a diamond results from both refraction and reflection.

Spectrometer

  • A spectrometer is a device for separating light into component wavelengths, acting as an investigative tool.

Spherical Mirrors

  • A spherical mirror is a section of a sphere defined by its radius and center of curvature.

  • Principal terms include:

    • Principal Axis: Line through to the mirror surface.

    • Vertex (V): Intersection of the principal axis with mirror's surface.

    • Focal Point (F) and Focal Length (f): Relationship, f = \frac{R}{2} or R = 2f.

Concave/Converging Mirror

  • A concave or converging mirror has its inside surface shaped inward, resembling a cave.

  • Light rays parallel to the principal axis converge at the focal point. Non-parallel incoming rays converge at the focal plane.

Convex/Diverging Mirror

  • A convex or diverging mirror has its outside surface bulging outward.

  • Parallel incoming rays reflect and appear to diverge from the focal point. Non-parallel rays become parallel to the principal axis, creating an expanded field of view.

Ray Diagrams for Mirrors

  • Ray diagrams solve spherical mirror image locations graphically. Conducted by:

    • Drawing two rays:

    1. A ray parallel to the principal axis, which reflects through the focal point.

    2. A ray through the center of curvature to the mirror surface, reflecting directly back.

  • Intersection of these rays indicates image position.

Image Characteristics

  • Images can be defined based on:

    • Real or Virtual:

    • Real Image: Light rays converge for image formation on a screen, located in front of the mirror.

    • Virtual Image: Light rays diverge; image forms behind or inside the mirror and cannot be screened.

    • Orientation: Upright (virtual images) or inverted (real images).

    • Size: Larger or smaller than the object.

Ray Diagrams - Concave Mirror Examples

  • Object Beyond Center of Curvature: Example shows an object place. The image is real and inverted.

  • Object Between F and C: An image formed in this scenario is real.

  • Object Inside the Focal Point (F): Produces a virtual image.

Ray Diagrams - Convex Mirror

  • Images from convex mirrors are always virtual, upright, and smaller than the object.

Lenses Overview

  • A lens refracts light waves and can create an image of an object. Types include:

    • Converging (Convex) Lens: Thicker in the center than edges.

    • Diverging (Concave) Lens: Thicker at the edges than at the center.

Ray Diagrams for Lenses

  • Images formed through lens refraction can be determined through a graphical method similar to mirror ray diagrams.

  • Ray Diagram Construction involves:

    • Drawing two rays:

    1. A ray parallel to the principal axis refracted through the focal point.

    2. A ray through the center of the lens, maintaining its direction.

  • The intersection marks the image position.

Ray Diagrams - Converging Lens Examples

  • If an object is beyond focal point (F), an inverted, real image forms.

  • When inside focal point (F), a virtual image arises.

Ray Diagrams - Diverging Lens Example

  • The image formed is always upright and smaller than the object.

Human Eye Structure

  • Photoreceptors:

    • Rods: More sensitive, responsible for light and dark vision.

    • Cones: Responsible for color vision.

  • Signals are transmitted to the brain via the optic nerve.

Vision Defects

  1. Nearsightedness: Clear vision for nearby objects, images form in front of the retina.

    • Corrected with diverging lenses.

  2. Farsightedness: Clear vision for distant objects, images form behind the retina.

    • Corrected with converging lenses.

Near Point and Aging

  • Near Point: Closest position where objects can be seen clearly, alters with age:

    • Children: around 10 cm

    • Young Adults: 12-15 cm

    • Adults past 40 may struggle to see objects closer than 25 cm due to less deformable lenses.

Polarization

  • Polarization: The preferential orientation of the light's electric field vectors. Polarization can be achieved by polarizing films, which allow light in a specific orientation to pass through.

  • The human eye cannot detect if light is polarized or unpolarized.

Polarized Light and Polarizers

  • Polarized Light Through Two Parallel Polarizers: The first polarizer transmits polarized light, while the aligned second allows it to pass as well.

  • Crossed Polarizers: Allow light from the first to pass, but block light from the second that is perpendicular in orientation.

Polarized Light

  • Polarization supports evidence that light is a transverse wave.

  • Longitudinal waves such as sound cannot be polarized.

Polarizing Sunglasses

  • Light reflecting off surfaces can become partially polarized and create glare. Polarizing sunglasses, aligned vertically, block horizontal light components, greatly reducing glare.

  • Longitudinal waves can not be polarized

Diffraction

  • The bending of waves passing through openings or around obstacles comparable to their wavelength size. All waves, including sound and light, exhibit this phenomenon.

  • Diffraction generally increases with wavelength compared to size of opening.

Light Diffraction Examples

  • Wavelength examples:

    • Audible sound waves: centimeters to meters.

    • Visible light: 400-700 nm

    • Usable diffraction is easily noticed in sound (ex: hearing around corners) contrary to light, which casts sharp shadows.

Radio Wave Diffraction

  • Radio waves have long wavelengths:

  • AM waves diffract more easily around buildings due to longer wavelengths, improving reception.

Interference

  • Interference: Occurs when waves overlap, creating a combined waveform.

  • Principle of Superposition: Combined waveform equals the sum of individual waves.

  • Two types of interference:

    • Constructive Interference: Wave pulses reinforce, amplifying the overall amplitude.

    • Destructive Interference: Wave pulses cancel, leading to lower overall amplitude. Energy is conserved; it is not destroyed.

Total Interference Types

  • Total Constructive: Peaks of overlapping waves lead to maximum amplitude.

  • Total Destructive: Overlapping troughs and peaks result in zero amplitude.

Thin-Film Interference

  • Displays of color from oil films or soap bubbles result from interference.

  • Light reflects off air-oil and oil-water surfaces, leading to constructive and destructive interference based on angles and thickness.

Diffraction Interference

  • Occurs when light bends at narrow slits. If employing two narrow slits, wavelength computation is possible via geometry. Multiple slits yield sharp lines aiding in light analysis.

Double-Slit Interference

  • Light waves passing through two slits interfere, creating bright and dark regions on a screen.

Diffraction Grating

  • The use of multiple slits produces sharper interference patterns compared to just two slits.